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9 atoms and molecules

  1. 1. C hapter 3 ATOMS AND MOLECULESAncient Indian and Greek philosophers have experimentations by Lavoisier 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 on Is 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 chemicals–Par manu. Another Indian philosopher, X YPakudha 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 nitrate sodium chlorideforms of matter. • Prepare separately a 5% solution of any Around the same era, ancient Greek one pair of substances listed under Xphilosophers – Democritus and Leucippus and 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 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 flaskthese indivisible particles atoms (meaning carefully; see that the solutions do notindivisible). All this was based on get 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 chemical Fig. 3.1: Ignition tube containing solution of X, dippedcombination were established after much in a conical flask containing solution of Y.
  2. 2. • 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 teacher at the age of mouth 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, John Daltonmass can neither be created nor destroyed physics and chemistry inin a chemical reaction. a college. He spent most of his life there teaching 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 two According to Dalton’s 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 of cannot be created or destroyed in ahydrogen and 8 g of oxygen are always chemical reaction.obtained. Similarly in ammonia, nitrogen and (iii) Atoms of a given element are identicalhydrogen 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 havefrom which it is obtained. dif ferent 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 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 by atoms are constant in a givenmass”. compound. The next problem faced by scientists was You 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 the ethanoic 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. Dalton’s ethanoate. Show that these 32 SCIENCE
  3. 3. 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 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 + carbon see 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 water. What mass of oxygen gas atoms. 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 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 HAT ARE THE MODERN DAYIt is a small grain of sand. Similarly, the SYMBOLS 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 than symbols for elements in a very specific sense.anything that we can imagine or compare When he used a symbol for an element hewith. 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. 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 nm the element. 1 m = 109 nmRelative SizesRadii (in m) Example10–10 Atom of hydrogen10–9 Molecule of water –810 Molecule of haemoglobin10–4 Grain of sand10–2 Ant Fig. 3.3: Symbols for some elements as proposed by10–1 Watermelon DaltonATOMS AND MOLECULES 33
  4. 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 from 3.2.2 ATOMIC MASSspecific colours. For example, gold was takenfrom the English word meaning yellow. The most remarkable concept that Dalton’sNow-a-days, IUPAC (International Union of atomic theory proposed was that of the atomicPure and Applied Chemistry) approves names mass. According to him, each element had aof elements. Many of the symbols are the first characteristic atomic mass. The theory couldone or two letters of the element’s 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, H masses 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 formed carbon 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 the with 4/3 times its mass of oxygen. Supposenames of elements in Latin, German or Greek.For example, the symbol of iron is Fe from its we define the atomic mass unit (earlierLatin name ferrum, sodium is Na from abbreviated as ‘amu’, but according to thenatrium, potassium is K from kalium. latest IUPAC recommendations, it is nowTherefore, each element has a name and a written as ‘u’ – unified mass) as equal to theunique chemical symbol. mass of one carbon atom, then we would 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 assign carbon an atomic mass of 1.0 u andto whenever you study about elements. Do oxygen an atomic mass of 1.33 u. However, itnot bother to memorise all in one go. With is more convenient to have these numbers 34 SCIENCE
  5. 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 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 14accepted atomic mass unit, carbon-12 isotope Oxygen 16was chosen as the standard reference formeasuring atomic masses. One atomic mass Sodium 23unit is a mass unit equal to exactly one- Magnesium 24twelfth (1/12 th) the mass of one atom of Sulphur 32carbon-12. The relative atomic masses of allelements have been found with respect to an Chlorine 35.5atom of carbon-12. Calcium 40 Imagine a fruit seller selling fruits withoutany standard weight with him. He takes awatermelon and says, “this has a mass equal 3.2.3 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 each independently. 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 his large numbers to form the matter that we canfruits by relative fruit mass unit (fmu), as in see, feel or touch. QFig. 3.4. uestions 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 definedFig. 3.4 : (a) Watermelon, (b) 12 pieces, (c) 1/12 of as 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 formatom of an element is defined as the average molecules.ATOMS AND MOLECULES 35
  6. 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 many Compound Combining 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, Oxygen 1:8consists of two atoms of oxygen and hence it Ammonia Nitrogen, Hydrogen 14:3is known as a diatomic molecule, O2. If 3 Carbonatoms of oxygen unite into a molecule, insteadof the usual 2, we get ozone. The number of Dioxide Carbon, Oxygen 3:8atoms constituting a molecule is known asits atomicity. Molecules of metals and some other Activity ______________ 3.2elements, such as carbon, do not have a • Refer to Table 3.4 for ratio by mass ofsimple structure but consist of a very large atoms 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 of Let us look at the atomicity of some elements in the molecules ofelements. compounds given in Table 3.4. • The ratio by number of atoms for a water molecule can be found as follows: Table 3.3 : Atomicity of some elements Element Ratio Atomic Mass Simplest by mass ratio/ ratio Types of Name Atomicity mass (u) atomic Element mass Non-Metal Argon Monoatomic 1 Helium Monoatomic H 1 1 =1 2 1 Oxygen Diatomic 8 1 Hydrogen Diatomic O 8 16 = 1 16 2 Nitrogen Diatomic Chlorine Diatomic • Thus, the ratio by number of atoms for water is H:O = 2:1. Phosphorus Tetra-atomic Sulphur Poly-atomic 3.3.3 WHAT IS AN ION? Metal Sodium Monoatomic Compounds composed of metals and non- Iron Monoatomic metals contain charged species. The charged Aluminium Monoatomic species are known as ions. An ion is a charged Copper Monoatomic particle 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 of charged 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. 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 a combining capacity of the elements.polyatomic ion. We shall learn more about The 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 Ionic Constituting Ratio compound. The valency of the atom of an Compound Elements by element can be thought of as hands or arms Mass 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 Magnesium Magnesium all the eight arms of the octopus and both sulphide and sulphur 3:4 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? The subscript 4 indicates the number of3.4 Writing Chemical Formulae humans held by the octopus.The chemical formula of a compound is a The 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 this chapter. Table 3.6: Some common, simple and polyatomic 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
  8. 8. The rules that you have to follow while writing 3. Formula of carbon tetrachloridea 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 The formulae of ionic compounds are simply (CuO) etc., where oxygen, chlorine, the whole number ratio of the positive to sulphur are non-metals and are written negative ions in the structure. on the right, whereas calcium, sodium, For magnesium chloride, we write the iron and copper are metals, and are symbol of cation (Mg2+) first followed by the written 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 chloride before writing the number to indicate the ratio.3.4.1 FORMULAE OF 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 write ion (Mg2+). The positive and negative chargesformulae for compounds. must balance each other and the overall While writing the chemical formulae for structure must be neutral. Note that in thecompounds, we write the constituent formula, 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 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
  9. 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 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 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 bracket In section 3.2.2 we discussed the concept ofaround OH with a subscript 2 indicates that atomic 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 are mass 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 : Na2CO3 Solution: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, 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 ofQ N+ 3 × the atomic mass of O uestions = 1 + 14 + 48 = 63 u 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 (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 compounds represented by the we calculate the molecular mass. The onlyATOMS AND MOLECULES 39
  10. 10. difference is that we use the word formula 3.5.3 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 formula and oxygen to form water:unit NaCl. Its formula unit mass can be 2H2+ O2 → 2H2O.calculated as– The above reaction indicates that 1 × 23 + 1 × 35.5 = 58.5 u (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 beQ characterised by its mass or the number of uestions 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, Cl 2, CO2, CH4, C2H6, Therefore, it is more convenient to refer to C2H4, NH3, CH3OH. the quantity of a substance in terms of the 2. Calculate the for mula 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
  11. 11. molecules, ions or particles) is that quantity for 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 chemist’s counting unit.is fixed, with a value of 6.022 × 1023. This is The word “mole” was introduced aroundan experimentally obtained value. This 1896 by Wilhelm Ostwald who derived thenumber is called the Avogadro Constant or term from the Latin word moles meaning aAvogadro Number (represented by N0), named ‘heap’ 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 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 3.3has one more advantage over a dozen or a 1. Calculate the number of moles for thegross. 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 atomsgrams. The atomic mass of an element gives (finding mole from number ofus the mass of one atom of that element in particles).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 gram Molar 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 molar ⇒n= = = 13 M 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. From given 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. N 12.044 × 1023 ⇒n= = =2 Chemists need the number of atoms and No 6.022 × 1023molecules while carrying out reactions, andATOMS AND MOLECULES 41
  12. 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 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 23 number of N 2 46 molecules (mass from number) ⇒N= × 6.022 × 1023 23Solutions: ⇒ N = 12.044 × 1023 (i) mass = molar mass × number of (ii) The number of molecules moles given mass ⇒ m = M × n = 28 × 0.5 = 14 g = × Avogadro number molar mass (ii) mass = molar mass × number of m ⇒N= × N0 moles M ⇒ m = M × n = 14 × 0.5 = 7 g atomic 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 number N0 8 ⇒N= × 6.022 × 1023 32 3.011 × 1023 = ⇒ N = 1.5055 × 1023 6.022 × 1023 ; 1.51 × 1023 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 × N0 = 0.1 x 6.022 × 1023 N = 6.022 × 1022 n= Q (iv) N0 ⇒m=M × N = 28 × 6.022 × 1023 uestions N0 6.022 × 1023 1. If one mole of carbon atoms = 28 × 1 = 28 g weighs 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 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
  13. 13. 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 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. 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 willATOMS AND MOLECULES 43
  14. 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 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
  15. 15. 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