CBSE Chemistry Revision Notes PDF

Chapter 1 SOME BASIC CONCEPTS OF CHEMISTRY Topic-1 General Introduction and Nature of Matter Revision Notes Chemistry is the branch of science that deals with Gases have neither definite volume nor definite the composition, structure and propertie...

Chapter 1 SOME BASIC CONCEPTS OF CHEMISTRY Topic-1 General Introduction and Nature of Matter Revision Notes Chemistry is the branch of science that deals with Gases have neither definite volume nor definite the composition, structure and properties of matter. shape. Nature of matter: Matter is anything that has mass The three states of matter are inter-convertible and occupies space. into each other by changing the conditions of Matter exist mainly in three physical states viz, solid, temperature and pressure. liquid and gas. The other two less common states of heat heat matter are Plasma and Bose-Einstein condensate state.  Solid    Liquid   Gas cool cool Solids have definite volume and definite shape. Liquids have definite volume but do not have Classification of matter: Matter can be categorised definite shape. as: The matter which contains two or more substances W=m×g is called mixture. where, m = mass, g = gravitational force The mixture in which the composition is uniform Note: The mass of substance is constant, whereas, throughout the mixture is called homogeneous its weight may vary from one place to another due mixture. e.g., sugar solution in water. to change in gravity. The mixture in which the composition is not uniform (b) Volume: SI unit of volume is m3. throughout the mixture and sometimes different (c) Density: components are visible is called heterogeneous Density = Mass/ Volume mixture. e.g., mixture of salt and sugar. d = m/V Pure substances have fixed composition. These can be classified into elements and compounds. l SI unit of density is kg m–3. An element contains only one type of particles. (d) The three common scales are used to measure These particles may be atoms or molecules. temperature: A compound is a combination of two or more atoms [e.g., Kelvin (K), degree Celsius (°C), degree of different elements in a definite ratio. Fahrenheit (°F)]. Properties of Matter and their Measurement: The (e) Length: general properties of matter are categorised into l Length is expressed in Angstrom (Å), nano- two types: metre (nm) and picometre (pm). These are (i) Physical properties related to the SI unit as follows: (ii) Chemical properties 1 Å = 10–10 m, Some Physical Quantities: 1 nm = 10–9 m, (a) Mass and Weight 1 pm = 10–12 m. (i) Mass: The standard unit of mass in the There are certain rules used for determining signifi- International System of Units (SI) is kilo- cant figures: gram (kg). (i) All non-zero digits are significant. (ii) Weight: In the International System of Units (ii) Zero preceding to first non-zero digit are not (SI), weight can be expressed in terms of the significant. force, i.e., Newton (N). 2 Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI (iii) Zero between two non-zero digits are signifi- cant. Key Equations (iv) Zeros at the end or right of a number are sig- Mass nificant, provided they are on the right side of Density = the decimal point. Volume Temperature: K = °C + 273.15; Laws of Chemical Combinations: Elements com- bine to form compounds in accordance with the fol- 9 °F  (C )  32 lowing five basic laws, called the laws of chemical 5 combinations. (i) Law of Conservation of Mass: Mass can neither be created nor destroyed. Key Facts (ii) Law of Definite Proportions/Composition: A given compound always contains exactly the The International System of Units (SI): The SI same proportion of elements by weight. system has seven base units. (iii) Law of Multiple Proportions: When two Volume: V = 1 dm3 = 103 cm3 = 10–3 m3. elements combine with each other to form Standard Temperature Pressure (STP): 0 °C more than one compound, the masses of one (273.15 K) temperature and 1 atmosphere pressure. element that combine with a fixed mass of the Normal Temperature Pressure (NTP): other element, are in the ratio of small whole numbers. 20 °C (293.15 K) temperature and 1 atmosphere (iv) Gay Lussac’s Law of Gaseous Volumes: When pressure. gases combine or are produced in a chemical Standard Ambient Temperature Pressure (SATP): reaction they do so in a simple ratio by volume, 25 °C (298.15 K) temperature and 1 atmosphere provided all the gases are at same temperature pressure. and pressure. (v) Avogadro's Law: At the same temperature and pressure, equal volume of all gases should contain equal number of molecules. Mnemonics Dalton’s Atomic Theory: l Matter consists of tiny indivisible particles. Concept Name: Metric System l Atoms are indestructible and cannot be divid- Mnemonics: The Great Morning King Henry ed into smaller particles. Mnemonics: Doesn't Usually Drink chocolate Milk l All atoms of a given element have identical Mnemonics: Mixed with Natural Powder properties, including identical mass. Atoms of Interpretation different elements differ in mass. The: Tera (×1012) l When atoms of different elements react to form Great: Giga (×109) compounds, their atoms combine a simple Morning: Mega (×106) fixed ratio. King: Kilo (×103) l Chemical reactions involve reorganisation of Henry: Hecto (×102) atoms. These are neither created nor destroyed in a chemical reaction. Doesn't: Deca (×10) Drawbacks of Dalton’s Atomic Theory: Usually: Unit (×1) l Theory does not explain the occurrence of Drink: Deci (×10–1) allotropes. Chocolate: Centi (×10–2) l Atom can be further divided into sub-parts i.e., Milk: Milli (×10–3) Electron, proton, neutron. Mixed with: Micro (×10–6) l The theory fails to explain how atoms combine with different elements to form a group. Natural: Nano (×10–9) Powder: Pico (×10–12) Atomic and molecular mass, mole concept, Topic-2 Percentage, empirical and Molecular Formula Revision Notes Atomic Mass: One atomic mass unit is defined as a carbon-12 atom. mass exactly equal to one-twelfth of the mass of one  Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI 3 Average mass of an atom×12 Number of moles of gas Relative Atomic Mass = Volume of the gas (STP) Mass of an atom of carbon-12 = Average Atomic Mass: The average atomic mass of 22.4 L an element refers to the atomic masses of the iso- Number of molecules = Number of moles × NA topes of the element, taking into account the abun- Where, NA = Avogadro's number = 6.022 × 1023 dances of the element’s isotopes. Number of atoms = Number of molecules Average Atomic Mass = × Atomicity Σ(Mass of isotopes × % natural abundance) Mass percentage = Total mass of element × 100 100 Molar mass Gram Atomic Mass: Gram atomic mass is the mass, in grams, of one mole of atoms in a monoatomic Percentage Composition: chemical element. Mass % of the element = Gram Atomic Mass = Atomic mass expressed in Mass of element in a molecule of the compound grams × 100 Molecular masss of the compound Molecular Mass: Molecular mass or molecular weight refers to the mass of a molecule. It is cal- Relationship between Empirical and Molecular culated as the sum of the mass of each constituent Formulae: atom multiplied by the number of atoms of that ele- ment in the molecular formula. Molecular Formula = n × Empirical formula Gram Molecular Mass or Gram Molecule: Gram Molar mass where n = molecular mass is the mass in grams of one mole of Empirical formula mass a molecular substance. Formula Mass: The formula mass of a molecule is the sum of the atomic weights of the atoms in the empirical formula of the compound. Key Term Mole Concept and Molar Mass: Mole is a funda- mental unit in the system. Molar mass is the mass Molar Volume (Vm): It is the volume occupied by of one mole of any substance, i.e., element or com- one mole of any substance. pound. Percentage Composition: Percentage composition of a compound is the ratio of the amount of each element to the total amount of individual elements Key Facts present in a compound multiplied by 100. Empirical Formula: A formula that gives the simplest 1 amu = 1.66056 × 10–24 g whole number ratio of various atoms in a compound. Mass of hydrogen atom in terms of amu Molecular Formula: It is a formula in which the 1.6736 × 10 −24 g exact number of different types of atoms present in = a molecule of a compound. 1.66056 × 10 −24 g/amu Steps for writing molecular formula: = 1.0078 amu (i) Conversion of mass percent to grams. Now, ‘amu’ has been replaced by ‘u’ which is (ii) Convert into number moles of each element. (iii) Divide each of the mole values obtained above called unified mass. by the smallest number amongst them. 1 mole of a substance = Molar mass of substance (iv) Write down the empirical formula by mention- = Avogadro’s number of chemical units = 22.4 L ing the numbers after writing the symbols of volume at STP of gaseous substance respective elements. e.g., 1 mole of CH4 = 16 g of CH4 = 6.022 × 1023 (v) Writing molecular formula: molecules of CH4 = 22.4 L at STP (a) Determine empirical formula mass by adding the atomic masses of various atoms w g VL (at STP) x particles MV present in the empirical formula. n= = = = Mm 22.4 L NA 1000 (b) Divide molar mass by empirical formula mass. Molar volume of gas = 22.4 L at STP (273K, 1 atm) (c) Multiply empirical formula by n obtained or 22.7 L at SATP (273 K, 1 bar) above to get the molecular formula. Calculating Molar Volume : PV = nRT Key Formulae nRT 1 mol × 0.0821 L atm K −1 mol −1 × 273 K ∴V= = P 1 atm Number of moles of atoms = Mass of element = 22.4 L Atomic mass Number of moles of molecules = Mass of molecules Molecular mass 4 Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI Chemical reactions, stoichiometry and Calculation Topic-3 based on Concentration Revision Notes Chemical Equation: A chemical equation is the correct formulas of the reactants and products. symbolic representation of a chemical reaction. Step 2: Write down the number of atoms per element. Step 3: Starting balancing with single element one by one. Stoichiometry and Stoichiometric Calculations: Step 4: Verify that the number of atoms of each l Stoichiometry is the calculation of masses element is balanced in the final equation. (sometimes volumes also) of the reactants and Mass-Mole Number Relationship: products involved in a chemical reaction. Mass of g l Balancing a chemical equation: Number of Moles = Molar mass in g mol –1 Step 1: Write down the chemical equation using Divided by Multiplied by molar mass Avogadro's constant Mass of Number of Mole substance Multiplied by Divided by Particles molar mass NA Ø Limiting Reagent: The reactant which gets consumed first, limits the amount of product formed and is, called the limiting reagent. It is present in smaller amount. l Mole Fraction of component: The ratio of the Strength of Solution: It is defined as the amount of number of moles of a given component of a solute dissolved per litre of solution. mixture to the total number of moles of all the Strength = M × Molecular weight components. M = Molarity Two components A and B Normally: It i defined as number of gram equivalents Mole fraction of A of solute dissolved per litre of solution Number of moles of A nA W × 1000 xA = = N= B Number of moles of solution nA + nB EB × V nA and nB are the mole of A and B respectively. Mole fraction of B, Number of moles of B nB Key Formulae xB = = Volume of solute Number of moles of solution nA + nB % by v strength = × 100 Also, remember that xA + xB = 1 V Volume of solution Molarity: It is defined as the number of moles of Weight of solute % by w strength = × 100 solute dissolve per 1000g or 1 kg of solvent V Volume of solution W × 1000 % Strength × density × 10 m= B M= M B × WA Molecular weight of solute Ø Modes of expressing concentration of solution: 1000 × M l Mass Percent: M= M × Molar mass of solute - 1000 d Mass of solute Mass percent = × 100 where, d = density Mass of solution M = Molarity Molality: The number of moles of solute per kilogram m = molality of solvent. Unit is mol kg–1 or m (molal). Mass of liquid Specific gravity = Vol. of liquid Molecular weight = 2 × Vapour density  Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI 5 Parts per million (ppm). If a solute is present in For dilution, molarity equation is: trace quantities, the concentration of the solution M1V1 = M2V2 is expressed in parts per million (ppm). Where, M 1 = initial molarity, V1 = initial volume, M2 = final molarity and V2 = final volume. Weight of solute × 10 6 ppm = Weight of solution Key Terms Limiting Reagent: It is the reactant which gets Mnemonics consumed first or limits the amount of product formed. Concept: Law of conservation of mass Mnemonics: BESABS Mass Percent: It is the mass of the solute in grams Interpretation: Balanced chemical equation has Same per 100 grams of the solution. number of Atoms on Both Sides Concept: Strength of solution in terms of molarity Mass of solute in g ×100 Mass % = Mnemonics: SSMM Mass of solution in g Interpretation: Strength of Solution is Molarity into Molecular weight  Chapter 2 STRUCTURE OF ATOM Fundamental Particles of an Atom, Dual Nature Topic-1 of Electromagnetic Radiation, Photoelectric Effect and Bohr Model Revision Notes Discovery of electron: In mid 1850s many scientists To vacuum pump Fluorescent mainly Faraday began to study electrical discharge coating in partially evacuated tubes, known as cathode cathode – ray discharge tubes. A cathode ray tube is made anode + of glass containing two thin pieces of metal, called electrodes, sealed in it. High voltage A cathode ray discharge tube with perforated Fig. 2: A cathode ray discharge anode tube with perforated node and zinc sulphide coating for phosphorescence The results of these experiments are summarised below: (i) The cathode rays start from cathode and move towards the anode. (ii) These rays themselves are not visible but Fig. 1: A cathode ray discharge tube their behaviour can be observed with the help of certain kind of materials (fluorescent or phosphorescent) which glow when hit by them. 6 Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI (iii) In the absence of electrical or magnetic field, electrical and magnetic field perpendicular to each these rays travel in straight lines. other as well as to the path of electron. Thomson (iv) In the presence of electrical or magnetic field, was able to determine the value of e/me as 1.758820 the behaviour of cathode rays are similar × 1011 C kg–1. to that expected from negatively charged Charge of electron: It was determined by R.A. particles, suggesting that the cathode rays Milikan by oil drop experiment. He found that consist of negatively charged particles, called the charge on the electron to be – 1.6 × 10–19 C. electrons. The present accepted value of electrical charge is (v) The characteristics of cathode rays (electrons) – 1.6022 × 10–19 C. do not depend upon the material of electrodes Discovery of proton: Also known as canal rays. and the nature of the gas present in the They were discovered through electrical discharge cathode ray tube. carried out in modified cathode ray tube. Charge to mass ratio: In 1897, J.J. Thomson Discovery of neutron: These particles were measured it by using cathode ray tube and applying discovered by Chadwick (1932) by bombarding a thin sheet of beryllium by a-particles. Table 2.1 Properties of Fundamental Particles Absolute Relative Approx. Name Symbol Mass/kg Mass/u charge/C charge mass/u Electron e –1.602176 × 1019 –1 9.109382 × 10–31 0.00054 0 –19 –27 Proton p +1.602176×10 +1 1.676216 × 10 1.00727 1 Neutron n 0 0 1.674927 × 10–27 1.00867 1 Thomson Model of Atom: J. J. Thomson, in 1898, Drawbacks of Rutherford’s model of atom: proposed that an atom possesses a spherical shape (i) It cannot explain the stability of an atom. in which the positive charge is uniformly distributed (ii) It says nothing about the electronic structure like plum pudding, raisin pudding or watermelon. of atoms. Rutherford Nuclear Model of Atom: Rutherford Atomic number (Z) = number of protons in the and his students bombarded very thin gold foil with nucleus of an atom = number of electrons in a a–particles. neutral atom It was observed that: Mass number (A) = number of protons (Z) + (i) Most of the a–particles passed through the number of neutrons (n) gold foil undeflected. Isobars: Isobars are the atoms with same mass (ii) A small fraction of the a–particles were number but different atomic number. For example, 14 14 deflected by small angles. 6 C and 7 N. (iii) A very few a–particles (~1 in 20,000) bounced Isotopes: Atoms with identical atomic number back, that is, were deflected by nearly 180°. but different atomic mass number are known as Rutherford drew the following conclusions: isotopes. For example, protium 11H, deuterium 12D (i) Most of the space in the atom is empty as and tritium 13T. most of the a–particles passed through the foil Developments Leading to the Bohr's Model of Atom undeflected. In order to improve Rutherford's atomic model, two (ii) A few positively charged a–particles were new concepts played a major role, these are: deflected. The positive charge has to be (i) Dual nature of electromagnetic radiation. concentrated in a very small volume that (ii) Experimental results regarding atomic spectra. repelled and deflected the positively charged Wave Nature of Electromagnetic Radiation: James a–particles. Maxwell (1870) suggested that when electrically (iii) Calculations by Rutherford showed that the charged particle moves under acceleration, alternating volume occupied by the nucleus is negligibly electrical and magnetic fields are produced and small as compared to the total volume of the transmitted. These fields are transmitted in the form atom. of waves called electromagnetic waves or electro According to Rutherford’s model of atom: magnetic radiation. (i) The positive charge and most of the mass of the l do not require any medium for their atom was densely concentrated in extremely propagation and can move in vacuum. small region called nucleus. l The increasing order of wavelengths of these (ii) The nucleus is surrounded by electrons that radiations is given as below : move around the nucleus with a very high Cosmic rays < g – rays < X – rays < Ultraviolet speed in circular paths called orbits. light < Visible light < Infrared rays < Micro waves < Radio waves. (iii) Electrons and the nucleus are held together by electrostatic forces of attraction.  Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI 7 l Electromagnetic radiation shows wave like properties during propagation and shows Mnemonics particle like properties on interaction with matter. Concept: Atomic Spectra Dual Nature of Electromagnetic Radiation: Mnemonics: Ronald McDonald Invented Very Photoelectric effect shows particle nature of light. Interpretation: Unusual & eXcellent Gherkins In the Diffraction and interference explain wave nature of order of increasing frequency or decreasing light. wavelength of electromagnetic waves The branch of science that deals with the study of Radio waves, Microwaves, Infrared, Visible spectra is called spectroscopy. light, Ultraviolet, X-Rays, Gamma Rays Spectra are classified as: (i) Emission spectra (ii) Absorption spectra. Characteristics of Waves: Emission Spectra: The spectrum of radiation l Wavelength (l): The distance between two emitted by a substance that has absorbed energy is successive crests or troughs. Unit: Meter (m). called an emission spectrum. l Frequency (u): It is defined as the number of Absorption Spectra: When the white light is first waves that pass a given point in one second. passed through the substance and the transmitted Unit: Hertz (Hz) or s–1. light is analysed we get absorption spectrum. It consists of dark lines separated by bright bands. l Wave number ( v ): The number of wavelengths Line spectra: The emission spectra of atoms in the per unit length. Unit: m–1. gas phase emit light only at specific wavelengths Relationship between Wavelength, Wave number, with dark spaces between them. Such spectra are Frequency and Velocity called line spectra or atomic spectra. l These characteristics are related as : Line Spectrum of Hydrogen: All series of lines in the c and hydrogen spectrum can be described by Johannes c=l×u or u= Rydberg expression: λ 1= υ \ n=cv υ =109, 677  12 – 12  cm –1 λ  n1 n2  Particle Nature of Electromagnetic Radiation: The where n1 = 1, 2,........ and n2 = n1 + 1, n1 + 2,............. phenomenon like diffraction and interference can The spectral Lines for Atomic Hydrogen are given be explained by wave nature of electromagnetic radiation but it could not explain black body as below: radiation and photoelectric effect. Series n1 n2 Spectral region Black Body Radiation and its Explanation: The Lyman 1 2, 3,..... Ultraviolet ideal body that emits and absorbs radiations of all Balmer 2 3, 4,..... Visible frequencies, is called black body and the radiation Paschen 3 4, 5,..... Infrared emitted by such a body is called black body radiation. l According to Planck’s quantum theory, atoms Brackett 4 5, 6,..... Infrared and molecules emit (or absorb) energy in the Pfund 5 6, 7,..... Infrared form of small discrete packets of energy, known From the line spectrum of different elements, it can as quantum. The energy of each quantum is be concluded that: given by the expression : (i) Line spectrum of an element is unique. E = hn (ii) There is a regularity in the line spectrum of where, h = Planck's constant (6·626 × 10–34 J s) each element. Bohr's Model for Hydrogen atom: Neils Bohr (1913) proposed a new model of atom based upon Planck's Mnemonics quantum theory quantitatively explaining the general features of the hydrogen atom structure Concept: Planck's Quantum theory and its spectrum. Mnemonics: Employee's Provident Fund (EPF) Postulates of Bohr's Atomic Model: Interpretation: Energy = Planck's constant × Frequency l In an atom, the electrons revolve around the DE = hn nucleus in certain definite circular paths of fixed radius called orbits or shells. Photoelectric Effect and its Explanation: l Each shell or orbit corresponds to a definite l The photoelectric effect is the observation energy. Therefore, these circular orbits are also that certain metals (e.g., potassium, rubidium, known as energy levels or energy shell. caesium, etc.) emit electrons when light shines Electron can move from one orbital to another upon them. Electrons emitted in this manner either by absorption of energy or by loss of can be called photoelectrons. energy. Total energy of photon = Work function + l The frequency of radiation absorbed or emitted Kinetic energy. is calculated by Bohr’s frequency rule : hn = hn0 + 1 mv2 ∆E E1 − E1 2 v= = h h 8 Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI where, E1 and E2 are the energies of the lower and higher allowed energy states respectively. Key Formulaes l The angular momentum of an electron is quantised: Angular momentum of orbits in c =l×v which electron revolves must be an integral 1 = υ multiple of h , where h is Planck's constant (h λ 2π = 6.626 × 10–34 Js). v =cυ \ me = n. h E = hv 2π 1 where n = 1, 2, 3,.............. hv = hv0 + mv2 According to Bohr's Theory for Hydrogen Atom: 2 (i) The stationary states for electron are numbered  1 1 –1 n = 1,2,3.......... These integral numbers are v = 109, 677  2 − 2  cm  n1 n2  known as Principal quantum numbers. (ii) The radii of stationary states are given by: ∆E E − E1 rn = n2a0 v = = 1 h h where a0 = 52·9 pm. As n increases the value of r will increase. h mevr = n. (iii) The energy of stationary state is given by: 2π En = – RH ( n1 ) 2 rn = n2a0  1 where n = 1, 2, 3,........ and RH (Rydberg En = – R H  2  n  constant) = 2·18 × 10–18 J. (iv) For hydrogen like ions i.e., one electron system (e.g., He+, Li2+, Be3+), the radii is given by, Key Terms 52.9 2 rn = n pm Electromagnetic Radiation: Energy emitted from Z any source in the form of waves in which electric and energies of the stationary states are given and magnetic fields oscillated perpendicular to by: each other and travel with a velocity of light is En = – 2.18 × 10–18 n 2 Z2 ( ) called electromagnetic radiation. Amplitude: The height of crest or depth of a (v) It is also possible to calculate the velocities of trough is called amplitude. It is denoted by the electrons moving in these orbits. symbol ‘a’. l If fails to explain zeeman effect (be splitting of Velocity: The distance travelled by a wave in one spectral line in the presence of magnetic field) second is called velocity. and tark effect be slitting of spectral line the Photon: A packet or particle of light energy is presence of electric field. called photon. Limitations of Bohr's Model: Photoelectric Effect: The phenomenon of l It fails to explain doublet observed in the hydrogen spectrum. ejection of electrons from a metal surface when a l It fails to explain the ability of atoms to form light radiation of suitable frequency falls on metal molecule by chemical bonds. surface is called photoelectric effect. l It fails to explain Zeeman effect (i.e., splitting of Threshold Frequency: The minimum frequency spectral line in the presence of magnetic field) (no) below which photoelectric effect is not and stark effect (i.e., splitting of spectral line in observed, is called threshold frequency. the presence of electric field) Mnemonics Key Facts 1. Concept: H-atom spectral lines Energy wise order of electromagnetic radiation : Mnemonics: Myan Mer Pasta Bread Fund Cosmic rays < g-rays < X-rays < UV light < Interpretation: Lyman (n1 = 1) Visible light < IR rays < Micro waves < Radio Balmer (n1 = 2) waves Paschen (n1 = 3) Wavelength (l) increases Brackett (n1 = 4) Frequency (n) decreases Pfund (n1 = 5) Wave number ( v ) decreases 2. Concept: Bohr Model of an atom Energy (e) decreases Mnemonics: Electronic video Recording (EVR) Interpretation: Energy (E) µ Z The value 109, 677 cm–1 is called the Rydberg Velocity (V) µ Z constant for hydrogen. Radius (R) µ n  Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI 9 de-Broglie Equation, Heisenberg's Uncertainty Topic-2 Principle and Quantum Mechanical Model Revision Notes Dual Behaviour of Matter: Einstein had suggested that 3. Both the exact position and exact velocity of light can behave as a wave as well as like a particle. i.e., an electron in an atom cannot be determined it has dual character. simultaneously. Therefore, only probability of finding an electron at different points is required. In 1924, de Broglie suggested that matter like radia- 4. An atomic orbital is the wave function y for an tions, should also exhibit a dual character – i.e., wave electron in an atom. and particle like properties. This means that just as 5. The probability of finding an electron at a point the photon has momentum as well as wavelength, within an atom is proportional to the square of electron should also have both. the orbital wave function, i.e., |y|2 at that point According to de-Broglie, the wavelength associated |y|2. It is also known as probability density. The with a particle of mass m, moving with velocity n value of probable density is always positive. is given by the relation, From the value of |y|2 at different points within h h an atom, it is possible to predict the region   around the nucleus where the electron will most mv p likely be found. where h = Planck's constant n = Velocity of particle (p = mv) = momentum of the particle. Key Terms Heisenberg's Uncertainty Principle Momentum : The quantity of motion of a moving body measured as a product of its mass and Heisenberg's uncertainty principle states that, velocity. "It is not possible to determine simultaneously both Uncertainty in position of the particle: If the the exact position and momentum (or velocity) of velocity of the electron is known precisely [D(vx) an electron, with absolute accuracy." is small], then the position of the electron will Mathematically, this law may be expressed as : be uncertain (Dx will be large), which is called h uncertainty in position of the particle. Dx × Dpx ³ 4p Uncertainty in velocity of the particle: If the h position of the electron is known with high Dx.D(mvx) ³ 4p degree of accuracy (Dx is small), then the velocity h of the electron will be uncertain [D(vx) is large], Or, Dx × Dvx ³ 4pm which is called uncertainty in velocity of the particle. where, Quantum mechanics : The branch of science that Dx = uncertainty in position, takes into account this dual behavior of matter is Dpx = uncertainty in momentum, called quantum mechanics. Dvx = uncertainty in velocity. Quantum Mechanical Model of Atom and Concept of Atomic Model: Key Facts Quantum mechanics was developed independently Electrons should have momentum as well as by Heisenberg and Schrodinger. wave length. ^ ^ Schrodinger equation is H y = Ey where H is a math- An atomic orbital is the wave function y for an ematical operator called Hamiltonian, E is the total electron in an atom. energy of the system and y is the wave function. Important Features of the Quantum Mechanical Model of Atom 1. The energy of the electrons in atoms is quantized. 2. The existence of quantized electronic energy level is a direct result of the wave like properties of electrons. 10 Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI Atomic Orbital, Quantum Numbers and Electronic Topic-3 Configuration Revision Notes Orbitals and Quantum Numbers Value Value Name of Number of the l Orbitals: A wave function for an electron in of n of l sub-shell sub-shells an atom is called an atomic orbital, which describes a region of space in which there is a 1 0 1s one high probability of finding the electrons. These 2 0,1 2s, 2p two can be distinguished by their size, shape and 3 0, 1, 2 3s, 3p, 3d three orientation. l Quantum Numbers: Electron have only certain 4 0, 1, 2, 3 4s, 4p, 4d, 4f four permissible values of energy and angular (c) Magnetic Orbital Quantum Number (ml) : This momentum. These permissible states of an describes the spatial orientation of the orbitals electron in any atom are called orbitals which along the standard set of co-ordinate axis. Co- are identified by a set of four numbers. These ordinate for a given value of l, the possible numbers are called Quantum Numbers. values of ml vary from – l, 0 to + l. The various quantum numbers are : (d) Spin Quantum Number (ms): This quantum (a) Principal quantum number (n) number describes the spin orientation of the (b) Azimuthal or angular momentum electron. Since the electron can spin in only quantum number (l). two ways- clockwise or anti-clockwise and, (c) Magnetic quantum number (ml). therefore, the spin quantum number can take (d) Spin quantum number (ms). only two values, +½ or – ½. (a) Principal quantum number (n): This quantum number determines the size energy associated with it. The principal quantum number ‘n’ is Mnemonics a positive integer with value of n = 1,2,3........ 1. Concept: Quantum Numbers The maximum number of electrons which a Mnemonics: SPAM shell can accommodate is 2n2. All the orbitals of a given value of ‘n’ constitute a single shell Interpretation: of atom and are represented by the following S – Spin Quantum no. (ms) letters P – Principal Quantum no. (n) n = 1 2 3 4............ A – Azimuthal Quantum no. (l) Shell = K L M N............ M – Magnetic Quantum no. (m) (b) Azimuthal or subsidiary or orbital angular 2. Concept: Sequence of orbitals quantum number (l): This quantum number Mnemonics: Sober Physicists Don't Find Giraffes defines the three dimensional shape of the orbital. It can have positive integer values from Hiding In Kitchen zero to (n – 1) where n is the principal quantum Interpretation: s,p,d,f,g,h,i,k number, i.e., l = 0, 1, 2, 3,..... (n – 1). Boundary Surface Diagrams: l Shape of s-orbitals: s-orbitals are non-directional and spherically symmetrical.  Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI 11 l Shape of p-orbitals: For p-orbitals (l = 1), there are p-orbitals in each p-sub-shell. three possible orientations corresponding to ml = These are designated as px, py, and pz. – 1, 0, + 1 values. This means that there are three Z Z Z X X X 2px-orbital 2py-orbital 2pz-orbital Y Y Y (x) (y) (z) Fig. 2.2: Shape of p-orbitals l Shape of d-orbitals: The five d-orbitals, designated is different from others but all the five d-orbitals are as dxy, dyz, dzx, dx2-y2, dz2. The shapes of first four d- equivalent in energy, i.e., degenerate. orbitals are similar to each other, whereas fifth one dz2 dz2 dxy dzx dyz dx2–y2 z z z z z x x x y x x y y y y (a) (b) (c) (e) (d) Fig. 2.3: Shape of five d-orbitals The total number of nodes are given by (n – 1), i.e., sum of l angular nodes and (n – l – 1) radial nodes. Nodes and Nodal Planes: The place or point where probability density function reduces to zero are called nodes or nodal surface. Degenerate Orbitals: The orbitals with same energy are called degenerate orbitals, e.g., p sub-shell has three degenerate orbitals namely px, py and pz. Shielding Effect: The term shielding effect refers to a decrease in attraction between electrons and the nucleus in an atom. Electrons are highly attracted to the nucleus, because they have a negative charge and the nucleus contains protons, which have a positive charge. n + l Rule: Orbitals are filled in the order of increasing n + l. For equal n + l values, the orbital with the lower n is most often filled first. Here, n is the principal quantum number and l is the angular momentum quantum number. Filling of Electrons in Atomic Orbitals: The filling of electrons in different orbitals is governed by the following rules: (i) Aufbau Principle: This principle states that the electrons are filled in the various orbitals in an Fig. 2.4: Order of filling of electrons in orbitals increasing order of their energies, i.e., orbital (ii) Pauli's Exclusion Principle: According to this having lowest energy will be filled first and the principle, no two electrons in an atom can have orbital having highest energy will be filled last. the same set of four quantum numbers. It can also be stated as only two electrons may exist in 12 Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI the same orbital and they must be of opposite The completely filled and completely half filled sub- spin quantum numbers’. The maximum number shells are stable due to the following reasons: of electrons in the shell with principal quantum 1. Symmetrical distribution of electrons number n is equal to 2n2. 2. Exchange energy: Higher maximum energy (iii) Hund's Rule of Maximum Multiplicity: No leads to higher stability. electron pairing takes place in p, d and f orbitals Electronic Configuration of Ions: until each orbital in the given sub-shell contains Examples: one electron having parallel spin, e.g., N2 (7) has Cl (No. of electrons = 17): 1s2, 2s2, 2p6, 3s2, 3px2 , 3py2 , electronic configuration 1s2, 2s2, 2px1, 2py1, 2pz1 3pz1 according to Hund's rule: Cl– (No. of electrons = 18): 1s2, 2s2, 2p6, 3s2, 3px2 , 3py2 , 3pz2 Mg (No. of electrons = 12): 1s2, 2s2, 2p6, 3s2 Mg2+ (No. of electrons = 10): 1s2, 2s2, 2p6 Electronic Configuration of Atoms: The distribution of electrons in different orbitals is Key Terms known as electronic configuration of the atom. Effective nuclear charge: It is the net positive Significance of Electronic Configuration: The num- charge an electron experiences in an atom with ber of valence electrons in an atom determines how multiple electrons. It may be approximated by the readily an atom will react with other atoms. If it has 7 equation or 1 valence electron, then it only needs to gain/loss 1 Zeff = Z – S electron which is relatively easier than gaining/losing where, Z = atomic number 3 or 4 electrons. S = number of shielding electrons  Chapter 3 CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES Topic-1 Classification of Elements and Periodic Table Revision Notes Classification of Elements: Some of the earlier important attempts to classify Arranging the elements in such a way that similar the elements are briefly summed up below: elements are placed together while dissimilar (i) Dobereiner’s triads: In 1829, a German elements are separated from one another is known scientist, Johann Dobereiner classified the as classification of elements. elements in groups of three elements called On the basis of this classification, periodic table may triads. be defined as: The elements in a triad had similar properties The arrangement of the known elements according and the atomic weight of the middle member to their properties in a tabular form. of each triad was very close to the arithmetic Historical Development of Periodic Table: mean of the other two elements. DOBEREINER’S TRIADS Li Lithium Ca Calcium Cl Chlorine Na Sodium Sr Strontium Br Bromine K Potassium Ba Barium I Iodine Elements Atomic weight Lithium 7 Sodium 23 Potassium 39  Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI 13 Atomic weight of Na = 23 7 + 39 Mean of other two elements = 2 = 23 (ii) Newlands’ Law of Octaves: In 1865, John Newlands' called this relation as the law of Alexander Newlands gave Law of Octaves which octaves due to similarity with the musical scale. state that "when the elements are arranged in the increasing order of atomic weights, the properties of every eighth element are similar to the first one." Table 3.2 : Newland's Octaves Element Li Be B C N O F At. wt. 7 9 11 12 14 16 19 Element Na Mg Al Si P S Cl At. wt. 23 24 27 29 31 32 35.5 Element K Ca At. wt. 39 40 (iii) Mendeleev’s Periodic Law: The physical and chemical properties of elements are periodic function of their atomic weights. Table 3.3 : Mendeleev's Prediction for the Elements Eka-aluminium (Gallium) and Eka-silicon (Germanium) Eka-aluminium Gallium Eka-silicon Germanium Property (predicated) (found) (predicted) (found) Atomic weight 68 70 72 72.6 3 Density/(g/cm ) 5.9 5.94 5.5 5.36 Melting point /K Low 302.93 High 1231 Formula of oxide E2O3 Ga2O3 EO2 GeO2 Formula of chloride ECl3 GaCl3 ECl4 GeCl4 Characteristics of Mendeleev’s Periodic Table: (iii) Based upon atomic weights, isotopes of an (i) Eight vertical columns, called groups: Except element could be assigned different groups for VIII group, each group is further sub- but as they are the atoms of same element (with divided in A and B. This sub-division is made only difference in their molecular mass) they on the basis of difference in their properties. must be in same group. (ii) Seven horizontal rows, called periods. (iv) Some dissimilar elements are grouped Significance of Mendeleev’s Periodic Table: together while some similar elements are (i) Instead of studying properties of elements placed in different groups. For example, alkali separately, they can be studied in groups metals in group IA which are highly reactive containing elements with same properties. It are in the same group as coinage metals like Cu, led to the systematic study of the elements. Ag, Au of group IB. At the same time, certain (ii) Prediction of new elements: At his time, only chemically similar elements like Cu (group 56 elements were known. He left blank spaces IB) and Hg (group IIB) have been placed in for unknown elements. different groups. (iii) Mendeleev’s periodic table corrected the (v) Position of elements of group VIII: No proper doubtful atomic weights. place was allotted to nine elements of group Defects in the Mendeleev’s Periodic Table: VIII which have been arranged in three triads (i) Hydrogen was placed in group IA: However, without any justification. it resembles both group IA elements (alkali Modern Periodic Law: It states that “The physical metals) and VII A (halogens). Therefore, the and chemical properties of the elements are periodic position of hydrogen in the periodic table is functions of their atomic number.” anomalous or controversial. According to the recommendations of IUPAC, the (ii) Anomalous pairs of elements: Some elements groups are numbered from 1 to 18 replacing the with higher atomic weight like Argon (39.9) older notation of groups IA … VIIA, VIII, IB … VIIB precede potassium (39.1) with lower atomic and 0. weight. There are 7 periods. The first period contains 2 elements. The subsequent periods contain 8, 8, 14 Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI 18, 18, 32 elements respectively. The 7th period (i) There are in all, 18 vertical columns or 18 is incomplete and the 6th period would have a groups in the long form periodic table. theoretical maximum of 32 elements. In this form of (ii) These groups are numbered from 1 to 18 periodic table, the elements of both sixth and seventh starting from the left. periods (lanthanoids and actinoids respectively) are (iii) There are seven horizontal rows called periods placed in separate panels at the bottom. in the long form of periodic table. Thus, there Long Form of Periodic Table: are seven periods in the long form of periodic General characteristics of the long form of the table. periodic table: First period 2 elements Shortest period Second and Third period 8 elements each Short period Fourth and Fifth period 18 elements each Long period Sixth period and Seventh period 32 elements each Longest period (iv) The elements of group 1, 2 and 13 to 17 are called the main group elements. These are Key Terms also called typical or representative or normal Periods: The horizontal rows in the periodic table elements. are called periods. (v) The elements of group 3 to 12 are called Groups: The vertical columns in the periodic transition elements. table are called groups. (vi) Elements with atomic number 58 to 71 (Ce to Lu) occurring after lanthanum (La) are called Lanthanoids. Elements with atomic number Key Facts 90 to 103 (Th to Lr) are called Actinoids. These elements are called f-block elements and also as At present, 118 elements are known. inner-transition elements. In modern periodic table, there are 18 groups. Defects of long form of the periodic table Group 1 and 2: s-block elements in which last l In periodic table, the position of hydrogen is electron entered in s-sub shell [ns1, ns2]. not clear. Group 3 to 12: d-block 'or transition' elements l It is placed in the separate box just above the elements in which last electron entered in d-sub group I. shell (n – 1)d–10 l Lanthanoids and actinoids have not been Group 13 to 18: p-block elements in which last merged with in the main body of the periodic electron entered in p-sub shell [np1 to np6]. table. Group 18: Noble gases. Group 1-2, 13-17 are known as typical or normal or representative Mnemonics elements. In ‘s’ and ‘p’ – block elements, the last electron Concept: Genesis of Periodic Classification enters in the outermost shell. Mnemonics: London and Turkey Donates Lots of In d-block elements, the last electron enters in the Nuts to Peru, Libya and Malaysia but denied to penultimate shell (n – 1). Mexico, Poland, Liberia and Maldives In f-block elements, the last electron enters in the Interpretation: Laws of Trieds – Dobereiner sub-penultimate shell (n – 2). Laws of Octaves – Newland Elements with atomic number 58 to 71 (Ce to Periodic Law – Mendeleev Lu) occurring after lanthanum (La) are called Lanthanoids. Elements with atomic number Modern Periodic Law – Moseley 90 to 103 (Th to Lr) are called Actinoids. These elements are called f-block elements and also as inner-transition elements.  Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI 15 Nomenclature and Relation between Electronic Topic-2 Configuration and Periodic Table Revision Notes Nomenclature of Elements with Atomic Number or man-made radioactive elements. Greater than 100: Group-wise Electronic Configuration: Elements in (i) The name is derived directly from the atomic a group have similar electronic configuration, same number of the element using the latin numeral number of electrons in valence orbitals and similar roots. properties, e.g., Group 1 (alkali) metals have general (ii) The symbol of the element is composed of the electronic configuration ns1. initial letters of the numeral roots which make Position of Helium: The electronic configuration up the name. of helium is 1s2. So it should be in s-block, but it is (iii) The root ‘un’ is pronounced with a long ‘u’ placed in p-block due to the presence of completely to rhyme with ‘moon’. In the element names, filled valence shell (1s2) and similarity in properties each root is to be pronounced separately. with other noble gases. Electronic Configuration of Elements and the Position of Hydrogen: Electronic configuration of Periodic Table: hydrogen is 1s1. Because of the presence of only We can classify the elements into four blocks viz, one electron in s-orbital, it can be placed in group 1 s-block, p-block, d-block and f-block depending on with alkali metals and it also achieves the electronic the type of atomic orbitals that are being filled with configuration of inert gas so, it can also be placed in electrons. 17 group. Electronic Configuration in Periods: The period s-Block Elements: indicates the value of n for the outermost or valence l The elements of group 1 (alkali metals) and shell. group 2 (alkaline earth metals) which have It can be readily seen that the number of elements ns1 and ns2 outermost electronic configuration in each period is twice the number of atomic orbitals belong to the s-block elements. available in the energy level that is being filled. l They are all reactive metals with low ionization (i) Elements in first period: The first period (n = 1) enthalpies. They lose the outermost electron(s) starts with the filling of the lowest level (1s) readily to form +1 ion (in the case of alkali and therefore has two elements – hydrogen metals) or +2 ion (in the case of alkaline earth (1s1) and helium (1s2) when the first shell (K) metals). is completed. l The metallic character and the reactivity increase (ii) Elements in second period: There are 8 as we go down the group. Because of high elements, in the second period n = 2 (L shell) reactivity they are never found pure in nature. starting from lithium 1s2, 2s1 to neon 1s2, 2s22p6. l The compounds of the s-block elements, with (iii) Elements in third period: There are 8 elements the exception of those of lithium and beryllium in third period n = 3 (M shell) starting from are predominantly ionic. Sodium 1s2, 2s2, 2p6, 3s1 to Argon, 1s2, 2s2, 2p6, p-Block Elements: 3s2 3p6 (iv) Elements in fourth period: There are 18 l The p-block elements comprise those belonging elements in n = 4 (N shell) starting from to groups 13 to 18 and these together with the Potassium 1s2, 2s2, 2p6, 3s2, 3p6, 4s1 to Krypton s-block elements are called the Representative 1s2, 2s2, 2p6, 3s2 3p6, 4s2, 3d10 4p6. Elements or Main Group Elements. l The outermost electronic configuration varies (v) Elements in fifth period: It starts with Rb (37) from ns2np1 to ns2np6 in each period. At the end with 5s orbital followed by filling of 4d orbitals, of each period is a noble gas element with a then 5p orbitals and ends at Xe (54), having closed valence shell ns2np6 configuration. All total 18 elements forming 4d transition series. the orbitals in the valence shell of the noble (vi) Elements in sixth period: It has 32 elements gases are completely filled by electrons and it forming lanthanoid series and successive is very difficult to alter this stable arrangement electrons are filled in 6s, 4f, 5d and 6p orbitals, by the addition or removal of electrons. The in that order. noble gases exhibit very low chemical reactivity (vii) Elements in seventh period: This period due to completely filled valence shell. corresponds to n = 7 forming actinoid series l Preceding the noble gas family are chemically and just like sixth period it involves the filling of important groups of non-metals, i.e., 7s, 5f, 6d and 7p. This period contains synthetic chalcogens and halogens. 16 Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI l The non-metallic character increases as we move from left to right across a period and Key Term metallic character increases as we go down the group. Transuranic Elements: The elements which follow d-Block Elements (Transition Elements): uranium [i.e., neptunium to Uub (Z = 112) of l These are the elements of Group 3 to 12 in the actinoids] in the periodic table are called transuranic centre of the Periodic Table. or man-made elements. These element do not l These are characterised by the filling of inner occur in nature because their half-life periods are (n – 1)d orbitals by electrons and are therefore so short. referred to as. These elements have the outer electronic configuration (n – 1)d1-10ns0-2. They are all metals. l However, Zn, Cd and Hg which have the Mnemonics electronic configuration, (n – 1)d10 ns2 do not show Concept: Classification of Elements and Periodicity in most of the properties of transition elements. Properties. f-Block Elements (Inner-Transition Elements): 1. Elements of Atomic No (1-18) l The two rows of elements at the bottom of the Mnemonics: Periodic Table, called Lanthanoids, Ce (Z = 58) Happy Harry Listen BBC Network Over French – Lu (Z = 71) and Actinoids, Th (Z = 90) – Lr (Z Network. Native Magpies Always Sit Peacefully = 103) are characterised by the outer electronic Searching Clear Areas configuration (n – 2)f1-14 (n – 1 )d0-1 ns2. Interpretation: l They all are metals. The chemistry of the H, He, Li, Be, B, C, N, O, F, Ne, Na, Al, Si, P, S, Cl, Ar early actinoids is more complicated than 2. Group I Elements the corresponding lanthanoids due to large Little Nasty Kids Rub Cats Fur number of oxidation states. Lithium (Li), Sodium (Na), Potassium (K), Rubidium l Actinoids are radioactive in nature. (Rb) Caesium (Cs), Francium (Fr) 3. Group II Elements Mnemonics: Beer Mugs Can Snap Bar's Reputation Interpretation: Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), Radium (Ra) 4. Group III Elements Mnemonics: BAGIT Interpretation: Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), Thallium (Tl) Division of Periodic Table into s, p, d, f-Blocks Metals, Non-metals and Metalloids: 5. Group IV B Elements (i) Metals: Mnemonics: Can Simple Germans Surprise Public l Metals comprise more than 78% of all Interpretation: Carbon (C), Silicon (Si), Germanium known elements and appear on the left side (Ge), Tin (Sn), Lead (Pb) of the Periodic Table. l Metals are usually solid at room temperature 6. Group V B Elements (Mercury is an exception ; gallium and New Police Assign Subordinate Bikram on Duty caesium also have very low melting point Nitrogen (N), Phosphorus (P), Arsenic (As), 303 K and 302 K). Antimony (Sb), Bismuth (Bi) l Metals usually have high melting and boiling point except gallium and caesium. 7. Group VI B Elements (ii) Non-Metals Old Saharanpur Seems Terribly Polluted l These are present only in p-block and appear Oxygen (O), Sulphur (S), Selenium (Se), Tellurium on the right hand side of the periodic table. (Te), Polonium (Po) l These can be either solid or gases at room temperature exceptional case is bromine 8. Group VII B Elements (which is liquid at room temperature). First Class Biryani In Australia l Melting and boiling points are very low Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), except boron and carbon. Astatine (Al) l They are brittle and neither malleable nor ductile. 9. Group VIII B/18 Elements (iii) Metalloids: The elements like silicon, germanium, He never Arrived; Karan exited with Rohan arsenic, antimony and tellurium (Si, Ge, As, Sb, Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Te) which show properties of both metals and Xenon (Ex), Radon (Rn) non–metals are called Metalloids.  Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI 17 10. 3d-Series 13. Lanthanides Scary Tiny Vicious Creatures are Ladies Can't Put Needles Properly in Mean females come to Night Club Zen Slot-machines. Every Girl Tries Daily Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, However, Every Time You'd be lose 11. 4d-Series La, Ce, Pr, Nd, Pm, Sm, Eu Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu Yes S(z)sir Nob. Most Technicians Rub Rod's Pale Sil- 14. Actinides series ver Cadillac Three Planets: Uranus, Neptune, and Pluto. Amy Y, Zr, Nb>Mo, Tc, Ru, Rh, Pd, Ag, Cd Cured Berkeley, California. Einstein and Fermi Made 12. 5d-Series NobleLaws.(Ac) Th Pa U Np Pu Am Cm Bk Cf Es Fm Late Harry Took Walk, Reached Md No Lr Office In Pajamas After an Hour La......., Hf, Ta, W, Re, OS, Ir, Pt, Au, Hg Topic-3 Periodic Properties and their Periodic Trends Revision Notes Atomic Radii: The term atomic radius means Covalent Radius: Covalent radius may be the distance from the centre of the nucleus to the defined as “one-half of the distance between the outermost shell of electrons. centre of nuclei of two similar atoms bonded by On the basis of the nature of combining atoms, a single covalent bond. atomic radius can be of following types: For homonuclear molecules, rcovalent = Internuclear distance between two bonded atoms 2 l Metallic Radius: Metallic radius is taken as The study of ionic radii leads to two important one-half of the internuclear distance between generalizations: the two neighbouring atoms of a metal in a (i) The radius of positive ion (cation) is always metallic lattice. smaller than that of the parent atom because it l van der Waals Radius: Noble gas atoms are has lesser electrons but same nuclear charge. held together by weak van der Waals forces of (ii) The radius of negative ion (anion) is larger than attraction. The van der Waals radius is half of the that of the parent atom because extra electrons distance between the centre of nuclei of atoms. are added due to which repulsion among Variation of Atomic Radii in the Periodic Table: electrons increases and effective nuclear charge l Variation in a period: In general, the atomic decrease. radii decrease with increase in atomic number Isoelectronic Species: These are the atoms or ions on going from left to right due to the increase having the same number of electrons. e.g., O2–, F–, of effective nuclear charge. Na+ and Mg2+ have 10 electrons. For example, in the second period, the atomic Ionization Enthalpy: It is defined as the amount radii decrease from Li to Ne through Be, B, C, of energy required to remove the loosely bound N, O and F. electron from isolated gaseous atom in its ground l Variation in a group: The atomic radii of state. elements increase from top to bottom in a group. On moving down a group, the effective The first ionization enthalpy (IE1) is the energy nuclear charge decreases with increase in required to remove the most loosely bound electron atomic number and we expect that the size of of the neutral atom and the second ionisation atom should increase. enthalpy (IE2) is the energy required to remove the Ionic Radii: The effective distance from the centre second electron from the resulting cation and so on. of the nucleus of the ion to which it has an influence Thus first ionisation enthalpy of an element (X) may in the ionic bond. be defined with the reaction represented as : The ionic radii show the same trends as atomic IE1 radius. X(g) → X+(g) + e– 18 Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI Factors Affecting Ionization Enthalpy: also depends upon the electronic configuration l Size of the atom: The ionisation enthalpy of the atom. depends upon the distance between the For example, half filled and completely filled electron and the nucleus, i.e., size of the atom. shells have extra stability associated with them. It decreases with increase in atomic size. (i) The noble gases have the most stable electronic configurations (ns2np6) in each l Charge on the nucleus: With the increase in period and therefore, have high ionization nuclear charge it becomes more difficult to enthalpies. remove an electron from valence shell and therefore ionization enthalpy increases. (ii) The elements like Be (1s2 2s2) and Mg (1s2 2s2 2p6 3s2) have completely filled orbitals l Screening effect: The reduction in force of and their ionisation enthalpies are large. attraction by the electrons of shells present in between the nucleus and valence electrons is Variation down a group: Within a group, there is a called screening effect or shielding effect. The gradual decrease in ionization enthalpy in moving greater the number of intervening electrons from top to bottom. between valence electron and nucleus, the The decrease in ionization enthalpy down a group greater will be shielding or screening effect. An can be explained as: increase in the number of inner electrons tends l While going from top to bottom in a group, the to decrease the ionization enthalpy. nuclear charge increases atomic size increases l Penetration effect of electrons: s-electrons and shielding effect increase. are more penetrating towards the nucleus l There is a gradual increase in atomic size due to than p-electrons and the penetration power an additional main energy shell (n). decreases in a given shell in the order. l There is increase in shielding effect on the s>p>d>f outermost electron due to increase in the For the same shell, the ionization enthalpy number of inner electrons that outweighs the would be more to remove an s-electron than increasing nuclear charge and removal of the the energy required to remove a p-electron, outermost electron requires less energy down a which in turn will be more than that for the group. removal of a d-electron and so-on. Variation along a period: In general, the ionisation enthalpy increases with increasing atomic number l Electronic Arrangement: The ionization enthalpy in a period. Variation of ionization enthalpy with atomic number in second period The general Increase of ionisation enthalpy along required to remove subsequent electrons from the a period car be explained on the basis of atomic & atom in the44 gaseous state are known as successive nuclear change: ionization enthalpies. (i) On moving across a period from left to right, It has been found that the effective nuclear charge increases. IE1 < IE2 < IE3 (ii) The atomic size decreases along a period Electron Gain Enthalpy (DegH): It is defined as the though the main energy level remains the enthalpy change, when a neutral gaseous atom same. takes up extra electrons to form an anion. Successive Ionization Enthalpies: The energies X(g) + e– → X–(g)  Oswaal CBSE Revision Notes, CHEMISTRY, Class-XI 19 The value of electron gain enthalpy depends upon gain enthalpies atomic size, nuclear charge, etc. With increase in size, l Electron gain enthalpy values of noble gases the electron gain enthalpy decreases as the nuclear are positive while those of Be, Mg, N and P are attraction decreases. Thus, electron gain enthalpy almost zero generally decreases in moving from top to bottom l Electron gain enthalpy of fluorine is in a group (F < Cl > Br > I). In a period from left to unexpectedly less negative than of chlorine: right, the electron gain enthalpy generally increases However, it is observed that F-atom has due to increase in effective nuclear charge. unexpectedly less negative electron gain Factors Affecting Electron Gain Enthalpy: enthalpy than Cl-atom. This is due to the l Nuclear charge: The electron gain enthalpy very small size of F-atom. As a consequence becomes more negative as the nuclear charge of small size, there are strong inter electronic increases. This is due to greater attraction for repulsions in the relatively compact the incoming electron if nuclear charge is high. 2p-subshell of fluorine and thus the incoming l Size of the atom: With the increase in size of the electron does not feel much attraction. atom, the distance between the nucleus and the Electronegativity: incoming electron increases consequently, the l It is a qualitative measure of the ability of an electron gain enthalpy becomes less negative. atom in a chemical compound to attract shared l Electronic configuration: The elements having pair of electrons towards itself. stable electronic configuration of half-filled and l Pauling scale is generally used on which completely filled valence sub shells show very fluorine (most electronegative element) was small tendency to accept additional electron assigned a value of 4.0 arbitrarily. and thus electron gain enthalpies are less l Electronegativity generally increases on negative. moving across a period because of increase in Important Trends in Electron Gain Enthalpy: effective nuclear charge. l Halogens ha