Tuition Classes IX (CBSE) Module-I

CHAPTER - 03
ATOMS AND MOLECULES

Ancient Greek philosophers (Democritus and Leucippus) suggested that, if we go on dividing matter,
a stage will come when particles obtained cannot be divided further. Democritus called these indivisible
particles atoms (meaning indivisible). Several Indian and Greek philosophers tried to explain the except
of atoms in several ways but their ideas could not satisfy modern scientists. The first meaningful effort
in this directions was made by John Dalton (1804). By the end of the 18th centuary scientists recognist
the difference between elements and compounds and naturally became interested in finding out how
and why elements combine and what happens when they combine.

Antoine Lavoisier laid the foundation of chemical science by establishing two important laws of chemical
combination. Lavoisier is regarded as father of chemistry.

Laws of chemical combination

Chemistry deals mainly with the reactions. In 18th century, chemist found that all chemical reactions
always takes place in accordance to certain laws. These laws referred to as laws of chemical
combination.

1. Law of conservation of mass (Lavoisier) - 1774.

Law of conservation of mass states that mass can neither be created nor be destroyed in a chemical
reaction.

OR

During a chemical or physical change, the total mass of the product obtained is always equal to the
total mass of the reactants taken at the beginning of the change ie total mass is conserved in a
reaction. Matter can be changed from one form to another.

This law can easily be understood by considering following examples :

(i) When calcium carbonate (CaCO3) is strongly heated, it decomposes to give CaO and CO2. If
100 g CaCO3 are strongly heated, we get 56 g CaO and 44 g CO2. The total mass of the product
(56 + 44 = 100 g) is exactly the same as the mass of CaCO3 taken in the beginning of the reaction.

Heat
CaCO3 ¾¾¾ ® CaO+ CO2
100 g 56 g 44 g

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2. Law of constant propotion (Joseph Proust) - 1799

Law of constant/definite propotion states that in a chemical substance the elements are always present
in definite proportions by mass.
For example, water (H2O) is composed of two elements hydrogen and oxygen in the ratio 1 : 8 by
mass, what ever the source of water.
The carbon dioxide contains only two elements carbon and oxygen combined together in the ratio 3 :
8 by mass.
DALTON’S ATOMIC THEORY - 1804
Laws of chemical combinations suggest that, the matter is composed of descrete particles which
donot break up during a chemical reaction. On the basis of these laws and the work of Greek
philosophers. Dalton proposed a very important theory known as Daltons atomic theory.
The basic postulates of this theory are the follows
1. All matter is made up of very tiny particles called atoms, which participate in chemical reactions
2. Atoms are indivisible particles which cannot be created or destroyed in a chemical reaction
3. Atoms of a given element are identical in mass and chemical properties
4. Atoms of the different elements have different masses and chemical properties
5. Atoms combine in the ratio of small whole numbers to form compounds
6. The relative number and kinds of atoms are constant in a given compound
7. Atom in the unit of a chemical reaction ie it is the smallest particles take part in the chemical
reaction.
ATOM
Atom is the smallest particle of the element and is the unit of chemical reaction. It may or may not
have independent existance.
Atoms of the element like hydrogen, oxygen etc cannot exist independently. Atoms of the element like
helium Argon, Xenon etc posses independent existance.
Atoms are very small, they are smaller than anything that are can imagine.
Atomic radius is measured in nanometers
1 nm = 10–9 m
1 m = 109 nm
SYMBOLS OF ATOMS OF DIFFERENT ELEMENT
Dalton was the first scientist to use the symbols for elements in a very specific sense.

eg : - Hydrogen - Sulphur

- Oxygen C - Copper
In the beginning, the name of the elements, were derived from the name of the place where they were
found for the first time eg : copper were taken from Cyprus. Some name were taken from specific
colums : eg Gold was taken from the English word meaning yellow.
Now a days IUPAC (International union of pure and applied chemistry) approves name of the
elements, symbols, and units. Many of the symbols are first one or two letters of the elements name
in English. First letter of the symbol is always written as a capital letter and second letter as small
letter.
eg : Hydrogen - H Aluminium - Al

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Cobalt - Co
Symbols of some elements are formed from first letter of the name and a letter appearing later in the
name.
Eg : Zinc - Zn Chlorine - Cl
Other symbols have been taken from the names of elements in Latin, German or Greek.
Eg : The symbol of iron is Fe from its Latin name ferrum, sodium is Na from natrium, potassium is K
from kalium.
Symbols of some elements
Aluminium - Al Copper - Cu
Arger - Ar Gold - Au
Barium - Ba Hydrogen - H
Bromine - Br Iron - Fe
Carbon - C Lead - Pb
Cobalt - Co Oxygen - O
ATOMIC MASS
Since determining the mass of individual atom was a relatively difficult task, relative atomic masses
were determined using the laws of chemical combinations and the compounds formed.
In 1961, C - 12 (the most stable isotope of carbon was adopted by international union of chemists to
calculate the relative atomic mass of the elements. The new scale adopted is called atomic mass unit
scale. On this scale, the relative atomic mass of an element is expressed in terms of atomic mass
unit (abbreviated as amu), now it is written as u - unified mass.
th
One atomic mass unit (1 amu) is defined as the mass equal to 1 mass of an atom of C - 12. The
12
relative atomic masses of all elements have been found with respect to an atom of C - 12.

1
1 amu = ´ mass of carbon - 12 atom = 1.66 ´ 10-23 g
12
The relative atomic mass of an element is the number of times the atomic mass of an element is
1
heavier than th mass of C - 12 atom.
12

1
Eg 1 : Mass of one atom of oxygen is 16 time heavier than th mass of C - 12 atom.
12
Therefore relative atomic mass of oxygen is 16.

1
Eg 2 : Mass of one atom of nitrogen is 14 times heavier than th mass of C - 12 atom.
12
Therefore relative atomic mass of nitrogen is 14.
On atomic mass unit scale, atomic mass of oxygen is 16 u and that of nitrogen 14 u.
Similarly, the relative atomic mass of the atom of an element is defined as the average mass of the
1
atom, as compared to th mass of one carbon atom.
12

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Element Atomic mass (u)

Hydrogen 1
Carbon 12
Nitrogen 14
Oxygen 16
Magnesium 24
Chlorine 35.5
EXISTANCE OF ATOM
Atoms of the most element are not able to exist independently. Atoms form molecules and ions.
These molecules and ions aggregate in large numbers to form the matter.
MOLECULES (Term molecule was first used by Avogadro)
A molecule in general a group of two or more atoms that are chemically bonded together by attractive
force.
A molecule is defined as the smallest particle of the substance which can exist independently and
show all the properties of that substance.
It can be subdivided as
1) Homoatomic molecules
Homoatomic molecules are the molecules of same element. Homoatomic molecules can be further
divided into monoatomic, diatomic and polyatomic molecules depending on the number of atoms.
Monoatomic molecules - He, Ne, Ar
Diatomic molecule - O2, N2, H2
Polyatomic molecules - P4, S8
2) Heteroatomic molecules
Heteroatomic molecules are molecules of compound or contain atoms of the different element. They
can be diatomic or polyatomic.
Eg : NO, HCl, H2SO4, PCl5
Molecules of the element
The molecules of an element constituted by the same type of atoms.
Monoatomic molecule - He, Ne, Ar, Kr
Diatomic molecules - N2, O2, H2
Triatomic molecule - O3
The number of atoms constituting a molecule is known as its atomicity.
Metal and some elements such as carbon do not have simple structure but consist of a very large and
indefinite number of atoms bonded together.

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Element Atomicity
Ar Monoatomic
He Monoatomic
Hydrogen (H2) Diatomic
Chlorine (Cl2) Diatomic
Phosphorus (P4) Tetraatomic
Molecules of compounds
Atoms of the different element join together in definite propotion to form molecules of the compound
Compound Elements combining Rate by mass
Water (H2O) Hydrogen, oxygen 1:8
Carbon dioxide (CO2) Carbon, Oxygen 3:8
IONS
An atom or group of atoms that carries positive or negative charge as a result of loss or gain of one or
more electrons. Compounds composed metals and non metals contain charged species. There are
two types of ions.
1. Cations
A positively charged ions are called cations.
eg : Na+, Mg2+, Ca2+, NH+4
2. Anions
A negatively charged ion are called anions

eg : Cl- ,Br - ,O2- ,SO24- ,PO34- ,CO32-

A group of atoms carrying charge is known as polyatomic ion. Consider sodium chloride (NaCl), its
constituents are positively charged sodium ion (Na+) and negatively charged chloride ions (Cl–)
Writing chemical formula
Chemical formula of a compound is a symbolic representation of its composition. The chemical formula
of different compounds can be written easily. For this we need to learn symbol and combining capacity
of the elements. The combining capacity of an element is known as its valency. The valencies of
some common ions are given in following table.

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Rules for writing chemical formula

1. The valencies or charges on the ion must balance.
2. When a compound consists of a metal and non-metal, the name or symbol of the metal is written
first.
eg : Calcium oxide (CaO)
Sodium chloride ( NaCl)
Copper oxide (CuO)
3. In compounds formed with polyatomic ions, the number of ions present in the compound is indicated
by enclosing the formula of the ion in the bracket and writting the number of ions outside the bracket
eg : Mg(OH)2
FORMULAE OF SIMPLE COMPOUNDS
While writting the chemical formulae for compounds, we write the constituent elements and their
valencies. Then we must crossover the valencies of the combining atoms.
eg :
1) Formula of hydrogen chloride

Symbol H Cl

Valency 1 1
Formula of the compound = HCl
2) Formula of hydrogen sulphide

Symbol H S

Valency 1 2
Formula of the compound = H2S
3) Formula of aluminium oxide

Symbol Al O

Valency 3 2
Formula of the compound = Al2O3
4) Formula of calcium oxide

Symbol Ca O

Valency 2 2
Formula of the compound = CaO
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Molecular mass

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule
of the substance. It is therefore the relative mass of a molecule expressed in atomic mass unit (u).

Eg : Relative molecular mass of HNO3

The molecular mass of HNO3 = The atomic mass of H + Atomic mass of N + 3 × Atomic mass of
oxygen

= 1 × 1 + 1 × 14 + 3 × 16 = 63 u

FORMULA UNIT MASS

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of
a compound. Formula unit mass is calculated in the same way as we calculate the molecular mass.
The only difference is that we use the word formula unit for those substance whose constituent particles
are ions.

Eg : Formula unit mass of NaCl = 1 × 23 + 1 × 35.5 = 58.5 u

MOLE CONCEPT (The word ‘mole’ was introduced by Wilhelm Ostwald)

One mole is defined as the amount of substance contains 6.022 × 1023 particles. Mole is the counting
unit in chemistry. Mole is the SI unit of amount of substance. Its symbol is ‘mol’.

The number 6.022 × 1023 is called Avogadro number and is represented by NA.

The amount of substance (symbol, n) of a system is a measure of the number of specified elementary
entities. An elementary entity may be atoms, molecules, an ion, an electron etc.

The mass of 1 mole of a substance is equal to its relative atomic mass or molecular mass in grams.
The atomic mass of an element gives in the mass of one atom of that element in atomic mass unit (u).
To get mass of one mole of a atom of that element (molar mass) we have to take the same numerical
value but change the unit from u to g.

Molar mass (mass 1 mole of atom) of atom is also known as gram atomic mass.

eg :

Atomic mass of hydrogen is = 1 u

Gram atomic mass of hydrogen = 1g

1 u hydrogen has only 1 atom of hydrogen.

1 g hydrogen contain 6.022 × 1023 / 1 mol of atoms of hydrogen

Molecular mass of water = 18 u

Gram molecular mass of water = 18 g

18 u water has only one molecules of water

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18 g water has 1 mol of molecules of water is 6.022 × 1023 molecules of water

1 mole = 6.022 × 1023 number = Relative mass in gram

1 mole carbon atoms = 6.022 × 1023 atoms of C = 12 g of carbon atoms

1 mol H2O molecule = 6.022 × 1023 molecule of H2O = 18 g of water

Given mass in gram

Number of moles =
Molar mass

m
n=
M
n = Number of moles m = Given mass in gram
M = Molar mass

Number of particles Atoms / molecule

Number of moles =
6.022 ´ 1023 NA

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QUESTIONS
1. A balanced chemical equation is in accordance with
A) Avogadros law
B) Law of constant proportion
C) Law of conservation of mass
D) Law of definite proportion
2. Two elements X and Y combine in gaseous state to form XY in the ratio 1 : 35.5 by mass. The mass
of Y that will be required to react with 2 g of X is
A) 7.1 g B) 3.55 g C) 46 g D) 71 g
3. Which of the following is not correct postulates of Dalton’s atomic theory ?
A) All matter is made up of tiny particles called atom
B) Atoms of the given element are identical in mass
C) Atoms of the different elements have same masses and same chemical properties
D) Atoms combine in the ratio of small whole numbers to form compounds
4. A sample of NH3 molecule irrespective of source contains 82.35 % nitrogen and 17.65 % of hydrogen
by mass. This data supports
A) Law of conservation of mass B) Law of multiple proportion
C) Avogadros law D) Law of definite proportion
5. Molecular mass is defined as the
A) Mass of one molecule of any substance compared with the mass of one atom of C - 12
B) Mass of one atom compared with the mass of one atom of hydrogen
C) Mass of one atom compared with mass of molecule
D) Mass of one molecule of compared with mass of one atom of nitrogen
6. Which of the following is not an element :
A) Diamond B) Graphite
C) Silica D) Ozone

th
7. An atom is 20 times heavier than 1 mass of the C - 12 atom. The atomic mass is
12
A) 10 B) 20 C) 240 D) 1.66
8. Which of the following statement is not true about an atom ?
A) Atoms are the basic units from which molecules and ions are formed
B) Atoms are always neutral in nature
C) Atoms of most elements are not able to exist independently
D) Atoms aggregate in large number to form matter that we can see, feel or touch
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9. An element X is divalent and another element Y is tetravalent. The compound formed by these two
element will be

A) XY B) XY2 C) XY4 D) X2Y

10. Which of the following is a wrong combination ?

A) 6.022 × 1023 molecule of oxygen = 32 g of oxygen

B) 6.022 × 1023 ions of sodium = 23 g of sodium

C) 6.022 × 1023 atom of carbon = 24 g of carbon

D) 6.022 × 1023 atom of hydrogen = 1 g of hydrogen atoms

11. Number of moles present in 42 g of nitrogen atoms are

A) 1 mole B) 1.5 mole

C) 2 mole D) 4.5 mole

12. What mass of carbon dioxide (CO2) will contain 3.011 × 1023 molecules ?

A) 11.0 g B) 22.0 g

C) 44.0 g D) 2.2 g

13. The formula of a chloride of a metal M is MCl2, the formula of the phosphate of metal will be

A) MPO4 B) M3(PO4)2

C) M2(PO4)3 D) M2PO4

14. Which of the following contains the largest number of molecules ?

A) 0.2 g H2 B) 0.8 g O2

C) 0.14 g N2 D) 0.9 g H2O

15. The molecular mass of X is 106. X can be

A) CaCO3 B) SO2

C) Na2CO3 D) NaCl

16. 1 U or 1 amu means

th
A) 1 mass of C - 12 atom
12

B) Mass of C - 12 atom

C) Mass of O - 16 atom

D) Mass of hydrogen molecule

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17. The percentage of hydrogen in H2O is

A) 8.88 % B) 11.12 %

C) 20.60 % D) 80.0 %

18. When an atom loses electrons, it is called a (an) ––––––––– and has a –––––––– change

A) Anion, positive B) Cation, positive

C) Cation, negative D) Anion, negative

19. Mass of 9.033 × 1023 methane (CH4) is

A) 24 g B) 16 g C) 32 g D) 48 g

20. The formula of the nitride of metal X is XN, the formula of metal sulphate is

A) X2(SO4)3 B) X3(SO4)2 C) X2SO4 D) X4(SO4)3

21. The molecular formula of magnesium nitrate

A) MgNO3 B) Mg2(NO3)3 C) Mg(NO3)2 D) Mg3(NO3)2

22. Which of the following represent a correct chemical formula ?

A) CaCl B) Na3(SO4)2 C) Na2S D) Al3O2

23. Which of the following pair of atoms of the elements represent a mole ratio of 1 : 1 ?

A) 20 g sodium and 20 g of calcium

B) 7 g of nitrogen and 12 g of sodium

C) 10 g of calcium and 6 g of carbon

D) 28g of nitrogen and 24 g of magnesium

24. A student heats 25 g of reactant A with 50 g of reactant B. He obtains 50 g of product C and recover
25 g unreacted B. Which of the following law is confirmed in the following reaction

A) Law of constant proportion

B) Law of conservation of mass

C) Law of conservation of mass and law of constant proportion

D) Law of multiple proportion

25. Number of atoms in 46 g Na is

A) 6.022 × 1023 B) 12.044 × 1023

C) 9.033 × 1023 D) 3.011 × 1024

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26. Number of moles of oxygen atoms in two moles of nitric acid is

A) 2 B) 4 C) 8 D) 6

27. Calculate the formula unit mass of MgCl2 ?

A) 36 B) 83 C) 59.5 D) 95

28. Which of the following does not represent molar mass of a substance ?

A) 1 mole of HCl B) 6.022 × 1023 molecule of He

C) 16 g of O2 D) 44 g of CO2

29. Which of the following does not change when a chemical reaction takes place ?

A) Volume B) Physical properties

C) Mass D) Chemical properties

30. The number of atoms present in 0.5 mole of nitrogen atoms is same as in
A) 24 g of Mg B) 32 g of S
C) 12 g of C D) 8 g of O

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