1.C hapter 3ATOMS ANDMOLECULESAncient Indian and Greek philosophers haveexperimentationsbyLavoisier andalways wondered about the unknown and Joseph L. Proust.unseen form of matter. The idea of divisibilityof matter was considered long back in India,3.1.1 LAW OF CONSERVATION OF MASSaround 500 BC. An Indian philosopherMaharishi Kanad, postulated that if we go onIs there a change in mass when a chemicaldividing matter (padarth), we shall get smaller change (chemical reaction) takes place?and smaller particles. Ultimately, a time willcome when we shall come across the smallest Activity ______________ 3.1particles beyond which further division will Take one of the following sets, X and Ynot be possible. He named these particles of chemicalsPar manu. Another Indian philosopher,XYPakudha Katyayama, elaborated this doctrine (i) copper sulphate sodium carbonateand said that these particles normally exist (ii) barium chloride sodium sulphatein a combined form which gives us various (iii) lead nitratesodium chlorideforms of matter. Prepare separately a 5% solution of anyAround the same era, ancient Greek one pair of substances listed under Xphilosophers Democritus and Leucippusand Y in water.suggested that if we go on dividing matter, a Take a little amount of solution of Y instage will come when particles obtaineda conical flask and some solution of X in an ignition tube.cannot be divided further. Democritus called Hang the ignition tube in the flaskthese indivisible particles atoms (meaning carefully; see that the solutions do notindivisible). All this was based onget mixed. Put a cork on the flaskphilosophical considerations and not much(see Fig. 3.1).experimental work to validate these ideascould be done till the eighteenth century.By the end of the eighteenth century,scientists recognised the difference betweenelements and compounds and naturallybecame interested in finding out how and whyelements combine and what happens whenthey combine.Antoine L. Lavoisier laid the foundationof chemical sciences by establishing twoimportant laws of chemical combination.3.1 Laws of Chemical CombinationThe following two laws of chemicalFig. 3.1: Ignition tube containing solution of X, dippedcombination were established after muchin a conical flask containing solution of Y.

2. Weigh the flask with its contents atomic theory provided an explanation for the carefully.law of conservation of mass and the law ofNow tilt and swirl the flask, so that the definite proportions. solutions X and Y get mixed.Weigh again. John Dalton was born inWhat happens in the reaction flask?a poor weavers family inDo you think that a chemical reaction1766 in England. He has taken place? began his career as aWhy should we put a cork on theteacher at the age of mouth of the flask?twelve. Seven years laterDoes the mass of the flask and itshe became a school contents change?principal. In 1793, Daltonleft for Manchester toLaw of conservation of mass states thatteach mathematics,John Daltonmass can neither be created nor destroyedphysics and chemistry inin a chemical reaction.a college. He spent most of his life thereteaching and researching. In 1808, he3.1.2 LAW OF CONSTANT PROPORTIONS presented his atomic theory which was aLavoisier, along with other scientists, noted turning point in the study of matter.that many compounds were composed of twoAccording to Daltons atomic theory, allor more elements and each such compound matter, whether an element, a compound orhad the same elements in the same a mixture is composed of small particlesproportions, irrespective of where the called atoms. The postulates of this theorycompound came from or who prepared it. may be stated as follows:In a compound such as water, the ratio of(i) All matter is made of very tiny particlesthe mass of hydrogen to the mass of oxygen called atoms.is always 1:8, whatever the source of water.(ii) Atoms are indivisible particles, whichThus, if 9 g of water is decomposed, 1 g ofcannot be created or destroyed in ahydrogen and 8 g of oxygen are alwayschemical reaction.obtained. Similarly in ammonia, nitrogen and (iii) Atoms of a given element are identicalhydrogen are always present in the ratio 14:3in mass and chemical properties.by mass, whatever the method or the source (iv) Atoms of different elements havefrom which it is obtained. dif ferent masses and chemicalThis led to the law of constant proportionsproperties.which is also known as the law of definite(v) Atoms combine in the ratio of smallproportions. 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 ofalways present in definite proportions byatoms are constant in a givenmass. compound.The next problem faced by scientists wasYou will study in the next chapter that all atoms are made up of still smaller particles. Qto give appropriate explanations of these laws.British chemist John Dalton provided thebasic theory about the nature of matter. uestionsDalton picked up the idea of divisibility of 1. In a reaction, 5.3 g of sodiummatter, which was till then just a philosophy.carbonate reacted with 6 g ofHe took the name atoms as given by theethanoic acid. The products wereGreeks and said that the smallest particles 2.2 g of carbon dioxide, 0.9 gof matter are atoms. His theory was based water and 8.2 g of sodiumon the laws of chemical combination. Daltons ethanoate. Show that these32 SCIENCE 3. observations are in agreementWe might think that if atoms are so with the law of conservation ofinsignificant in size, why should we care mass.about them? This is because our entire world sodium carbonate + ethanoic acid is made up of atoms. We may not be able to sodium ethanoate + carbonsee them, but they are there, and constantly dioxide + wateraffecting whatever we do. Through modern2. 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 water. What mass of oxygen gas atoms. would be required to react completely with 3 g of hydrogen gas?3. Which postulate of Daltons atomic theory is the result of the law of conservation of mass?4. Which postulate of Daltons atomic theory can explain the law of definite proportions?3.2 What is an Atom?Have you ever observed a mason buildingwalls, from these walls a room and then a Fig. 3.2: An image of the surface of siliconcollection of rooms to form a building? Whatis the building block of the huge building?What about the building block of an ant-hill? 3.2.1 W HATARE THE MODERN DAYIt is a small grain of sand. Similarly, theSYMBOLS OF ATOMS OF DIFFERENTbuilding blocks of all matter are atoms. ELEMENTS?How big are atoms?Dalton was the first scientist to use theAtoms are very small, they are smaller thansymbols for elements in a very specific sense.anything that we can imagine or compareWhen he used a symbol for an element hewith. More than millions of atoms whenalso meant a definite quantity of that element,stacked would make a layer barely as thickthat is, one atom of that element. Berziliusas 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 nmthe element.1 m = 109 nmRelative SizesRadii (in m)Example1010 Atom of hydrogen109Molecule of water 810Molecule of haemoglobin104Grain of sand102AntFig. 3.3: Symbols for some elements as proposed by101WatermelonDaltonATOMS AND MOLECULES33 4. In the beginning, the names of elements the passage of time and repeated usage youwere derived from the name of the place where will automatically be able to reproduce thethey were found for the first time. For symbols).example, the name copper was taken fromCyprus. Some names were taken from3.2.2 ATOMIC MASSspecific colours. For example, gold was takenfrom the English word meaning yellow. The most remarkable concept that DaltonsNow-a-days, IUPAC (International Union of atomic theory proposed was that of the atomicPure and Applied Chemistry) approves namesmass. According to him, each element had aof elements. Many of the symbols are the firstcharacteristic atomic mass. The theory couldone or two letters of the elements name in explain the law of constant proportions soEnglish. The first letter of a symbol is always well that scientists were prompted to measurewritten as a capital letter (uppercase) and the the atomic mass of an atom. Sincesecond letter as a small letter (lowercase).determining the mass of an individual atomFor example was a relatively difficult task, relative atomic (i) hydrogen, Hmasses were determined using the laws of(ii) aluminium, Al and not AL chemical combinations and the compounds (iii) cobalt, Co and not CO. formed.Let us take the example of a compound, Symbols of some elements are formedcarbon monoxide (CO) formed by carbon andfrom the first letter of the name and a letter, oxygen. It was observed experimentally thatappearing later in the name. Examples are:3 g of carbon combines with 4 g of oxygen to(i) chlorine, Cl, (ii) zinc, Zn etc.form CO. In other words, carbon combines Other symbols have been taken from thewith 4/3 times its mass of oxygen. Supposenames of elements in Latin, German or Greek.For example, the symbol of iron is Fe from itswe define the atomic mass unit (earlierLatin name ferrum, sodium is Na fromabbreviated as amu, but according to thenatrium, potassium is K from kalium.latest IUPAC recommendations, it is nowTherefore, each element has a name and awritten as u unified mass) as equal to theunique chemical symbol. mass of one carbon atom, then we would Table 3.1: Symbols for some elementsElementSymbol Element SymbolElement SymbolAluminiumAl CopperCuNitrogen NArgonAr FluorineF Oxygen OBarium Ba GoldAuPotassiumKBoronBHydrogenH SiliconSiBromineBr IodineI Silver AgCalciumCa IronFeSodium NaCarbon CLeadPbSulphurSChlorine Cl Magnesium MgUraniumUCobalt Co NeonNeZinc Zn (The above table is given for you to refer assign carbon an atomic mass of 1.0 u andto whenever you study about elements. Dooxygen an atomic mass of 1.33 u. However, itnot bother to memorise all in one go. Withis more convenient to have these numbers34SCIENCE 5. as whole numbers or as near to a whole mass of the atom, as compared to 1/12th thenumbers as possible. While searching for mass of one carbon-12 atom.various atomic mass units, scientists initiallytook 1/16 of the mass of an atom of naturally Table 3.2: Atomic masses ofoccurring oxygen as the unit. This was a few elementsconsidered relevant due to two reasons:oxygen reacted with a large number of ElementAtomic Mass (u) elements and formed compounds.this atomic mass unit gave masses of Hydrogen1 most of the elements as whole numbers. Carbon 12However, in 1961 for a universallyNitrogen14accepted atomic mass unit, carbon-12 isotopeOxygen 16was chosen as the standard reference formeasuring atomic masses. One atomic massSodium 23unit is a mass unit equal to exactly one- Magnesium 24twelfth (1/12 th) the mass of one atom ofSulphur32carbon-12. The relative atomic masses of allelements have been found with respect to an Chlorine35.5atom of carbon-12.Calcium40Imagine a fruit seller selling fruits withoutany standard weight with him. He takes awatermelon and says, this has a mass equal3.2.3 HOW DO ATOMS EXIST?to 12 units (12 watermelon units or 12 fruitmass units). He makes twelve equal pieces of Atoms of most elements are not able to existthe watermelon and finds the mass of eachindependently. Atoms form molecules andfruit he is selling, relative to the mass of one ions. These molecules or ions aggregate inpiece of the watermelon. Now he sells hislarge numbers to form the matter that we canfruits by relative fruit mass unit (fmu), as insee, feel or touch. QFig. 3.4. uestions1. 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 definedFig. 3.4 : (a) Watermelon, (b) 12 pieces, (c) 1/12 ofas the smallest particle of an element or a watermelon, (d) how the fruit seller cancompound 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 thedifferent elements can join together to formatom of an element is defined as the average molecules.ATOMS AND MOLECULES 35 6. 3.3.1 MOLECULES OF ELEMENTS Table 3.4 : Molecules of someThe molecules of an element are constituted compoundsby the same type of atoms. Molecules of manyCompoundCombining Ratioelements, such as argon (Ar), helium (He) etc.Elements byare made up of only one atom of that element. MassBut this is not the case with most of the non-metals. For example, a molecule of oxygen Water Hydrogen, Oxygen1:8consists of two atoms of oxygen and hence itAmmonia Nitrogen, Hydrogen14:3is known as a diatomic molecule, O2. If 3Carbonatoms of oxygen unite into a molecule, insteadof the usual 2, we get ozone. The number of Dioxide Carbon, Oxygen3:8atoms constituting a molecule is known asits atomicity.Molecules of metals and some other Activity ______________ 3.2elements, such as carbon, do not have aRefer to Table 3.4 for ratio by mass ofsimple structure but consist of a very largeatoms present in molecules and Tableand indefinite number of atoms bonded 3.2 for atomic masses of elements. Findtogether. the ratio by number of the atoms ofLet us look at the atomicity of someelements in the molecules ofelements. compounds given in Table 3.4. The ratio by number of atoms for awater molecule can be found as follows: Table 3.3 : Atomicity of some elementsElement Ratio Atomic Mass Simplestbymass ratio/ratio Types ofName Atomicitymass(u) atomic Elementmass Non-MetalArgonMonoatomic1Helium Monoatomic H 11=12 1OxygenDiatomic 81HydrogenDiatomicO 816 = 1 16 2NitrogenDiatomicChlorineDiatomic Thus, the ratio by number of atoms forwater is H:O = 2:1.Phosphorus Tetra-atomicSulphurPoly-atomic 3.3.3 WHAT IS AN ION? MetalSodium Monoatomic Compounds composed of metals and non-Iron Monoatomic metals contain charged species. The chargedAluminiumMonoatomicspecies are known as ions. An ion is a chargedCopper Monoatomicparticle and can be negatively or positively charged. A negatively charged ion is called an anion and the positively charged ion, a3.3.2 MOLECULES OF COMPOUNDS cation. Take, for example, sodium chlorideAtoms of different elements join together in (NaCl). Its constituent particles are positivelydefinite proportions to form molecules ofcharged sodium ions (Na+) and negativelycompounds. Few examples are given in charged chloride ions (Cl). Ions may consistTable 3.4. of a single charged atom or a group of atoms 36 SCIENCE 7. that have a net charge on them. A group of exercise, we need to learn the symbols andatoms carrying a charge is known as acombining capacity of the elements.polyatomic ion. We shall learn more aboutThe combining power (or capacity) of anthe 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 another element to for m a chemical IonicConstitutingRatio compound. The valency of the atom of an Compound Elementsby element can be thought of as hands or armsMass of that atom. Calcium oxideCalcium andHuman beings have two arms and anoxygen5:2octopus has eight. If one octopus has to catch hold of a few people in such a manner that MagnesiumMagnesium all the eight arms of the octopus and both sulphide and sulphur 3:4arms of all the humans are locked, how many Sodium Sodium humans do you think the octopus can hold? chloride and chlorine23:35.5Represent the octopus with O and humans with H. Can you write a formula for this combination? Do you get OH4 as the formula? The subscript 4 indicates the number of3.4 Writing Chemical Formulaehumans held by the octopus.The chemical formula of a compound is aThe valencies of some simple andsymbolic representation of its composition.polyatomic ions are given in Table 3.6. WeThe chemical for mulae of dif ferent will learn more about valency in the nextcompounds can be written easily. For thischapter. Table 3.6: Some common, simple and polyatomic ions Vale- Name of Symbol Non-Symbol Polyatomic Symbol ncy ionmetallicionselement1.SodiumNa+ HydrogenH+ Ammonium+ NH4Potassium K+Hydride H- Hydroxide OHSilverAg+ ChlorideCl-Nitrate NO3Copper (I)* Cu+ Bromide Br-HydrogenIodideI carbonate HCO32.Magnesium Mg2+Oxide O2-Carbonate CO32Calcium Ca2+SulphideS2-SulphiteSO32ZincZn2+ SulphateSO42Iron (II)*Fe2+Copper (II)*Cu2+3.Aluminium Al3+Nitride N3-Phosphate PO43Iron (III)* Fe3+* Some elements show more than one valency. A Roman numeral shows their valency in a bracket.ATOMS AND MOLECULES 37 8. The rules that you have to follow while writing3. Formula of carbon tetrachloridea chemical formula are as follows: the valencies or charges on the ion mustbalance. when a compound consists of a metaland a non-metal, the name or symbol ofthe metal is written first. For example:calcium oxide (CaO), sodium chloride(NaCl), iron sulphide (FeS), copper oxide The formulae of ionic compounds are simply(CuO) etc., where oxygen, chlorine, the whole number ratio of the positive tosulphur are non-metals and are writtennegative ions in the structure.on the right, whereas calcium, sodium,For magnesium chloride, we write theiron and copper are metals, and are symbol of cation (Mg2+) first followed by thewritten on the left.symbol of anion (Cl-). Then their charges are in compounds formed with polyatomic criss-crossed to get the formula.ions, the ion is enclosed in a bracket 4. Formula of magnesium chloridebefore writing the number to indicate theratio.3.4.1 FORMULAEOF SIMPLE COMPOUNDSThe simplest compounds, which are made up Formula : MgCl2of two different elements are called binary Thus, in magnesium chloride, there arecompounds. Valencies of some ions are given two chloride ions (Cl-) for each magnesiumin Table 3.6. You can use these to writeion (Mg2+). The positive and negative chargesformulae for compounds.must balance each other and the overallWhile writing the chemical formulae forstructure must be neutral. Note that in thecompounds, we write the constituentformula, the charges on the ions are notelements and their valencies as shown below.indicated.Then we must crossover the valencies of thecombining atoms.Some more examples (a) Formula for aluminium oxide:Examples 1. Formula of hydrogen chlorideFormula : Al2O3 (b) Formula for calcium oxide: Formula of the compound would be HCl. 2. Formula of hydrogen sulphideHere, the valencies of the two elementsare the same. You may arrive at the formulaCa2O2. But we simplify the formula as CaO.38 SCIENCE 9. (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 mchemical 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 MoleNote that the for mula of calcium Concepthydroxide is Ca(OH)2 and not CaOH2. We use 3.5.1 MOLECULAR MASSbrackets when we have two or more of thesame ions in the formula. Here, the bracketIn section 3.2.2 we discussed the concept ofaround OH with a subscript 2 indicates thatatomic mass. This concept can be extendedthere are two hydroxyl (OH) groups joined to to calculate molecular masses. The molecularone calcium atom. In other words, there aremass of a substance is the sum of the atomictwo atoms each of oxygen and hydrogen in masses of all the atoms in a molecule of thecalcium 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 : Na2CO3Solution:In the above example, brackets are not(a) Atomic mass of hydrogen = 1u,needed if there is only one ion present.oxygen = 16 u (f) Formula of ammonium sulphate:So the molecular mass of water, whichcontains two atoms of hydrogen andone atom of oxygen is = 2 1+ 116= 18 u(b) The molecular mass of HNO3 = theFormula : (NH4)2SO4 atomic mass of H + the atomic mass ofQN+ 3 the atomic mass of Ouestions = 1 + 14 + 48 = 63 u1. Write down the formulae of (i) sodium oxide 3.5.2 FORMULA UNIT MASS(ii) aluminium chlorideThe formula unit mass of a substance is a (iii) sodium suphidesum of the atomic masses of all atoms in a (iv) magnesium hydroxideformula unit of a compound. Formula unit2. Write down the names of mass is calculated in the same manner ascompounds represented by the we calculate the molecular mass. The onlyATOMS AND MOLECULES39 10. difference is that we use the word formula3.5.3 MOLE CONCEPTunit for those substances whose constituentparticles are ions. For example, sodium Take an example of the reaction of hydrogenchloride as discussed above, has a formulaand oxygen to form water:unit NaCl. Its formula unit mass can be 2H2+ O2 2H2O.calculated asThe above reaction indicates that1 23 + 1 35.5 = 58.5 u (i) two molecules of hydrogen combine Example 3.2 Calculate the formula unitwith one molecule of oxygen to formmass of CaCl2. two molecules of water, or(ii) 4 u of hydrogen molecules combine Solution: with 32 u of oxygen molecules to formAtomic mass of Ca36 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 beQcharacterised by its mass or the number of uestions molecules. But, a chemical reaction equationindicates directly the number of atoms or1. Calculate the molecular masses molecules taking part in the reaction. of H2, O2, Cl 2, CO2, CH4, C2H6, Therefore, it is more convenient to refer to C2H4, NH3, CH3OH.the quantity of a substance in terms of the2. Calculate the for mula unitnumber 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 40SCIENCE 11. molecules, ions or particles) is that quantityfor this they need to relate the mass in gramsin number having a mass equal to its atomic to the number. It is done as follows:or molecular mass in grams. 1 mole = 6.022 1023 number The number of particles (atoms, molecules = Relative mass in grams.or ions) present in 1 mole of any substance Thus, a mole is the chemists counting unit.is fixed, with a value of 6.022 1023. This is The word mole was introduced aroundan experimentally obtained value. This1896 by Wilhelm Ostwald who derived thenumber is called the Avogadro Constant or term from the Latin word moles meaning aAvogadro Number (represented by N0), namedheap or pile. A substance may be consideredin honour of the Italian scientist, as a heap of atoms or molecules. The unitAmedeo Avogadro.mole was accepted in 1967 to provide a simple1 mole (of anything) = 6.022 1023 in number,way of reporting a large number the massiveas, 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 3.3has one more advantage over a dozen or a 1. Calculate the number of moles for thegross. This advantage is that mass of 1 molefollowing: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 atomsgrams. The atomic mass of an element gives(finding mole from number ofus the mass of one atom of that element inparticles).atomic mass units (u). To get the mass of1 mole of atom of that element, that is, molar Solutions:mass, we have to take the same numerical No. of moles=nvalue but change the units from u to g.Given mass= mMolar mass of atoms is also known as gramMolar mass= Matomic mass. For example, atomic mass of Given number of particles = Nhydrogen=1u. So, gram atomic mass of Avogadro number of particles= N0hydrogen = 1 g.1 u hydrogen has only 1 atom of hydrogen (i)Atomic mass of He= 4u1 g hydrogen has 1 mole atoms, that is, Molar mass of He = 4g6.022 1023 atoms of hydrogen.Thus, the number of molesSimilarly,given mass16 u oxygen has only 1 atom of oxygen,= molar mass16 g oxygen has 1 mole atoms, that is,6.022 1023 atoms of oxygen. m 52 To find the gram molecular mass or molarn== = 13M 4mass of a molecule, we keep the numericalvalue the same as the molecular mass, but(ii) we know,simply change units as above from u to g. 1 mole = 6.022 1023For example, as we have already calculated, The number of molesmolecular mass of water (H2O) is 18 u. Fromgiven number of particleshere we understand that =18 u water has only 1 molecule of water,Avogadro number18 g water has 1 mole molecules of water,that is, 6.022 1023 molecules of water.N12.044 1023 n== =2 Chemists need the number of atoms and No 6.022 1023molecules while carrying out reactions, andATOMS AND MOLECULES 41 12. Example 3.4 Calculate the mass of the Solutions: following: (i) The number of atoms (i) 0.5 mole of N2 gas (mass from molegiven massof molecule) = Avogadro number molar mass(ii) 0.5 mole of N atoms (mass frommole of atom) m(iii) 3.011 1023 number of N atoms N= N0M(mass from number)(iv) 6.022 10 23 number of N 2 46molecules (mass from number) N= 6.022 1023 23Solutions: N = 12.044 1023(i)mass = molar mass number of(ii) The number of moleculesmoles given mass m = M n = 28 0.5 = 14 g = Avogadro number molar mass(ii) mass = molar mass number ofm N= N0moles M m = M n = 14 0.5 = 7 gatomic mass of oxygen = 16 u(iii) The number of moles, n molar mass of O2 molecules= 16 2 = 32g given number of particles N ==Avogadro numberN0 8 N= 6.022 1023 32 3.011 1023 = N = 1.5055 1023 6.022 1023; 1.51 10233.011 1023 m = M n = 14 (iii) The number of particles (atom) =6.022 1023number of moles of particles = 14 0.5 = 7 gAvogadro numberN = n N0 = 0.1 x 6.022 1023 N= 6.022 1022 n=Q(iv) N0 m=M N = 28 6.022 1023uestionsN06.022 10231. If one mole of carbon atoms= 28 1 = 28 gweighs 12 gram, what is the mass (in gram) of 1 atom of carbon?Example 3.5 Calculate the number of 2. Which has more number of particles in each of theatoms, 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 13. Whatyou havelearntDuring 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 can 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.Exercises1. 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 willATOMS AND MOLECULES43 14. 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 followingcompounds. (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, HNO37.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 in16 g of solid sulphur. 11.Calculate the number of aluminium ions present in 0.051 g ofaluminium 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)44SCIENCE 15. Group ActivityPlay 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: Na2SO4Do it yourself :Now, write the formula of sodium phosphate.ATOMS AND MOLECULES 45