Class 9th Structure of the Atom PDF

Structure of the Atom  Topics to be Covered Dalton’s atomic theory Discovery of Electron Discovery of Proton Discovery of Neutron Thomson’s atomic model Rutherford’s atomic model Neil Bohr’s atomic model Atomic Number Mass Number Valency Isotopes and Isobars  Dalton’s Atomi...

Structure of the Atom  Topics to be Covered Dalton’s atomic theory Discovery of Electron Discovery of Proton Discovery of Neutron Thomson’s atomic model Rutherford’s atomic model Neil Bohr’s atomic model Atomic Number Mass Number Valency Isotopes and Isobars  Dalton’s Atomic Theory In 1808, Dalton's atomic theory described matter as composed of indivisible atoms, forming the basis of modern chemistry. However, by the late 19th century, experiments revealed subatomic particles—electrons, protons, and neutrons—leading to the revision of his concept.  Discovery of Electron J. J. Thomson discovered the existence of electrons by performing a experiment called Cathode Ray Experiment. Cathode Ray Tube: A vacuum tube with electrodes (cathode and anode) used to produce a beam of electrons. High Voltage: Electricity flows from the negative (cathode) to the positive (anode) electrode. The properties of cathode rays are the same, irrespective of the gas or electrode material used. Discovery: Proved that atoms are divisible and contain subatomic particles like electrons.  Characteristics of an Electron Electrons are the negatively charged sub-atomic particles of an atom. Charge on electron = −1.6 × 10⁻¹⁹ C Mass of electron = 9.1 × 10⁻³¹ kg The symbol for an electron is e–. Electrons are extremely small. They are found outside the nucleus.  Discovery of Proton E. Goldstein by his famous anode rays/canal rays (positively charged radiations) experiment was able to detect presence of positively charged particles called protons in the atom. Characteristics of a Protons Charge on proton = + 1.6 × 10⁻¹⁹ C Mass of proton = 1.673 × 10⁻²⁴ gm i.e., Mass of proton ≅ 2000 × Mass of electron  Discovery of Neutrons J. Chadwick bombarded light elements (e.g., lithium, boron) with α-particles, discovering neutral particles with mass equal to protons, called neutrons. Since, mass of electrons are negligible as compared to that of proton and neutrons hence, sum of masses of protons and neutrons in an atom will compose its atomic mass. Property Electron (e⁻) Proton (p⁺) Neutron (n⁰) Charge Negative (-1) Positive (+1) Neutral (0) Negligible (~1/2000 of Mass 1 unit ~1 unit proton) Discoverer J.J. Thomson (1897) E. Goldstein (1886) J. Chadwick (1932)  Thomson’s Model of an Atom First model proposed and considered. Analogy: Atom compared to a watermelon: Positive charge = Red edible part. Electrons = Black seeds embedded in it. Alternative Comparison: Known as the plum pudding model.  Postulates of Thomson’s atomic model: Q. What are the two postulates proposed by Thomson? (i) An atom consists of a positively charged sphere and the electrons are embedded in it. (ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral. Q. On the basis of Thomson’s model of an atom, explain how the atomic neutral as a whole. Answer: The negatively charged particles are embedded in the positively charged sphere. These negative and positive charges are equal in magnitude. Thus, by counterbalancing each other's effect, they make an atom neutral.  Rutherford’s Model of Atom Gold Foil Experiment: Used a thin gold foil, only 1000 atoms thick, to observe alpha particle behavior. Alpha Particles: Doubly charged helium ions. Mass = 4u, fast-moving, with significant energy. Q. Why did Rutherford select a gold foil in his α–ray scattering experiment? Answer: Rutherford wanted a metal sheet that could be as thin as possible for the scattering experiment. Gold is the most malleable of all the metals known to man. It’s simple to make thin sheets out of it. As a result, for his alpha-ray scattering experiment, Rutherford used gold foil.  Observations: (i) Most of the alpha particles passed straight through that gold foil. (ii) There was a deflection at a small angle by some of the alpha particles. (iii) A very small amount of alpha particles rebounded.  Conclusions: Q. What were the observations obtained from Rutherford’s alpha particle experiment? 1. Mostly Empty Space: Most alpha particles passed through the gold foil undeflected, indicating atoms are predominantly empty space. 2. Positively Charged Nucleus: Few alpha particles deflected, and about 1 in 12,000 bounced back, revealing a small, dense, positively charged nucleus. 3. Nucleus Size: Nucleus is extremely small, about 1/10⁵ the size of the atom, occupying negligible space compared to the empty space in an atom. 4. Mass Concentration: Entire atomic mass is concentrated in the nucleus.  NUCLEUS: The following final model was put by Rutherford after all the observations: The nucleus is at the centre and is positively charged and nearly all the mass of the nucleus resides in the nucleus. Nucleus of an atom is very dense. The size of the nucleus is very less as Rutherford’s compared to the size of the atom. Model of Atom  Rutherford’s Model of Atom Features of Rutherford proposed model of atom: (i) There is positively placed nucleus in an atom. Nearly all the mass resides in nucleus (Proton + Neutron). (ii) Electrons revolves round the nucleus in well defined orbits. (iii) Size of nucleus is very small compared to the size of atom.  Drawbacks of Rutherford’s Model: Q. What were the drawbacks of Rutherford’s model of an atom? According to Rutherford, electrons revolve round the nucleus in well- defined orbits, but electrons being charged particles will lose their energy and finally will fall into the nucleus. This will make atom highly unstable. This was the major drawback of Rutherford which was unexplained by him. To overcome drawbacks of Rutherford’s Model, Neil Bohr in 1912 proposed modified model of structure of atom.  Bohr’s Model of Atom Q. List the postulates stated by Niels Bohr to overcome the drawbacks of Rutherford's model. Niels Bohr’s postulates: Bohr in 1913 Electrons revolve around the nucleus in stable orbits without the emission of radiant energy. Each orbit has a definite energy and is called an energy shell or energy level. An orbit or energy level is designated as K, L, M, and N shells. When the electron is in the lowest energy level, it is said to be in the ground state.  Bohr’s Model of Atom An electron emits or absorbs energy when it jumps from one orbit or energy level to another. When it jumps from a higher energy level to a lower energy level, it emits energy, while it absorbs energy when it jumps from a lower energy level to a higher energy level. Electrons absorbs enegry and jump to other level of energy.  Atomic Number The total number of proton lying in the nucleus of any atom is called the atomic number. An atomic number is the identity of an atom, changing atomic number means changing the atom. Atomic number is denoted by ‘Z’. Atomic number = no. of protons or a neutral atom, no. of protons and electrons are equal. How are Electrons Distributed in Different Orbits? The distribution of electrons in various shells is done in accordance to ‘Bohr-Bury Scheme’. (i) The filling of electrons in an atom is done in accordance to ‘2n²’, where ‘n’ is the number of shell and ‘2n²’ represents the total number of electrons that can be accommodated in that particular shell. Maximum number of electrons that can be filled in particular shell: If n = 1, i.e., K = shell, 2n² = 2×1² = 2 electrons If n = 2, i.e., L = shell, 2n² = 2×2² = 8 electrons If n = 3, i.e., M = shell, 2n²2 = 2×3² = 18 electrons If n = 4, i.e., N = shell, 2n² = 2×4² = 32 electrons (ii) The outermost shell can’t hold more than 8 electrons, while second last shell can’t have more than 18 electrons, even though they may have capacity to hold more electrons. Example: ‘Ca₂₀’, the electron distribution will be : Ca₂₀ = 2(K), 8(L), 8(M), 2(N) → But Ca₂₀ = 2, 8, 10 is wrong although ‘M’ shell can contain upto 18 electrons. (iii) The outermost shell can’t hold more than 2 electrons and the penultimate shell can’t hold more than 8 electrons unless the preceding inner shell (antepenultimate shell) is filled completely obeying ‘2n² ’ rule. Some examples: (i) Ka₁₉ = 2, 8, 8, 1 (ii) Al₁₃ = 2, 8, 3 (iii) F₉ = 2, 7 (iv) Ne₁₀ = 2, 8 (v) Na₁₁ = 2, 8, 1 Checkpoint!!!  Elements with their Electronic Configuration  Why do Atoms Combine? “The atoms combine to attain a noble or inert gas electronic configuration, in order to complete their octet by formation of a chemical bond either by sharing, losing or gaining electrons.”  Valency The electrons present in the outermost shell of an atom are known as the valence electrons. The combining capacity of the atoms or their tendency to react and form molecules with atoms of the same or different elements is known as the valency of the atom. Atoms of elements, having a completely filled outermost shell, show little chemical activity. Their combining capacity or valency is zero.  Valence Shell and Valence Electrons Outer Shell Limit: Max 8 electrons can occupy the outermost shell. Octet Rule: Atoms strive for 8 electrons in their outermost shell by gaining or losing electrons. Valency: The number of electrons lost or gained to achieve stability. Exceptions: Elements like H, He, Li, Be, B aim for 2 electrons (duplet rule) and adjust valency accordingly. Q. Write down the electron distribution of chlorine atom. How many electrons are there in the L shell? (Atomic number of chlorine is 17). Q. Write the electronic configuration of a positively charged sodium ion (Na+). Atomic number of sodium is 11. Q. An atom of element X has 7 electrons in its outermost shell. To achieve a noble gas configuration, how many electrons will it gain, and what will be the resulting charge on the ion?  Mass Number It is the sum of total number of protons and no. of neutrons lying in the nucleus of an atom. It is denoted by ‘A’. Mass number = no. of protons + no. ] neutrons Relationship between Mass number & Atomic number: Representation of an Element: Q. Write the correct representation of an element ‘X’ which contains 15 electrons and sixteen neutrons. Q. What are the atomic number, mass number and valency of atoms X, Y and Z? Q. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have? Q. Calculate the number of neutrons, atomic number and mass number present in the nucleus of an element X which is represented as ³¹ ₁₅ X  ISOTOPES Isotopes are the atoms in which the number of neutrons differs and the number of protons is the same. Isotopes are those elements having the same atomic number and different mass numbers. Isotopes of Hydrogen: In protium the number of neutrons is zero. In deuterium, the number of neutrons is one. In tritium, the number of neutrons is two.  Fractional Atomic Mass Fractional atomic mass/ Average atomic mass is when the atomic mass of an element is a fraction instead of a whole number. If the element exists in isotopic forms, the average atomic mass is determined by considering the relative abundances of each isotope. This is done by calculating the weighted average based on the percentage of each isotope. 35 e.g. 17 Cl with an abundance of 75% 37 17 Cl with an abundance of 25%  Chemical Properties: Isotopes have the same atomic number and valence electrons, leading to identical chemical behavior. Physical Properties: Isotopes differ in mass numbers due to varying neutrons, affecting physical properties (e.g., protium, deuterium, tritium). Application of Isotopes: (i) Uranium isotope is used as fuel in nuclear reactor. (ii) Isotope of cobalt is useful in treatment of cancer. (iii) An isotope of iodine is used in the treatment of goiter.  ISOBARS Isobars are atoms (nuclides) of different chemical elements which differs in the chemical property but has the same physical property. Isobars are those elements which have a different atomic number but the same mass number. Isobars have the same atomic mass but different atomic numbers, resulting in different numbers of electrons and distinct chemical properties. Q. Will ³⁵Cl and ³⁷Cl have different valencies? Justify your answer. Q. The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of both these elements is 40. What is the name given to such a pair of elements?  1. Atomic Number of an element is equal to: (a) Number of Protons (b) Number of electrons (c) Number of neutrons (d) Both (a) and (b) 2. Isotopes of an element have: (a) The same physical properties (b) Different chemical properties (c) Different number of neutrons (d) Different atomic numbers 3. Which of the following correctly represent the electronic distribution in the Mg atom? (a) 3, 8, 1 (b) 2, 8, 2 (c) 1, 8, 3 (d) 8, 2, 2 4. Rutherford’s alpha particles scattering experiment’ showed that (i) electrons have negative charge (ii) the mass and positive charge of the atom is concentrated in the nucleus (iii) neutron exists in the nucleus (iv) most of the space in atom is empty Which of the following is correct? (a) (i) and (iii) (b) (ii) and (iv) (c) (i) and (iv) (d) (iii) and (iv) 5. Q. An element X has a mass number of 4 and an atomic number of 2. What is the valency of this element? (a) 0 (b) 1 (c) 2 (d) 4 "Stay positive at the center and let the negativity orbit around you!"