Atoms and Molecules - Chapter 3 PDF

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

Chapter 3 ATOMS AND MOLECULES Ancient Indian and Greek philosophers have experimentations by Lavoisier and Joseph always wondered about the unknown and 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 time 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– Parmanu. Another Indian philosopher, X Y Pakudha Katyayama, elaborated this doctrine (i) copper sulphate sodium carbonate and said that these particles normally exist (ii) barium chloride sodium sulphate in a combined form which gives us various (iii) lead nitrate sodium chloride forms of matter. Prepare separately a 5% solution of any Around the same era, ancient Greek one pair of substances listed under X philosophers – Democritus and Leucippus and Y in water. suggested that if we go on dividing matter, a Take a little amount of solution of Y in stage will come when particles obtained a conical flask and some solution of X in an ignition tube. cannot be divided further. Democritus called Hang the ignition tube in the flask these indivisible particles atoms (meaning carefully; see that the solutions do not indivisible). All this was based on get mixed. Put a cork on the flask philosophical considerations and not much (see Fig. 3.1). experimental work to validate these ideas could be done till the eighteenth century. By the end of the eighteenth century, 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 much in a conical flask containing solution of Y.  Weigh the flask with its contents atomic theory provided an explanation for the carefully. law of conservation of mass and the law of Now tilt and swirl the flask, so that the definite proportions. solutions X and Y get mixed. Weigh again. John Dalton was born in What happens in the reaction flask? a poor weaver’s family in Do you think that a chemical reaction 1766 in England. He has taken place? began his career as a Why should we put a cork on the mouth teacher at the age of of the flask? twelve. Seven years later Does the mass of the flask and its he became a school contents change? principal. In 1793, Dalton left for Manchester to Law of conservation of mass states that teach mathematics, mass can neither be created nor destroyed in John Dalton physics and chemistry in a chemical reaction. a college. He spent most of his life there teaching and researching. In 1808, he 3.1.2 LAW OF CONSTANT PROPORTIONS presented his atomic theory which was a Lavoisier, along with other scientists, noted turning point in the study of matter. 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 called atoms. is always 1:8, whatever the source of water. (ii) Atoms are indivisible particles, which Thus, if 9 g of water is decomposed, 1 g of cannot be created or destroyed in a hydrogen and 8 g of oxygen are always chemical reaction. obtained. Similarly in ammonia, nitrogen and (iii) Atoms of a given element are identical hydrogen are always present in the ratio 14:3 in mass and chemical properties. by mass, whatever the method or the source (iv) Atoms of different elements have from which it is obtained. different masses and chemical This led to the law of constant proportions properties. which is also known as the law of definite (v) Atoms combine in the ratio of small proportions. This law was stated by Proust whole numbers to form compounds. as “In a chemical substance the elements are (vi) The relative number and kinds of always present in definite proportions by atoms are constant in a given mass”. compound. The next problem faced by scientists was 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 uestions Q basic theory about the nature of matter. Dalton picked up the idea of divisibility of 1. In a reaction, 5.3 g of sodium matter, which was till then just a philosophy. carbonate reacted with 6 g of He took the name ‘atoms’ as given by the ethanoic acid. The products were Greeks and said that the smallest particles of 2.2 g of carbon dioxide, 0.9 g matter are atoms. His theory was based on water and 8.2 g of sodium the laws of chemical combination. Dalton’s ethanoate. Show that these 32 SCIENCE  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 + ethanoic acid made up of atoms. We may not be able to see → sodium ethanoate + 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 H A T ARE 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 as this sheet of paper. that is, one atom of that element. Berzilius 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 10–2 Ant Fig. 3.3: Symbols for some elements as proposed by 10–1 Watermelon Dalton ATOMS AND MOLECULES 33  In the beginning, the names of elements the passage of time and repeated usage you were derived from the name of the place where will 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. 3.2.2 ATOMIC MASS For example, gold was taken from the English word 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) approves names of elements. Many mass. According to him, each element had a of the symbols are the first one or two letters characteristic atomic mass. The theory could of the element’s name in English. The first explain the law of constant proportions so well letter of a symbol is always written as a capital that scientists were prompted to measure the letter (uppercase) and the second letter as a atomic mass of an atom. Since determining small letter (lowercase). the mass of an individual atom was a relatively For example difficult task, relative atomic masses were (i) hydrogen, H determined using the laws of chemical (ii) aluminium, Al and not AL combinations and the compounds formed. (iii) cobalt, Co and not CO. Let us take the example of a compound, carbon monoxide (CO) formed by carbon and Symbols of some elements are formed oxygen. It was observed experimentally that from the first letter of the name and a letter, 3 g of carbon combines with 4 g of oxygen to appearing later in the name. Examples are: form CO. In other words, carbon combines (i) chlorine, Cl, (ii) zinc, Zn etc. with 4/3 times its mass of oxygen. Suppose Other symbols have been taken from the we define the atomic mass unit (earlier names of elements in Latin, German or Greek. For example, the symbol of iron is Fe from its abbreviated as ‘amu’, but according to the Latin name ferrum, sodium is Na from latest IUPAC recommendations, it is now natrium, potassium is K from kalium. written as ‘u’ – unified mass) as equal to the Therefore, each element has a name and a mass of one carbon atom, then we would unique chemical symbol. assign carbon an atomic mass of 1.0 u and 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 oxygen an atomic mass of 1.33 u. However, it to whenever you study about elements. Do is more convenient to have these numbers as not bother to memorise all in one go. With whole numbers or as near to a whole numbers 34 SCIENCE as possible. While searching for various mass of the atom, as compared to 1/12th the atomic mass units, scientists initially took 1/ mass of one carbon-12 atom. 16 of the mass of an atom of naturally occurring oxygen as the unit. This was Table 3.2: Atomic masses of considered relevant due to two reasons: a few elements oxygen reacted with a large number of elements and formed compounds. Element Atomic Mass (u) this atomic mass unit gave masses of most of the elements as whole numbers. Hydrogen 1 However, in 1961 for a universally Carbon 12 accepted atomic mass unit, carbon-12 isotope Nitrogen 14 was chosen as the standard reference for Oxygen 16 measuring atomic masses. One atomic mass unit is a mass unit equal to exactly one-twelfth Sodium 23 (1/12th) the mass of one atom of carbon-12. Magnesium 24 The relative atomic masses of all elements Sulphur 32 have been found with respect to an atom of carbon-12. Chlorine 35.5 Imagine a fruit seller selling fruits without Calcium 40 any standard weight with him. He takes a watermelon and says, “this has a mass equal to 12 units” (12 watermelon units or 12 fruit 3.2.3 HOW DO ATOMS EXIST? mass units). He makes twelve equal pieces of the watermelon and finds the mass of each Atoms of most elements are not able to exist fruit he is selling, relative to the mass of one independently. Atoms form molecules and piece of the watermelon. Now he sells his fruits ions. These molecules or ions aggregate in by relative fruit mass unit (fmu), as in Fig. large numbers to form the matter that we can 3.4. see, feel or touch. Fig. 3.4 : (a) Water melon, (b) 12 pieces, (c) 1/12 of 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 water melon, (d) how the fruit seller can as the smallest particle of an element or a weigh the fruits using pieces of compound that is capable of an independent water melon 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 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 Hydrogen, Oxygen 1:8 metals. For example, a molecule of oxygen Ammonia Nitrogen, Hydrogen 14:3 consists of two atoms of oxygen and hence it Carbon is known as a diatomic molecule, O2. If 3 dioxide Carbon, Oxygen 3:8 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone. 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 T able 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 by mass ratio/ ratio Table 3.3 : Atomicity of some mass (u) atomic elements mass Type of Name Atomicity 1 Element H 1 1 =1 2 1 Non-Metal Argon Monoatomic 8 1 O 8 16 = 1 Helium Monoatomic 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. An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an 3.3.2 MOLECULES OF COMPOUNDS ‘anion’ and the positively charged ion, a ‘cation’. Take, for example, sodium chloride Atoms of different elements join together in (NaCl). Its constituent particles are positively definite proportions to form molecules of charged sodium ions (Na+ ) and negatively compounds. Few examples are given in Table charged chloride ions (Cl–). Ions may consist 3.4. of a single charged atom or a group of atoms 36 SCIENCE that have a net charge on them. A group of exercise, we need to learn the symbols and atoms carrying a charge is known as a combining capacity of the elements. polyatomic ion (Table 3.6). We shall learn more The combining power (or capacity) of an about the formation 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 for m a chemical Compound Elements by compound. The valency of the atom of an Mass element can be 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 Magnesium Magnesium hold of a few people in such a manner that all sulphide and sulphur 3:4 the eight arms of the octopus and both arms 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 valencies of some common ions are The chemical formulae of different given in Table 3.6. We will learn more about compounds can be written easily. For this 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 HCO–3 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 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, For magnesium chloride, we write the sulphur are non-metals and are written symbol of cation (Mg2+) first followed by the on the right, whereas calcium, sodium, symbol of anion (Cl- ). Then their charges are iron and copper are metals, and are criss-crossed to get the formula. written on the left. 4. Formula of magnesium chloride in compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio. In case the number of polyatomic ion is one, the bracket is not required. For example, NaOH. Formula : MgCl2 3.4.1 FORMULAE OF SIMPLE COMPOUNDS Thus, in magnesium chloride, there are The simplest compounds, which are made up two chloride ions (Cl-) for each magnesium of two different elements are called binary ion (Mg2+). The positive and negative charges compounds. Valencies of some ions are given must balance each other and the overall in Table 3.6. You can use these to write structure must be neutral. Note that in the 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 must crossover the valencies of the combining Some more examples atoms. (a) Formula for aluminium oxide: Examples 1. Formula of hydrogen chloride Formula : Al 2O3 (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 Ca2O 2. But we simplify the formula as CaO. 38 SCIENCE  (c) Formula of sodium nitrate: following formulae: (i) Al 2(SO4)3 (ii) CaCl2 (iii) K 2SO4 (iv) KNO3 (v) CaCO3. Formula : NaNO3 3. What is meant by the term chemical formula? (d) Formula of calcium hydroxide: 4. How many atoms are present in a (i) H 2S molecule and (ii) PO 43– ion? Formula : Ca(OH)2 3.5 Molecular Mass and Mole Note that the formula 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 2O). (b) Calculate the molecular mass of HNO3. Formula : Na2CO 3 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 HNO3 = 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 3.5.2 FORMULA UNIT MASS (i) sodium oxide (ii) aluminium chloride The formula unit mass of a substance is a (iii) sodium suphide sum of the atomic masses of all atoms in a (iv) magnesium hydroxide formula unit of a compound. Formula unit 2. Write down the names of mass is calculated in the same manner as we compounds represented by the calculate the molecular mass. The only ATOMS AND MOLECULES 39 difference is that we use the word formula 3.5.3 MOLE CONCEPT unit for those substances whose constituent particles are ions. For example, sodium Take an example of the reaction of hydrogen chloride as discussed above, has a formula and oxygen to form water: unit NaCl. Its formula unit mass can be 2H2+ O2 → 2H 2O. calculated as– 1 × 23 + 1 × 35.5 = 58.5 u The above reaction indicates that (i) two molecules of hydrogen combine Example 3.2 Calculate the formula unit with one molecule of oxygen to form mass of CaCl2. two molecules of water, or (ii) 4 u of hydrogen molecules combine Solution: with 32 u of oxygen molecules to form Atomic mass of Ca 36 u of water molecules. + (2 × atomic mass of Cl) We can infer from the above equation that = 40 + 2 × 35.5 = 40 + 71 = 111 u the quantity of a substance can be characterised by its mass or the number of uestions molecules. But, a chemical reaction equation Q indicates directly the number of atoms or 1. Calculate the molecular masses molecules taking part in the reaction. of H2, O 2, 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 2. Calculate the formula unit number of its molecules or atoms, rather than masses of ZnO, Na2O, K2CO3, their masses. So, a new unit “mole” was given atomic masses of Zn = 65 u, introduced. One mole of any species (atoms, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u. Fig. 3.5: Relationship between mole, Avogadro number and mass 40 SCIENCE molecules, ions or particles) is that quantity to the number. It is done as follows: in number having a mass equal to its atomic 1 mole = 6.022 × 1023 number or molecular mass in grams. = Relative mass in grams. The number of particles (atoms, molecules Thus, a mole is the chemist’s counting unit. or ions) present in 1 mole of any substance is The word “mole” was introduced around fixed, with a value of 6.022 × 1023. This is an 1896 by Wilhelm Ostwald who derived the experimentally obtained value. This number term from the Latin word moles meaning a is called the Avogadro Constant or Avogadro ‘heap’ or ‘pile’. A substance may be considered Number (represented by N0), named in honour as a heap of atoms or molecules. The unit of the Italian scientist, Amedeo Avogadro. mole was accepted in 1967 to provide a simple 1 mole (of anything) = 6.022 × 10 23 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 (finding mole from number of us the mass of one atom of that element in particles). atomic mass units (u). To get the mass of 1 mole of atom of that element, that is, molar Solutions: mass, we have to take the same numerical No. of moles = n value but change the units from ‘u’ to ‘g’. Molar Given mass = m mass of atoms is also known as gram atomic Molar mass = M mass. For example, atomic mass of Given number of particles = N hydrogen=1u. So, gram atomic mass of Avogadro number of particles = N0 hydrogen = 1 g. 1 u hydrogen has only 1 atom of hydrogen (i) Atomic mass of He = 4u 1 g hydrogen has 1 mole atoms, that is, Molar mass of He = 4g 6.022 × 10 23 atoms of hydrogen. Thus, the number of moles Similarly, given mass 16 u oxygen has only 1 atom of oxygen, = molar mass 16 g oxygen has 1 mole atoms, that is, 6.022 × 10 23 atoms of oxygen. m 52 To find the gram molecular mass or molar ⇒n = = = 13 M 4 mass of a molecule, we keep the numerical value the same as the molecular mass, but (ii) we know, simply change units as above from u to g. For 1 mole = 6.022 × 1023 example, as we have already calculated, The number of moles molecular mass of water (H2O) is 18 u. From given number of particles here we understand that = 18 u water has only 1 molecule of water, Avogadro number 18 g water has 1 mole molecules of water, that is, 6.022 × 1023 molecules of water. N 12.044 × 10 23 ⇒n = = =2 Chemists need the number of atoms and No 6.022 × 1023 molecules while carrying out reactions, and for this they need to relate the mass in grams ATOMS AND MOLECULES 41 Example 3.4 Calculate the mass of the Solutions: following: ( i) The number of atoms (i) 0.5 mole of N 2 gas (mass from mole given mass of molecule) = × Avogadro number molar mass (ii) 0.5 mole of N atoms (mass from mole of atom) m (iii) 3.011 × 1023 number of N atoms ⇒N = × N0 M (mass from number) (iv) 6.022 × 10 2 3 number of N 2 46 molecules (mass from number) ⇒N = × 6.022 × 102 3 23 Solutions: ⇒ N = 12.044 × 10 23 (i) mass = molar mass × number of (ii) The number of molecules moles ⇒ m = M × n = 28 × 0.5 =14 g given mass = × Avogadro number molar mass (ii) mass = molar mass × number of m ⇒N = × N0 moles M ⇒ m = M × n = 14 × 0.5 = 7 g atomic massof oxygen = 16 u (iii) The number of moles, n ∴ molar mass of O2 molecules = 16 × 2 = 32g given number of particles N = = Avogadro number N0 8 ⇒N = × 6.022 × 1023 32 3.011 × 10 23 = ⇒ N = 1.5055 × 102 3 6.022 × 1023 ≃ 1.51 × 102 3 23 3.011 × 10 ⇒ m = M × n = 14 × (iii) The number of particles (atom) = 6.022 × 102 3 number of moles of particles × = 14 × 0.5 = 7 g Avogadro number N = n × N0 = 0.1 x 6.022 × 1023 N (iv) n = N = 6.022 × 10 22 0 Q ⇒ m= M × N = 28 × 6.022 × 10 23 uestions N0 6.022 × 10 23 1. If one mole of carbon atoms = 28 × 1 = 28 g weighs 12 grams, what is the mass (in grams) of 1 atom of carbon? Example 3.5 Calculate the number of 2. Which has more number of particles in each of the atoms, 100 grams of sodium or following: 100 grams of iron (given, atomic (i) 46 g of Na atoms (number from mass of Na = 23 u, Fe = 56 u)? mass) (ii) 8 g O 2 molecules (number of molecules from mass) (iii) 0.1 mole of carbon atoms (number from given moles) 42 SCIENCE  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 that contains the same number of particles (atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g of carbon-12. 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  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, S 8 (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  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+ SO42- P043- 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