Atoms and Molecules PDF

Chapter 3 ATOMS AND MOLECULES Ancient Indian and Greek philosophers have much experimentations by Lavoisier and always wondered about the unknown and Joseph L. Proust. unseen form of matter. The idea of divisibilit...

Chapter 3 ATOMS AND MOLECULES Ancient Indian and Greek philosophers have much experimentations by Lavoisier and always wondered about the unknown and Joseph L. Proust. unseen form of matter. The idea of divisibility of matter was considered long back in India, 3.1.1 LAW OF CONSERVATION OF MASS around 500 BC. An Indian philosopher Maharishi Kanad, postulated that if we go on Is there a change in mass when a chemical dividing matter (padarth), we shall get smaller change (chemical reaction) takes place? and smaller particles. Ultimately, a stage will come when we shall come across the smallest Activity ______________ 3.1 particles beyond which further division will Take one of the following sets, X and Y not be possible. He named these particles of chemicals– Parm anu. Another Indian philosopher, X Y Pakudha Katyayama, elaborated this doctrine (i) copper sulphate sodium carbonate and said that these particles normally exist 1.25 g 1.43 g in a combined form which gives us various (ii) barium chloride sodium sulphate forms of matter. 1.22 g 1.53 g Around the same era, ancient Greek (iii) lead nitrate sodium chloride philosophers – Democritus and Leucippus 2.07 g 1.17 g Prepare separately a 5% solution of suggested that if we go on dividing matter, a any one pair of substances listed stage will come when particles obtained under X and Y each in 10 mL in water. cannot be divided further. Democritus called Take a little amount of solution of Y in these indivisible particles atoms (meaning a conical flask and some solution of indivisible). All this was based on X in an ignition tube. philosophical considerations and not much Hang the ignition tube in the flask experimental work to validate these ideas carefully; see that the solutions do not could be done till the eighteenth century. get mixed. Put a cork on the flask By the end of the eighteenth century, (see Fig. 3.1). scientists recognised the difference between elements and compounds and naturally became interested in finding out how and why elements combine and what happens when they combine. Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination. 3.1 Laws of Chemical Combination The following two laws of chemical Fig. 3.1: Ignition tube containing solution of X, dipped combination were established after in a conical flask containing solution of Y. 2022-23  Weigh the flask with its contents conservation of mass and the law of definite carefully. proportions. Now tilt and swirl the flask, so that the solutions X and Y get mixed. John Dalton was born in Weigh again. a poor weaver’s family in What happens in the reaction flask? 1766 in England. He Do you think that a chemical reaction began his career as a has taken place? teacher at the age of Why should we put a cork on the mouth twelve. Seven years later of the flask? he became a school Does the mass of the flask and its principal. In 1793, Dalton contents change? left for Manchester to teach mathematics, John Dalton Law of conservation of mass states that physics and chemistry in mass can neither be created nor destroyed in a college. He spent most of his life there a chemical reaction. teaching and researching. In 1808, he presented his atomic theory which was a 3.1.2 LAW OF CONSTANT PROPORTIONS turning point in the study of matter. Lavoisier, along with other scientists, noted that many compounds were composed of two According to Dalton’s atomic theory, all or more elements and each such compound matter, whether an element, a compound or had the same elements in the same a mixture is composed of small particles called proportions, irrespective of where the atoms. The postulates of this theory may be compound came from or who prepared it. stated as follows: In a compound such as water, the ratio of (i) All matter is made of very tiny particles the mass of hydrogen to the mass of oxygen is called atoms, which participate in always 1:8, whatever the source of water. Thus, chemical reactions. if 9 g of water is decomposed, 1 g of hydrogen (ii) Atoms are indivisible particles, which and 8 g of oxygen are always obtained. cannot be created or destroyed in a Similarly in ammonia, nitrogen and hydrogen chemical reaction. are always present in the ratio 14:3 by mass, (iii) Atoms of a given element are identical whatever the method or the source from which in mass and chemical properties. it is obtained. (iv) Atoms of different elements have This led to the law of constant proportions dif ferent masses and chemical which is also known as the law of definite properties. proportions. This law was stated by Proust as (v) Atoms combine in the ratio of small “In a chemical substance the elements are whole numbers to form compounds. always present in definite proportions by (vi) The relative number and kinds of atoms are constant in a given mass”. The next problem faced by scientists was compound. You will study in the next chapter that all to give appropriate explanations of these laws. atoms are made up of still smaller particles. British chemist John Dalton provided the basic theory about the nature of matter. uestions Q Dalton picked up the idea of divisibility of matter, which was till then just a philosophy. 1. In a reaction, 5.3 g of sodium He took the name ‘atoms’ as given by the carbonate reacted with 6 g of Greeks and said that the smallest particles of acetic acid. The products were matter are atoms. His theory was based on the 2.2 g of carbon dioxide, 0.9 g laws of chemical combination. Dalton’s atomic water and 8.2 g of sodium theory provided an explanation for the law of acetate. Show that these 32 SCIENCE 2022-23  observations are in agreement We might think that if atoms are so with the law of conservation of insignificant in size, why should we care about mass. them? This is because our entire world is sodium carbonate + acetic acid made up of atoms. We may not be able to see → sodium acetate + carbon them, but they are there, and constantly dioxide + water affecting whatever we do. Through modern 2. Hydrogen and oxygen combine in techniques, we can now produce magnified the ratio of 1:8 by mass to form images of surfaces of elements showing atoms. water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas? 3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass? 4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions? 3.2 What is an Atom? Have you ever observed a mason building walls, from these walls a room and then a Fig. 3.2: An image of the surface of silicon collection of rooms to form a building? What is the building block of the huge building? What about the building block of an ant-hill? 3.2.1 W HAT A R E THE MODERN DAY It is a small grain of sand. Similarly, the SYMBOLS OF ATOMS OF DIFFERENT building blocks of all matter are atoms. ELEMENTS ? How big are atoms? Dalton was the first scientist to use the Atoms are very small, they are smaller than symbols for elements in a very specific sense. anything that we can imagine or compare When he used a symbol for an element he with. More than millions of atoms when also meant a definite quantity of that element, stacked would make a layer barely as thick that is, one atom of that element. Berzilius as this sheet of paper. suggested that the symbols of elements be Atomic radius is measured in nanometres. made from one or two letters of the name of 1/10 9 m = 1 nm the element. 1 m = 109 nm Relative Sizes Radii (in m) Example 10–10 Atom of hydrogen 10–9 Molecule of water 10–8 Molecule of haemoglobin –4 10 Grain of sand 10–3 Ant –1 Fig. 3.3: Symbols for some elements as proposed by 10 Apple Dalton ATOMS AND MOLECULES 33 2022-23  In the beginning, the names of elements passage of time and repeated usage you will were derived from the name of the place where automatically be able to reproduce the they were found for the first time. For example, symbols). the name copper was taken from Cyprus. Some names were taken from specific colours. For example, gold was taken from the English word 3.2.2 ATOMIC MASS meaning yellow. Now-a-days, IUPAC The most remarkable concept that Dalton’s (International Union of Pure and Applied atomic theory proposed was that of the atomic Chemistry) is an international scientific organisation which approves names of mass. According to him, each element had a elements, symbols and units. Many of the characteristic atomic mass. The theory could symbols are the first one or two letters of the explain the law of constant proportions so well element’s name in English. The first letter of a that scientists were prompted to measure the symbol is always written as a capital letter atomic mass of an atom. Since determining the (uppercase) and the second letter as a small mass of an individual atom was a relatively letter (lowercase). difficult task, relative atomic masses were For example determined using the laws of chemical (i) hydrogen, H combinations and the compounds formed. (ii) aluminium, Al and not AL Let us take the example of a compound, (iii) cobalt, Co and not CO. carbon monoxide (CO) formed by carbon and Symbols of some elements are formed from oxygen. It was observed experimentally that 3 the first letter of the name and a letter, g of carbon combines with 4 g of oxygen to appearing later in the name. Examples are: (i) form CO. In other words, carbon combines chlorine, Cl, (ii) zinc, Zn etc. Other symbols have been taken from the with 4/3 times its mass of oxygen. Suppose names of elements in Latin, German or Greek. we define the atomic mass unit (earlier For example, the symbol of iron is Fe from its abbreviated as ‘amu’, but according to the Latin name ferrum, sodium is Na from natrium, latest IUPAC recommendations, it is now potassium is K from kalium. Therefore, each written as ‘u’ – unified mass) as equal to the element has a name and a unique chemical mass of one carbon atom, then we would symbol. Table 3.1: Symbols for some elements Element Symbol Element Symbol Element Symbol Aluminium Al Copper Cu Nitrogen N Argon Ar Fluorine F Oxygen O Barium Ba Gold Au Potassium K Boron B Hydrogen H Silicon Si Bromine Br Iodine I Silver Ag Calcium Ca Iron Fe Sodium Na Carbon C Lead Pb Sulphur S Chlorine Cl Magnesium Mg Uranium U Cobalt Co Neon Ne Zinc Zn (The above table is given for you to refer to assign carbon an atomic mass of 1.0 u and whenever you study about elements. Do not oxygen an atomic mass of 1.33 u. However, it bother to memorise all in one go. With the is more convenient to have these numbers as 34 SCIENCE 2022-23 whole numbers or as near to a whole numbers mass of the atom, as compared to 1/12th the as possible. While searching for various mass of one carbon-12 atom. atomic mass units, scientists initially took 1/ 16 of the mass of an atom of naturally Table 3.2: Atomic masses of occurring oxygen as the unit. This was a few elements considered relevant due to two reasons: oxygen reacted with a large number of Element Atomic Mass (u) elements and formed compounds. this atomic mass unit gave masses of Hydrogen 1 most of the elements as whole numbers. Carbon 12 However, in 1961 for a universally Nitrogen 14 accepted atomic mass unit, carbon-12 isotope Oxygen 16 was chosen as the standard reference for measuring atomic masses. One atomic mass Sodium 23 unit is a mass unit equal to exactly one-twelfth Magnesium 24 (1/12th) the mass of one atom of carbon-12. Sulphur 32 The relative atomic masses of all elements have been found with respect to an atom of Chlorine 35.5 carbon-12. Calcium 40 Imagine a fruit seller selling fruits without any standard weight with him. He takes a watermelon and says, “this has a mass equal 3.2.3 HOW DO ATOMS EXIST? to 12 units” (12 watermelon units or 12 fruit mass units). He makes twelve equal pieces of Atoms of most elements are not able to exist the watermelon and finds the mass of each fruit independently. Atoms form molecules and he is selling, relative to the mass of one piece ions. These molecules or ions aggregate in of the watermelon. Now he sells his fruits by large numbers to form the matter that we can relative fruit mass unit (fmu), as in Fig. 3.4. see, feel or touch. uestions Q 1. Define the atomic mass unit. 2. Why is it not possible to see an atom with naked eyes? 3.3 What is a Molecule? A molecule is in general a group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces. A molecule can be defined as Fig. 3.4 : (a) Watermelon, (b) 12 pieces, (c) 1/12 of the smallest particle of an element or a watermelon, (d) how the fruit seller can compound that is capable of an independent weigh the fruits using pieces of watermelon existence and shows all the properties of that substance. Atoms of the same element or of Similarly, the relative atomic mass of the different elements can join together to form atom of an element is defined as the average molecules. ATOMS AND MOLECULES 35 2022-23 3.3.1 MOLECULES OF ELEMENTS Table 3.4 : Molecules of some The molecules of an element are constituted compounds by the same type of atoms. Molecules of many Compound Combining Ratio elements, such as argon (Ar), helium (He) etc. Elements by are made up of only one atom of that element. Mass But this is not the case with most of the non- Water (H2O) Hydrogen, Oxygen 1:8 metals. For example, a molecule of oxygen Ammonia (NH3) Nitrogen, Hydrogen 14:3 consists of two atoms of oxygen and hence it Carbon is known as a diatomic molecule, O2. If 3 dioxide (CO2) Carbon, Oxygen 3:8 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone, O3. The number of atoms constituting a molecule is known as Activity ______________ 3.2 its atomicity. Refer to Table 3.4 for ratio by mass of Metals and some other elements, such as atoms present in molecules and Table carbon, do not have a simple structure but 3.2 for atomic masses of elements. Find the ratio by number of the atoms of consist of a very large and indefinite number elements in the molecules of of atoms bonded together. compounds given in Table 3.4. Let us look at the atomicity of some The ratio by number of atoms for a non-metals. water molecule can be found as follows: Element Ratio Atomic Mass Simplest Table 3.3 : Atomicity of some by mass ratio/ ratio elements mass (u) atomic mass Type of Name Atomicity Element 1 H 1 1 =1 2 1 Non-Metal Argon Monoatomic 8 1 Helium Monoatomic O 8 16 = 1 16 2 Oxygen Diatomic Thus, the ratio by number of atoms for Hydrogen Diatomic water is H:O = 2:1. Nitrogen Diatomic Chlorine Diatomic 3.3.3 WHAT IS AN ION? Phosphorus Tetra-atomic Compounds composed of metals and non- Sulphur Poly-atomic metals contain charged species. The charged species are known as ions. Ions may consist of a single charged atom or a group of atoms that have a net charge on them. An ion can be negatively or positively charged. A negatively 3.3.2 MOLECULES OF COMPOUNDS charged ion is called an ‘anion’ and the Atoms of different elements join together positively charged ion, a ‘cation’. Take, for in definite proportions to form molecules example, sodium chloride (NaCl). Its of compounds. Few examples are given in constituent particles are positively charged Table 3.4. sodium ions (Na+) and negatively charged 36 SCIENCE 2022-23 chloride ions (Cl–). A group of atoms carrying learn the symbols and combining capacity of a charge is known as a polyatomic ion (Table the elements. 3.6). We shall learn more about the formation The combining power (or capacity) of an of ions in Chapter 4. element is known as its valency. Valency can be used to find out how the atoms of an Table 3.5: Some ionic compounds element will combine with the atom(s) of Ionic Constituting Ratio another element to form a chemical compound. Compound Elements by The valency of the atom of an element can be Mass thought of as hands or arms of that atom. Calcium oxide Calcium and Human beings have two arms and an oxygen 5:2 octopus has eight. If one octopus has to catch hold of a few people in such a manner that all Magnesium Magnesium the eight arms of the octopus and both arms sulphide and sulphur 3:4 of all the humans are locked, how many Sodium Sodium humans do you think the octopus can hold? chloride and chlorine 23:35.5 Represent the octopus with O and humans with H. Can you write a formula for this combination? Do you get OH4 as the formula? 3.4 Writing Chemical Formulae The subscript 4 indicates the number of The chemical formula of a compound is a humans held by the octopus. symbolic representation of its composition. The The valencies of some common ions are chemical formulae of different compounds can given in Table 3.6. We will learn more about be written easily. For this exercise, we need to valency in the next chapter. Table 3.6: Names and symbols of some ions Vale- Name of Symbol Non- Symbol Polyatomic Symbol ncy ion metallic ions element 1. Sodium Na+ Hydrogen H+ Ammonium NH4+ Potassium K+ Hydride H- Hydroxide OH– Silver Ag+ Chloride Cl- Nitrate NO3– Copper (I)* Cu+ Bromide Br- Hydrogen Iodide I– carbonate HCO3– 2. Magnesium Mg2+ Oxide O2- Carbonate CO32– Calcium Ca2+ Sulphide S2- Sulphite SO32– Zinc Zn2+ Sulphate SO42– Iron (II)* Fe2+ Copper (II)* Cu2+ 3. Aluminium Al3+ Nitride N3- Phosphate PO43– Iron (III)* Fe3+ * Some elements show more than one valency. A Roman numeral shows their valency in a bracket. ATOMS AND MOLECULES 37 2022-23 The rules that you have to follow while writing 3. Formula of carbon tetrachloride a chemical formula are as follows: the valencies or charges on the ion must balance. when a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO) etc., where oxygen, chlorine, sulphur are non- For magnesium chloride, we write the metals and are written on the right, symbol of cation (Mg2+) first followed by the whereas calcium, sodium, iron and symbol of anion (Cl-). Then their charges are copper are metals, and are written on the criss-crossed to get the formula. left. in compounds formed with polyatomic ions, 4. Formula of magnesium chloride the number of ions present in the compound is indicated by enclosing the formula of ion in a bracket and writing the number of ions outside the bracket. For example, Mg (OH)2. In case the number of polyatomic ion is one, the bracket is not Formula : MgCl2 required. For example, NaOH. Thus, in magnesium chloride, there are 3.4.1 FORMULAE OF SIMPLE COMPOUNDS two chloride ions (Cl-) for each magnesium The simplest compounds, which are made up ion (Mg2+). The positive and negative charges of two different elements are called binary must balance each other and the overall compounds. Valencies of some ions are given structure must be neutral. Note that in the in Table 3.6. You can use these to write formulae for compounds. formula, the charges on the ions are not While writing the chemical formulae for indicated. compounds, we write the constituent elements and their valencies as shown below. Then we Some more examples must crossover the valencies of the combining (a) Formula for aluminium oxide: atoms. Examples 1. Formula of hydrogen chloride Formula : Al2O3 (b) Formula for calcium oxide: Formula of the compound would be HCl. 2. Formula of hydrogen sulphide Here, the valencies of the two elements are the same. You may arrive at the formula Ca2O2. But we simplify the formula as CaO. 38 SCIENCE 2022-23  (c) Formula of sodium nitrate: following formulae: (i) Al2(SO4)3 (ii) CaCl2 (iii) K2SO4 (iv) KNO3 (v) CaCO3. Formula : NaNO3 3. What is meant by the ter m chemical formula? (d) Formula of calcium hydroxide: 4. How many atoms are present in a (i) H2S molecule and (ii) PO43– ion? Formula : Ca(OH)2 3.5 Molecular Mass and Mole Note that the for mula of calcium Concept hydroxide is Ca(OH)2 and not CaOH2. We use 3.5.1 MOLECULAR MASS brackets when we have two or more of the same ions in the formula. Here, the bracket In section 3.2.2 we discussed the concept of around OH with a subscript 2 indicates that atomic mass. This concept can be extended there are two hydroxyl (OH) groups joined to to calculate molecular masses. The molecular one calcium atom. In other words, there are mass of a substance is the sum of the atomic two atoms each of oxygen and hydrogen in masses of all the atoms in a molecule of the calcium hydroxide. substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u). (e) Formula of sodium carbonate: Example 3.1 (a) Calculate the relative molecular mass of water (H 2 O). (b) Calculate the molecular mass of HNO3. Formula : Na2CO3 Solution: In the above example, brackets are not needed (a) Atomic mass of hydrogen = 1u, if there is only one ion present. oxygen = 16 u (f) Formula of ammonium sulphate: So the molecular mass of water, which contains two atoms of hydrogen and one atom of oxygen is = 2 × 1+ 1×16 = 18 u (b) The molecular mass of HNO 3 = the Formula : (NH4)2SO4 atomic mass of H + the atomic mass of N+ 3 × the atomic mass of O = 1 + 14 + 48 = 63 u uestions Q 1. Write down the formulae of (i) sodium oxide 3.5.2 FORMULA UNIT MASS (ii) aluminium chloride The formula unit mass of a substance is a sum (iii) sodium suphide of the atomic masses of all atoms in a formula (iv) magnesium hydroxide unit of a compound. Formula unit mass is 2. Write down the names of calculated in the same manner as we calculate compounds represented by the the molecular mass. The only difference is that ATOMS AND MOLECULES 39 2022-23 we use the word formula unit for those 3.5.3 MOLE CONCEPT substances whose constituent particles are ions. For example, sodium chloride as Take an example of the reaction of hydrogen discussed above, has a formula unit NaCl. Its and oxygen to form water: formula unit mass can be calculated as– 2H2+ O2 → 2H2O. 1 × 23 + 1 × 35.5 = 58.5 u The above reaction indicates that Example 3.2 Calculate the formula unit (i) two molecules of hydrogen combine mass of CaCl2. with one molecule of oxygen to form two molecules of water, or Solution: (ii) 4 u of hydrogen molecules combine with Atomic mass of Ca 32 u of oxygen molecules to form 36 u + (2 × atomic mass of Cl) of water molecules. = 40 + 2 × 35.5 = 40 + 71 = 111 u We can infer from the above equation that the quantity of a substance can be characterised by its mass or the number of uestions Q molecules. But, a chemical reaction equation indicates directly the number of atoms or 1. Calculate the molecular masses molecules taking part in the reaction. of H2, O2, Cl2, CO2, CH4, C2H6, Therefore, it is more convenient to refer to the C 2H4, NH3, CH3OH. quantity of a substance in terms of the number 2. C a l c u l a t e t h e f o r m u l a u n i t of its molecules or atoms, rather than their masses of ZnO, Na2O, K2CO3, masses. So, a new unit “mole” was introduced. given atomic masses of Zn = 65 u, The mole, symbol mol, is the SI unit of amount Na = 23 u, K = 39 u, C = 12 u, of substance. One mole contains exactly and O = 16 u. Fig. 3.5: Relationship between mole, Avogadro number and mass 40 SCIENCE 2022-23 6.02214076 × 1023 elementary entities. This for this they need to relate the mass in grams number is the fixed numerical value of the to the number. It is done as follows: Avogadro constant, NA, when expressed in the 1 mole = 6.022 × 1023 number unit mol–1 and is called the Avogadro number*. = Relative mass in grams. The amount of substance, symbol n, of a Thus, a mole is the chemist’s counting unit. system is a measure of the number of specified The word “mole” was introduced around elementary entities. An elementary entity may 1896 by Wilhelm Ostwald who derived the be an atom, a molecule, an ion, an electron, term from the Latin word moles meaning a any other particle or specified group of ‘heap’ or ‘pile’. A substance may be considered particles. The mole is the amount of substance as a heap of atoms or molecules. The unit of a system that contains 6.02214076 × 1023 specified elementary entities. mole was accepted in 1967 to provide a simple 1 mole (of anything) = 6.022 × 1023 in number, way of reporting a large number– the massive as, 1 dozen = 12 nos. heap of atoms and molecules in a sample. 1 gross = 144 nos. Besides being related to a number, a mole Example 3.3 has one more advantage over a dozen or a 1. Calculate the number of moles for the gross. This advantage is that mass of 1 mole following: of a particular substance is also fixed. (i) 52 g of He (finding mole from The mass of 1 mole of a substance is equal mass) to its relative atomic or molecular mass in (ii) 12.044 × 1023 number of He atoms grams. The atomic mass of an element gives us the mass of one atom of that element in (finding mole from number of atomic mass units (u). To get the mass of 1 particles). mole of atom of that element, that is, molar Solutions: mass, we have to take the same numerical value but change the units from ‘u’ to ‘g’. Molar No. of moles = n mass of atoms is also known as gram atomic Given mass = m mass. For example, atomic mass of Molar mass = M hydrogen=1u. So, gram atomic mass of Given number of particles = N hydrogen = 1 g. Avogadro number of particles = NA 1 u hydrogen has only 1 atom of hydrogen (i) Atomic mass of He = 4u 1 g hydrogen has 1 mole atoms, that is, 6.022 × 1023 atoms of hydrogen. Molar mass of He = 4g Similarly, Thus, the number of moles 16 u oxygen has only 1 atom of oxygen, given mass 16 g oxygen has 1 mole atoms, that is, = molar mass 6.022 × 1023 atoms of oxygen. To find the gram molecular mass or molar m 52 ⇒n= = = 13 mass of a molecule, we keep the numerical M 4 value the same as the molecular mass, but simply change units as above from u to g. For (ii) we know, example, as we have already calculated, 1 mole = 6.022 × 1023 molecular mass of water (H2O) is 18 u. From The number of moles here we understand that given number of particles 18 u water has only 1 molecule of water, = 18 g water has 1 mole molecules of water, that Avogadro number is, 6.022 × 1023 molecules of water. Chemists need the number of atoms and molecules while carrying out reactions, and * Avogadro Constant or Avogadro Number was named in honor of Italian scientist, Amedeo Avogadro. ATOMS AND MOLECULES 41 2022-23 Example 3.4 Calculate the mass of the (iii) 0.1 mole of carbon atoms (number following: from given moles) (i) 0.5 mole of N2 gas (mass from mole Solutions: of molecule) (ii) 0.5 mole of N atoms (mass from (i) The number of atoms mole of atom) given mass = × Avogadro number (iii) 3.011 × 1023 number of N atoms molar mass (mass from number) (iv) 6.022 × 10 23 number of N 2 molecules (mass from number) Solutions: 46 ⇒N= × 6.022 × 1023 23 (i) mass = molar mass × number of ⇒ N = 12.044 × 1023 moles ⇒ m = M × n = 28 × 0.5 = 14 g (ii) The number of molecules (ii) mass = molar mass × number of moles ⇒ m = M × n = 14 × 0.5 = 7 g (iii) The number of moles, n given number of particles N ∴ molar mass of O2 molecules = = = 16 × 2 = 32g Avogadro number NA 3.011 × 10 23 = 6.022 × 10 23 3.011 × 1023 ⇒ m = M × n = 14 × (iii) The number of particles (atom) = 6.022 × 1023 number of moles of particles × = 14 × 0.5 = 7 g Avogadro number N N = n × NA = 0.1 x 6.022 × 1023 (iv) n= = 6.022 × 1022 NA uestions Q = 28 × 1 = 28 g 1. If one mole of carbon atoms weighs 12 grams, what is the mass (in grams) of 1 atom of Example 3.5 Calculate the number of carbon? particles in each of the 2. Which has more number of following: atoms, 100 grams of sodium or (i) 46 g of Na atoms (number from 100 grams of iron (given, atomic mass) mass of Na = 23 u, Fe = 56 u)? (ii) 8 g O 2 molecules (number of molecules from mass) 42 SCIENCE 2022-23  What you have learnt During a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is known as the Law of Conservation of Mass. In a pure chemical compound, elements are always present in a definite proportion by mass. This is known as the Law of Definite Proportions. An atom is the smallest particle of the element that cannot usually exist independently and retain all its chemical properties. A molecule is the smallest particle of an element or a compound capable of independent existence under ordinary conditions. It shows all the properties of the substance. A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element. Clusters of atoms that act as an ion are called polyatomic ions. They carry a fixed charge on them. The chemical formula of a molecular compound is determined by the valency of each element. In ionic compounds, the charge on each ion is used to determine the chemical formula of the compound. Scientists use the relative atomic mass scale to compare the masses of different atoms of elements. Atoms of carbon-12 isotopes are assigned a relative atomic mass of 12 and the relative masses of all other atoms are obtained by comparison with the mass of a carbon-12 atom. The Avogadro constant 6.022 × 1023 is defined as the number of atoms in exactly 12 g of carbon-12. The mole is the amount of substance of a system that contains 6.02214076 × 1023 specified elementary entities. Mass of 1 mole of a substance is called its molar mass. Exercises 1. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight. 2. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will ATOMS AND MOLECULES 43 2022-23  be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer? 3. What are polyatomic ions? Give examples. 4. Write the chemical formulae of the following. (a) Magnesium chloride (b) Calcium oxide (c) Copper nitrate (d) Aluminium chloride (e) Calcium carbonate. 5. Give the names of the elements present in the following compounds. (a) Quick lime (b) Hydrogen bromide (c) Baking powder (d) Potassium sulphate. 6. Calculate the molar mass of the following substances. (a) Ethyne, C2H2 (b) Sulphur molecule, S8 (c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31) (d) Hydrochloric acid, HCl (e) Nitric acid, HNO3 7. What is the mass of— (a) 1 mole of nitrogen atoms? (b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)? (c) 10 moles of sodium sulphite (Na2SO3)? 8. Convert into mole. (a) 12 g of oxygen gas (b) 20 g of water (c) 22 g of carbon dioxide. 9. What is the mass of: (a) 0.2 mole of oxygen atoms? (b) 0.5 mole of water molecules? 10. Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur. 11. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide. (Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u) 44 SCIENCE 2022-23  Group Activity Play a game for writing formulae. Example1 : Make placards with symbols and valencies of the elements separately. Each student should hold two placards, one with the symbol in the right hand and the other with the valency in the left hand. Keeping the symbols in place, students should criss-cross their valencies to form the formula of a compound. Example 2 : A low cost model for writing formulae: Take empty blister packs of medicines. Cut them in groups, according to the valency of the element, as shown in the figure. Now, you can make formulae by fixing one type of ion into other. For example: Na+ SO 4 2- P0 43- Formula for sodium sulphate: 2 sodium ions can be fixed on one sulphate ion. Hence, the formula will be: Na2SO4 Do it yourself : Now, write the formula of sodium phosphate. ATOMS AND MOLECULES 45 2022-23