NIOS Lesson 1 - ATOMS, MOLECULES AND CHEMICAL ARITHMETIC

1.1 SCOPE OF CHEMISTRY

1.1.1 Health and Medicine

1.1.2 Energy and the Environment

1.1.3 Materials and Technology

1.1.4 Food and Agriculture

INTEXT QUESTIONS 1.1

1.2 PARTICULATE NATURE OF MATTER

INTEXT QUESTIONS 1.2

1.3 LAWS OF CHEMICAL COMBINATIONS

INTEXT QUESTIONS 1.3

1.4 DALTON’S ATOMIC THEORY

1.4.1 Postulates of Dalton's Atomic Theory

1.4.2 What is an Atom?

1.4.3 Molecules

1.4.4 Elements

INTEXT QUESTIONS 1.4

1.5 SI UNITS

INTEXT QUESTIONS 1.5

1.6 RELATIONSHIP BETWEEN MASS AND NUMBER OF PARTICLES

INTEXT QUESTIONS 1.6

1.7 MOLE – A NUMBER UNIT

INTEXT QUESTIONS 1.7

1.8 AVOGADRO’S CONSTANT

Significance of Avogadro’s Constant

INTEXT QUESTIONS 1.8

1.9 MOLE, MASS AND NUMBER RELATIONSHIPS

1.9.1 Atomic Mass Unit

1.9.2 Relative Atomic and Molecular Masses

1.9.3 Atomic, Molecular and Formula Masses

1.9.4 Molar Masses

(i) Molar mass of an element

(ii) Molar mass of a molecular substance

(iii) Molar masses of ionic compounds

INTEXT QUESTIONS 1.9

1.10 MASS, MOLAR MASS AND NUMBER OF MOLES

INTEXT QUESTIONS 1.10

1.11 MOLAR VOLUME, Vm

INTEXT QUESTIONS 1.11

1.12 MOLCULAR AND EMPIRICAL FORMULAE

INTEXT QUESTIONS 1.12

1.13 CHEMICAL COMPOSITION AND FORMULAE

1.13.1 Percentage Composition

INTEXT QUESTIONS 1.13

1.14 DETERMINATION OF EMPIRICAL FORMULAE – FORMULA STOICHIOMETRY

INTEXT QUESTIONS 1.14

1.15 CHEMICAL EQUATION AND REACTION STOICHIOMETRY

1.15.1 Microscopic Quantitative Information

1.15.2 Macroscopic Quantitative Information

(a) Mole Relationships

(b) Mass Relationships

(c) Volume Relationships

INTEXT QUESTIONS 1.15

1.16 LIMITING REAGENT

INTEXT QUESTIONS 1.16

WHAT YOU HAVE LEARNT

TERMINAL EXERCISE

Chemistry is the study of matter and the changes it undergoes. 

Chemistry is often called the central science, because a basic knowledge of chemistry is

essential for the study of biology, physics, geology, ecology, and many other subjects.

Although chemistry is an ancient science, its modern foundation was laid in the nineteenth century, when intellectual and technological advances enabled scientists to break down substances into ever smaller components and consequently to explain many of their physical and chemical characteristics.

Chemistry plays a pivotal role in many areas of science and technology e.g.in health, medicine, energy and environment, food, agriculture and new materials.

As you are aware, atoms and molecules are so small that we cannot see them with our naked eyes or even with the help of a microscope. 

Any sample of matter which can be studied consists of extremely large number of atoms or molecules.

In chemical reactions, atoms or molecules combine with one another in a definite number ratio. Therefore, it would be pertinent if we could specify the total number of atoms or molecules in a given sample of a substance. 

We use many number units in our daily life. 

For example, we express the number of bananas or eggs in terms of ‘dozen’. 

In chemistry we use a number unit called mole which is very large.

With the help of mole concept, it is possible to take a desired number of atoms/ molecules by weighing. Now, in order to study chemical compounds and reactions in the laboratory, it is necessary to have adequate knowledge of the quantitative relationship among the amounts of the reacting substances that take part and products formed in the chemical reaction. This relationship is known as stoichiometry. 

Stoichiometry (derived from the Greek Stoicheion = element andmetron = measure) is the term we use to refer to all the quntatitative aspects of chemical compounds and reactions. 

In the present lesson, you will see how chemical formulae are determined and how chemical equations prove useful in predicting the proper amounts of the reactants that must be mixed to carry out a complete reaction. 

In other words we can take reactants for a reaction in such a way that none of the reacting substances is in excess. This aspect is very vital in chemistry and has wide application in industries.

OBJECTIVES

After reading this lesson you will be able to :

􀁺 explain the scope of chemistry;

􀁺 explain the atomic theory of matter;

􀁺 state the laws of chemical combination;

􀁺 explain Dalton’s atomic theory;

􀁺 define the terms element, atoms and molecules.

􀁺 state the need of SI units;

􀁺 list base SI units;

􀁺 explain the relationship between mass and number of particles;

􀁺 define Avogadro’s constant and state its significance;

􀁺 calculate the molar mass of different elements and compounds;

􀁺 define molar volume of gases at STP.

􀁺 define empirical and molecular formulae;

􀁺 differentiate between empirical and molecular formulae;

􀁺 calculate percentage by mass of an element in a compound and also work out empirical formula from the percentage composition;

􀁺 establish relationship between mole, mass and volume;

􀁺 calculate the amount of substances consumed or formed in a chemical reaction using a balanced equation and mole concept, and

􀁺 explain the role of limiting reagent in limiting the amount of the products formed.

1.1 SCOPE OF CHEMISTRY

Chemistry plays an important role in all aspects of our life. Let us discuss role

of chemistry in some such areas.

1.1.1 Health and Medicine

Three major advances in this century have enabled us to prevent and treat

diseases. Public health measures establishing sanitation systems to protect vast

numbers of people from infectious diseases; surgery with anesthesia, enabling

physicians to cure potentially fatal conditions, such as an inflamed appendix; and

the introduction of vaccines and antibiotics that made it possible to prevent

diseases spread by microbes. Gene therapy promises to be the fourth revolution

in medicine. (A gene is the basic unit of inheritance.) Several thousand known

conditions, including cystic fibrosis and hemophilia, are carried by inborn

damage to a single gene. Many other ailments, such as cancer, heart disease,

AIDS, and arthritis, result to an extent from impairment of one or more genes

involved in the body’s defenses. In gene therapy, a selected healthy gene is

delivered to a patient’s cell to cure or ease such disorders. To carry out such

a procedure, a doctor must have a sound knowledge of the chemical properties

of the molecular components involved.

Chemists in the pharmaceutical industry are researching potent drugs with few

or no side effects to treat cancer, AIDS, and many other diseases as well as

drugs to increase the number of successful organ transplants. On a broader scale,

improved understanding of the mechanism of ageing will lead to a longer and

healthier lifespan for the world’s population.

11.2 Energy and the Environment

Energy is a by-product of many chemical processes, and as the demand for

energy continues to increase, both in technologically advanced countries like the

United States and in developing ones like India. Chemists are actively trying

to find new energy sources. Currently the major sources of energy are fossil

fuels (coal, petroleum, and natural gas). The estimated reserves of these fuels

will last us another 50-100 years at the present rate of consumption, so it is

urgent that we find alternatives.

Solar energy promises to be a viable source of energy for the future. Every year

earth’s surface receives about 10 times as much energy from sunlight as is

contained in all of the known reserves of coal, oil, natural gas, and uranium

combined. But much of this energy is “wasted” because it is reflected back into

space. For the past thirty years, intense research efforts have shown that solar

energy can be harnessed effectively in two ways. One is the conversion of

sunlight directly to electricity using devices called photovoltaic cells. The other

is to use sunlight to obtain hydrogen from water. The hydrogen can then be

fed into a fuel cell to generate electricity. Although our understanding of the

scientific process of converting solar energy to electricity has advanced, the

technology has not yet improved to the point where we can produce electricity

on a large scale at an economically acceptable cost. By 2050, however, it has

been predicted that solar energy will supply over 50 percent of our power needs.

Another potential source of energy is nuclear fission, but because of environmental

concerns about the radioactive wastes from fission processes, the future of the

nuclear industry is uncertain. Chemists can help to devise better ways to dispose

of nuclear waste. Nuclear fusion, the process that occurs in the sun and other

stars, generates huge amounts of energy without producing much dangerous

radioactive waste. In another 50 years, nuclear fusion will likely be a significant

source of energy.

Energy production and energy utilization are closely tied to the quality of our

environment. A major disadvantage of burning fossil fuels is that they give off

carbon dioxide, which is a greenhouse gas (that is, it promotes the heating of

Earth’s atmosphere), along with sulfur dioxide and nitrogen oxides, which result

in acid rain and smog. Harnessing solar energy has no such detrimental effects

on the environment. By using fuel-efficient automobiles and more effective

catalytic converters, we should be able to drastically reduce harmful auto

emissions and improve the air quality in areas with heavy traffic. In addition,

electric cars, powered by durable, long-lasting batteries, should be more

prevalent in the next century, and their use will help to minimize air pollution.

1.1.3 Materials and Technology

Chemical research and development in the twentieth century have provided us

with new materials that have profoundly improved the quality of our lives and

helped to advance technology in countless ways. A few examples are polymers

(including rubber and nylon), ceramics (such as cookware), liquid crystals (like

those in electronic displays), adhesives, and coatings (for example, latex paint).

What is in store for the near future? One likely possibility is room-temperature

superconductors. Electricity is carried by copper cables, which are not perfect

conductors. Consequently, about 20 percent of electrical energy is lost in the

form of heat between the power station and our homes. This is a tremendous

waste. Superconductors are materials that have no electrical resistance and can

therefore conduct electricity with no energy loss.

1.1.4 Food and Agriculture

How can the world’s rapidly increasing population be fed? In poor countries,

agricultural activities occupy about 80 percent of the workforce and half of an

average family budget is spent on foodstuffs. This is a tremendous drain on a

nation’s resources. The factors that affect agricultural production are the

richness of the soil, insects and diseases that damage crops, and weeds that

compete for nutrients. Besides irrigation, farmers rely on fertilizers and

pesticides to increase crop yield.

1.2 PARTICULATE NATURE OF MATTER

Chemistry deals with study of structure and composition of matter. Since ancient

time people have been wondering about nature of matter. Suppose we take a

piece of rock and start breaking it into smaller and smaller particles can this

process go on far ever resulting in smaller and smaller particles or would it come

to stop when such particles are formed which can no longer to broken into still

smaller particles? Many people including Greek philosophers Plato and Aristotle

believed that matter is continuous and the process of subdivision of matter can

go on.

On the other hand, many people believed that the process of subdivision of mater

can be repeated only a limited nuimber of times till such particles are obtained

which cannot be further subdivided. They believed that mattr is composed of

large number of very tiny particles and thus has particle naturew. The smallest

indivisible particles of matter were given the name ‘atom’ from the Greek word

“atoms” meaning ‘indivisible’. It is generally agreed that the Greek philosopher

Leucippus and his student Democritus were the first to propose this idea, about

440 B.C.. However, Maharshi Kanad had propounded the atomic concept of

matter earlier (500 BC) and had named the smallest particle of matter as

"PARMANU".

1.3 LAWS OF CHEMICAL COMBINATIONS

There was tremendous progress in Chemical Sciences after 18th century. It arose

out of an interest in the nature of heat and the way things burn. Major progress

was made through the careful use of chemical balance to determine the change

in mass that occurs in chemical reactions. The great French Chemist Antoine

Lavoisier used the balance to study chemical reactions. He heated mercury in

a sealed flask that contained air. After several days, a red substance mercury

(II) oxide was produced. The gas remaining in the flask was reduced in mass.

The remaining gas was neither able to support combustion nor life. The

remaining gas in the flask was identified as nitrogen. The gas which combined

with mercury was oxygen. Further he carefully performed the experiment by

taking a weighed quantity of mercury (II) oxide. After strong heating, he found

that mercury (II) oxide, red in colour, was decomposed into mercury and

oxygen. He weighed both mercury and oxygen and found that their combined

mass was equal to that of the mercury (II) oxide taken. Lavoisier finally came

to the conclusion that in every chemical reaction, total masses of all the

reactants is equal to the masses of all the products. This law is known as the

law of conservation of mass.

There was rapid progress in science after chemists began accurate determination

of masses of reactants and products. French chemist Claude Berthollet and

Joseph Proust worked on the ratio (by mass) of two elements which combine

to form a compound. Through a careful work, Proust demonstrated the

fundamental law of definite or constant proportions in 1808. In a given

chemical compound, the proportions by mass of the elements that compose

it are fixed, independent of the origin of the compound or its mode of

preparation.

In pure water, for instance, the ratio of mass of hydrogen to the mass of oxygen

is always 1:8 irrespective of the source of water. In other words, pure water

contains 11.11% of hydrogen and 88.89% of oxygen by mass whether water

is obtained from well, river or from a pond. Thus, if 9.0 g of water are

decomposed, 1.0 g of hydrogen and 8.0 g of oxygen are always obtained.

Furthermore, if 3.0 g of hydrogen are mixed with 8.0 g of oxygen and the mixture

is ignited, 9.0 g of water are formed and 2.0 g of hydrogen remains unreacted.

Similarly sodium chloride contains 60.66% of chlorine and 39.34% of sodium

by mass whether we obtained it from salt mines or by crytallising it from water

of ocean or inland salt seas or synthesizing it from its elements sodium and

chlorine. Of course, the key word in this sentence is ‘pure’. Reproducible

experimental results are highlights of scientific thoughts. In fact modern science

is based on experimental findings. Reproducible results indirectly hint for a

truth which is hidden. Scientists always worked for findings this truth and in

this manner many theories and laws were discovered. This search for truth plays

an important role in the development of science.

The Dalton’s atomic theory not only explained the laws of conservations of mass

and law of constant proportions but also predicted the new ones. He deduced

the law of multiple proportions on the basis of his theory. The law states that

when two elements form more than one compound, the masses of one

element in these compound for a fixed mass of the other element are in

the ratio of small whole numbers. For example, carbon and oxygen form two

compounds: carbon monoxide and carbon dioxide. Carbon monoxide contains

1.3321 g of oxygen for each 1.0000 g of carbon, whereas carbon dioxide

contains 2.6642 g of oxygen for 1.0000 g of carbon. In other words, carbon

dioxide contains twice the mass of oxygen as is contained in carbon monoxide

(2.6642 g = 2 × 1.3321 g) for a given mass of carbon. Atomic theory explains

this by saying that carbon dioxide contains twice as many oxygen atoms for a

given number of carbon atoms as does carbon monoxide. The deduction of law

of multiple proportions from atomic theory was important in convincing

chemists of the validity of the theory.

1.4 DALTON’S ATOMIC THEORY

As we learnt earlier, Lavosier laid the experimental foundation of modern

chemistry. But the British chemist John Dalton (1766–1844) provided the basic

theory; all matter – whether element, compound, or mixture –is composed of

small particles called atoms. The postulates, or basic assumptions of Dalton's

theory are presented below in this section.

1.4.1 Postulates of Dalton's Atomic Theory

The English scientist John Dalton was by no means the first person to propose

the existence of atoms, as we have seen in the previous section, such ideas date

back to classical times. Dalton’s major contribution was to arrange those ideas

in proper order and give evidence for the existence of atoms. He showed that

the mass relationship expressed by Lavoisier and Proust (in the form of law of

conservation of mass and law of constant proportions) could be interpreted most

suitably by postulating the existence of atoms of the various elements.

In 1803, Dalton published a new system of chemical philosophy in which the

following statements comprise the atomic theory of matter:

1. Matter consists of indivisible atoms.

2. All the atoms of a given chemical element are identical in mass and in all

other properties.

3. Different chemical elements have different kinds of atoms and in particular

such atoms have different masses.

4. Atoms are indestructible and retain their identity in chemical reactions.

5. The formation of a compound from its elements occurs through the

combination of atoms of unlike elements in small whole number ratio.

Dalton’s fourth postulate is clearly related to the law of conservation of mass.

Every atom of an element has a definite mass. Also in a chemical reaction there

is rearrangement of atoms. Therefore after the reaction, mass of the product

should remain the same. The fifth postulate is an attempt to explain the law of

definite proportions. A compound is a type of matter containing the atoms of

two or more elements in small whole number ratio. Because the atoms have

definite mass, the compound must have the elements in definite proportions by

mass.

The Dalton’s atomic theory not only explained the laws of conservations of mass

and law of constant proportions but also predicted the new ones. He deduced

the law of multiple proportions on the basis of his theory. The law states that

when two elements form more than one compound, the masses of one

element in these compound for a fixed mass of the other element are in

the ratio of small whole numbers. For example, carbon and oxygen form two

compounds: Carbon monoxide and carbon dioxide. Carbon monoxide contains

1.3321 g of oxygen for each 1.000g of carbon, whereas carbon dioxide contains

2.6642 g of oxygen for 1.0000 g of carbon. In other words, carbon dioxide

contains twice the mass of oxygen as is contained in carbon monoxide (2.6642

g = 2 × 1.3321 g) for a given mass of carbon. Atomic theory explains this by

saying that carbon dioxide contains twice as many oxygen atoms for a given

number of carbon atoms as does carbon monoxide. The deduction of law of

multiple proportions from atomic theory was important in convincing chemists

of the validity of the theory.

1.4.2 What is an Atom?

As you have just seen in the previous section that an atom is the smallest particle

of an element that retains its (elements) chemical properties. An atom of one

element is different in size and mass from the atoms of the other elements. These

atoms were considered ‘indivisible’ by Indian and Greek ‘Philosophers’ in the

beginning and the name ‘atom’ was given as mentioned earlier. Today, we know

that atoms are not indivisible. They can be broken down into still smaller particles

although they lose their chemical identity in this process. But inspite of all these

developments atom still remains a building block of matter.

1.4.3 Molecules

A molecule is an aggregate of at least two atoms in a definite arrangement

held together by chemical forces (also called chemical bonds). It is smallest

particle of matter, an element or a compound, which can exist independently.

A molecule may contain atoms of the same element or atoms of two or more

elements joined in a fixed ratio, in accordance with the law of definite

proportions stated. Thus, a molecule is not necessarily a compound, which, by

definition, is made up of two or more elements. Hydrogen gas, for example,

is a pure element, but it consists of molecules made up of two H atoms each.

Water, on the other hand, is a molecular compound that contains hydrogen and

oxygen in a ratio of two H atoms and one O atom. Like atoms, molecules are

electrically neutral.

The hydrogen molecule, symbolized as H2, is called a diatomic molecule

because it contains only two atoms. Other elements that normally exist as

diatomic molecules are nitrogen (N2) and oxygen (O2), as well as the Group

17 elements-fluorine (F2), chlorine (Cl2), bromine (Br2), and iodine (I2). Of

course, a diatomic molecule can contain atoms of different elements. Examples

are hydrogen chloride (HCl) and carbon monoxide (CO).

The vast majority of molecules contain more than two atoms. They can be atoms

of the same element, as in ozone (O3), which is made up of three atoms of

oxygen, or they can be combinations of two or more different elements.

Molecules containing more than two atoms are called polyatomic molecules.

Like ozone, water (H2O) and ammonia (NH3) are polyatomic molecules.

1.4.4 Elements

Substances can be either elements or compounds. An element is a substance

that cannot be separated into simpler substances by chemical means. To date,

118 elements have been positively identified. Eighty-three of them occur

naturally on Earth. The others have been created by scientists via nuclear

processes.

For convenience, chemists use symbols of one or two, letters to represent the

elements. The first letter of a symbol is always capitalized, but the following

letter is not. For example, Co is the symbol for the element cobalt, whereas CO

is the formula for the carbon monoxide molecule. Table 1.l shows the names

and symbols of some of the more common elements; a complete list of the

elements and their symbols appears inside the front cover of this book. The

symbols of some elements are derived from their Latin names for example, Au

from auram (gold), Fe from ferrurn. (iron), and Na from natrium (sodium) while

most of them come from their English names.

Chemists use chemical formulas to express the composition of molecules and

ionic compounds in terms of chemical symbols. By composition we mean not

only the elements present but also the ratios in which the atoms are combined.

INTEXT QUESTIONS 1.1

1. Chemistry plays a vital role in many areas of science and technology. What

are those areas?

2. Who proposed the particulate nature of matter?

3. What is law of conservation of mass?

4. What is an atom?

5. What is a molecule?

6. Why is the symbol of sodium Na?

7. How is an element different from a compound?

1.5 SI UNITS (REVISITED)

Measurement is needed in every walk of life. As you know that for every

measurement a ‘unit’ or a ‘reference standard’ is required. In different countries,

different systems of units gradually developed. This created difficulties whenever

people of one country had to deal with those of another country. Since scientists

had to often use each other’s data, they faced a lot of difficulties. For a practical

use, data had to be first converted into local units and then only it could be used.

In 1960, the ‘General Conference of Weights and Measures’, the international

authority on units proposed a new system which was based upon the metric system.

This system is called the ‘International System of Units’ which is abbreviated as

SI units from its French name, Le Système Internationale d’Unitès. You have

learned about SI units in your earlier classes also and know that they are based

upon seven base units corresponding to seven base physical quantities. Units

needed for various other physical quantities can be derived from these base SI

units. The seven base SI units are listed in Table 1.2.

1.6 RELATIONSHIP BETWEEN MASS AND NUMBER

OF PARTICLES

Suppose you want to purchase 500 screws. How, do you think, the shopkeeper

would give you the desired quantity? By counting the screws individually? No,

he would give the screws by weight because it will take a lot of time to count

them. If each screw weighs 0.8 g, he would weigh 400 g screws because it is the

mass of 500 screws (0.8 × 500 = 400 g). You will be surprised to note that the

Reserve Bank of India gives the desired number of coins by weight and not by

counting.This process of counting by weighing becomes more and more labour

saving as the number of items to be counted becomes large. We can carry out the

reverse process also. Suppose we take 5000 very tiny springs (used in watches)

and weigh them. If the mass of these springs is found to be 1.5 g, we can conclude

that mass of each spring is 1.5 ÷ 5000 = 3 × 10–4 g.

Thus, we see that mass and number of identical objects or particles are interrelated.

Since atoms and molecules are extremely tiny particles it is impossible to

weigh or count them individually. Therefore we need a relationship between the

mass and number of atoms and molecules (particles). Such a relationship is

provided by ‘mole concept’.

1.7 MOLE – A NUMBER UNIT

Mass of an atom or a molecule is an important property. However, while discussing

the quantitative aspects of a chemical reaction, the number of reacting atoms or

molecules is more significant than their masses.ACTIVITY

It is observed experimently that iron and sulphur do not react with each other in

a simple mass ratio. When taken in 1:1 ratio by mass (Fe:S), some sulphur is left

unreacted and when taken in 2:1 ratio by mass (Fe:S) some iron is left unreacted.

Let us now write the chemical equation of this reaction

Fe + S → FeS

From the above chemical equation, it is clear that 1 atom of iron reacts with 1

atom of sulphur to form 1 molecule of iron (II) sulphide (FeS). It means that if

we had taken equal number of atoms of iron and sulphur, both of them would

have reacted completely. Thus we may conclude that substances react in a simple

ratio by number of atoms or molecules.

From the above discussion it is clear that the number of atoms or molecules of a

substance is more relevant than their masses. In order to express their number we

need a number unit. One commonly used number unit is ‘dozen’, which, as you

know, means a collection of 12. Other number units that we use are ‘score’ (20)

and ‘gross’(144 or 12 dozens). These units are useful in dealing with small numbers

only. The atoms and molecules are so small that even in the minute sample of any

substance, their number is extremely large. For example, a tiny dust particle

contains about 1016 molecules. In chemistry such large numbers are commonly

represented by a unit known as mole. Its symbol is ‘mol’ and it is defined as.

A mole is the amount of a substance that contains as many elementary entities

(atoms, molecules or other particles) as there are atoms in exactly 0.012 kg

or 12 g of the carbon-12 isotope.

The term mole has been derived from the Latin word ‘moles’ which means

a ‘heap’ or a ‘pile’. It was first used by the famous chemist Wilhelm Ostwald

more than a hundred years ago.

Here you should remember that one mole always contains the same number of

entities, no matter what the substance is. Thus mole is a number unit for dealing

with elementary entities such as atoms, molecules, formula units, electrons etc.,

just as dozen is a number unit for dealing with bananas or oranges. In the next

section you will learn more about this number.

1.8 AVOGADRO’S CONSTANT

In the previous section we have learned that a mole of a substance is that amount

which contains as many elementary entities as there are atoms in exactly 0.012

kilogram or 12 gram of the carbon-12 isotope. This definition gives us a method

by which we can find out the amount of a substance (in moles) if we know the

number of elementary entities present in it or vice versa. Now the question arises

how many atoms are there in exactly 12 g of carbon-12. This number is determined

experimentally and its currently accepted value is 6.022045 × 1023. Thus 1 mol =

6.022045 × 1023 entities or particles, or atoms or molecules.

For all practical purposes this number is . rounded off to 6.022 × 1023.

The basic idea of such a number was first conceived by an Italian scientist

Amedeo Avogadro. But, he never determined this number. It was

determinned later and is known as Avogadro’s constant in his honour.

This number was earlier known as Avogadro’s number. This number alongwith

the unit, that is, 6.022 × 1023 mol–1 is known as Avogadro constant. It is

represented by the symbol NA. Here you should be clear that mathematically a

number does not have a unit. Avogadro’s number 6.022 × 1023 will not have any

unit but Avogradro’s constant will have unit of mol–1. Thus Avogradro’s constant,

NA = 6.022 × 1023 mol–1.

Significance of Avogadro’s Constant

You know that 0.012 kg or 12 g of carbon –12 contains its one mole of carbon

atoms. A mole may be defined as the amount of a substance that contains 6.022 ×

1023 elementary entities like atoms, molecules or other particles. When we say

one mole of carbon –12, we mean 6.022 × 1023 atoms of carbon –12 whose mass

is exactly 12 g. This mass is called the molar mass of carbon-12. The molar mass

is defined as the mass ( in grams) of 1 mole of a substance. Similarly, a mole of

any substance would contain 6.022 × 1023 particles or elementary entities. The

nature of elementary entity, however,depends upon the nature of the substance as

given below :

S.No. Type of Substance Elementary Entity

1. Elements like Na, K, Cu which Atom

exist in atomic form

2. Elements like O, N, H, which Molecule

exist in molecular form (O2, N2, H2)

3. Molecular compounds like NH3, H2O, CH4 Molecule

4. Ions like Na+, Cu2+, Ag+, Cl–, O2– Ion

5. Ionic compounds like NaCl, NaNO3, K2SO4 Formula unit

Formula unit of a compound contains as many atoms or ions of different types

as is given by its chemical formula. The concept is applicable to all types of

compounds. The following examples would clarify the concept.

Formula Atoms/ions present in one formula unit

H2O Two atoms of H and one atom of O

NH3 One atom of N and three atoms of H

NaCl One Na+ ion and one Cl– ion

NaNO3 One Na+ ion and one NO3

– ion

K2SO4 Two K+ ions and one 2—

4 SO ion

Ba3(PO4)2 Three Ba2+ ions and two 3—

4 PO ions

Now, let us take the examples of different types of substances and correlate their

amounts and the number of elementary entities in them.

1 mole C = 6.022 × 1023 C atoms

1 mole O2 = 6.022 × 1023 O2 molecules

1 mole H2O = 6.022 × 1023 H2O molecules

1 mole NaCl = 6.022 × 1023 formula units of NaCl

1 mole Ba2+ ions = 6.022 × 1023 Ba2+ ions

We may choose to take amounts other than one mole and correlate them with

number of particles present with the help of relation :

Number of elementary entities = number of moles × Avogadro’s constant

1 mole O2 = 1 × (6.022 × 1023) = 6.022 × 1023 molecules of O2

0.5 mole O2 = 0.5 × (6.022 × 1023) = 3.011 × 10

23

molecules of O2

0.1 mole O2 = 0.1 × (6.022 × 1023) = 6.022 ×10

22

molecules of O2

INTEXT QUESTIONS 1.3

1. A sample of nitrogen gas consists of 4.22 × 1023 molecules of nitrogen.

How many moles of nitrogen gas are there?

2. In a metallic piece of magnesium, 8.46 × 1024 atoms are present. Calculate

the amount of magnesium in moles.

3. Calculate the number of Cl2 molecules and Cl atoms in 0.25 mol of Cl2 gas.