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 This presentation is created with the sole intention of
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3. Contents :
 Laws of Chemical Combination
 About Atoms
 About Molecules
 About Ions
 Writing Chemical Formulae
 Molecular Mass & Mole Concept
 Multiple Choice Questions
 Some Interesting Pictures

4. Laws of Chemical Composition
 The following two laws of chemical
combination were established after much
experiments by Lavoisier and Joseph L
Proust :
(i) Law of conservation of mass :
- Law of constant of mass states that
mass can neither be created nor
destroyed in a chemical reaction.

5. (ii) Law of Constant Proportions
- Lavoisier, along with other scientists,
noted that many compounds were
composed of two or more elements and
each such compound had the same
elements in the same proportions,
irrespective of where the compound came
from or who prepared it.

6. In a compound such as water, the ratio
of the mass of hydrogen to the mass of
oxygen is always 1:8, whatever the source
of water. Similarly in ammonia, nitrogen
and hydrogen are always present in the
ratio 14:3 by mass, whatever the method
or the source from which it is obtained.
This led to the law of constant
proportions which is also known as the
Law of Definite Proportions.

7. About Atoms
According to Dalton’s atomic theory, all
matter whether an element, a compound
or a mixture is composed of small particles
called Atoms.

8. The postulates of this theory may be
started as follows :
(i)All matter is made of very tiny particles
called Atoms.
(ii) Atoms are indivisible particles, which
cannot be created or destroyed in a
chemical reaction.
(iii) Atoms of a given element are identical in
mass and chemical properties.
(iv)Atoms of different elements have
different masses and chemical properties.

9. (v) Atoms combine in the ratio of small
whole numbers to form compounds.
(vi) The relative number and kinds of atoms
are constant in a given compound.

10.  The building blocks of all matter are atoms.
 Atoms are very small. They are smaller than
anything we can imagine or compare with.

11. ► Atomic radius is measured in nanometers.

12. Relative Sizes :

14. Modern Day Symbols of
Atoms of Different Elements

16. Symbols for Some Elements
 Aluminum : Al

17.  Argon : Ar

18.  Barium : Ba

19.  Boron : B

20.  Bromine : Br

21.  Calcium : Ca

22.  Carbon : C

23. Chlorine : Cl

24. Cobalt : Co

25. • Copper : Cu

26. ► Fluorine :F

27. ► Gold : Au

28.  Hydrogen : H

29.  Iodine : I

30.  Iron : Fe

31.  Lead : Pb

32.  Magnesium : Mg

33. • Neon : Ne

34.  Nitrogen : N

35.  Oxygen : O

36.  Potassium : K

37.  Silicon : Si

38.  Silver : Ag

39.  Sodium : Na

40. • Sulphur : S

41. ► Uranium :U

42.  Zinc : Zn

43.  Phosphorus : P

44.  Platinum : Pt

45.  Mercury : Hg

46. • Nickel : Ni

47.  Tin : Sn

48.  Bismuth : Bi

49.  Antimony : Sb

50.  Arsenic : As

51.  Manganese : Ma

52.  Tungsten : W

53.  Titanium : Ti

54. • Cerium : Ce

55.  Strontium : Sr

56.  Beryllium : Be

57.  Zirconium :

58.  In 1961 a university accepted atomic mass
unit, carbon-12 isotope was chosen as the
standard reference for measuring atomic
masses.

59.  One atomic mass unit is a mass unit equal
to exactly one twelfth the mass of one
atom of carbon-12.

60. Atomic Masses of Few Elements
Element Atomic Mass (u)
(i) Hydrogen 1
(ii) Carbon 12
(iii) Nitrogen 14
(iv) Oxygen 16
(v) Sodium 23
(vi) Magnesium 24

61. Element Atomic Mass (u)
(vii) Sulphur 32
(viii) Chlorine 35.5
(ix) Calcium 40

62.  Atoms of most elements are not able
to exist independently.

63.  Atoms form molecules and ions.
 These molecules or ions aggregate in
large numbers to form the matter that we
can see, feel or touch.

64. About Molecules
 A molecule is a general group of two or
more atoms that are chemically bonded
together, that is, tightly held together by
attractive forces.

65. A molecule can be defined as the smallest
particle of an element or a compound & is
capable of an independent existence and
shows all the properties of that substance.

66. Atoms of the same element or of different
elements can join together to form
molecules.

67. Molecules of Elements
• The molecules of an element are
constituted by the same type of atoms.

68. ► Moleculesof an element are constituted
by the same type of atoms.

69. ► Moleculesof many elements, such as
argon(Ar), helium(He),…etc. are made up
of only one atom of that element.
► But,
this is not the case with most of the
non-metals.

70.  For example, a molecule of oxygen consists of
two atoms of oxygen and hence it is known as
a diatomic module O2.
 If 3 atoms of oxygen unite into a molecule,
instead of the usual 2, we get ozone.

71.  The number of atoms constituting a
molecule is known as its Atomicity.

72. Atomicity of Some Elements
 Type of Element : Non – Metal
Name Atomicity
(i) Argon - Monoatomic
(ii) Helium - Monoatomic

73.  Type of Element : Non – Metal
Name Atomicity
(i) Oxygen Diatomic
(ii) Hydrogen Diatomic
(iii) Nitrogen Diatomic
(iv) Chlorine Diatomic

74.  Type of Element : Non – Metal
Name Atomicity
(i) Phosphorus Tetra - atomic

75.  Type of Element : Non – Metal
Name Atomicity
(i) Sulphur Poly-atomic

76.  Atoms of different elements join together
in definite proportions to form molecules
of compounds.

77. Molecules of Some Compounds
 Compound : Water
Combining Elements : Hydrogen, Oxygen
Ratio by Mass : 1:8

78.  Compound : Ammonia
Combining Elements : Nitrogen, Hydrogen
Ratio by Mass : 14:8

79.  Compound : Carbon Dioxide
Combining Elements : Carbon,
Oxygen
Ratio by mass : 3:8

80. About Ions
 Compounds composed of metals and
non metals contain charged species.
The charged species are known as
Ions.

81. An ion is a charged particle and can be
negatively or positively charged.

82. A negatively charged ion is called an
‘Anion’ and the positively charged ion is
called a ‘Cation’.

83. Some Ionic Compounds
• Ionic Compound : Calcium Oxide
Constituting Elements : Calcium &
Oxygen
Ratio by Mass : 5:2

84. ► IonicCompound : Magnesium Sulphide
Constituting Elements : Magnesium &
Sulphur
Ratio by Mass : 3:4

85. ► IonicSolution : Sodium Chloride
Constituting Elements : Sodium &
Chlorine
Ratio by Mass : 23:35.5

86. Molecular Mass & Mole Concept
 The molecular mass of a substance is the sum
of the atomic masses of all the atoms in a
molecule of the substance.

87.  It is therefore the relative mass of a
molecule expressed in atomic mass units
(u).

88.  The formula unit mass of a substance is a
sum of the atomic masses of all atoms in a
formula unit of a compound.

89.  Formula unit mass is calculated in the same
manner as we calculate the molecular mass.
 The only difference is that we use the word
formula unit for those substances whose
constituent particles are ions.

90.  EXAMPLE : Reaction of hydrogen and
oxygen to form water:
2H2 + O2  2H2O
The above reaction indicates that :
(i) Two molecules of hydrogen combine with
one molecule of oxygen to form two
molecules of water. Or,
(ii) 4 u of hydrogen molecules combine with
32 u of oxygen molecules to form 36 u of
water molecules.

91.  We can infer from the above equation that
the quantity of a substance can be
characterised by its mass or the number
of molecules.
 But, a chemical reaction equation indicates
directly the number of atoms or molecules
taking part in the reaction.

92.  Therefore, it is more convenient to refer to the
quantity of a substance in terms of the number
of its molecules or atoms, rather than their
masses.

93.  So, a new unit “Mole” was introduced.

94. Multiple Choice Questions
 Q 1 : A molecule does not signify
(a) Atomic mass unit
(b) 22.4 litres of a gas at STP
(c) Gram molecular mass
(d) None of the above

95. Answer :
Option :
(a) Atomic mass unit

96.  Q 2 : Which of the following does
not change when a chemical
reaction takes place ?
(a) Volume
(b) Mass
(c) Physical properties
(d) Chemical properties

97. Answer :
Option :
(b) Mass

98. Q 3 : Molecules of the following
element are made up of only one
atom of that element:
(a) Iron
(b) Sodium
(c) Helium
(d) Chlorine

99. Answer :
Option :
(c) Helium

100.  Q 4 : Law of definite proportion was
given by
(a) John Dalton
(b) Lavoisier
(c) Joseph Proust
(d) Ritcher

101. Answer :
Option :
(b) Lavoisier

102. Q 5 : Argon is
(a) Monoatomic
(b) Diatomic
(c) Triatomic
(d) Polyatomic

103. Answer :
Option :
(a) Monoatomic

104. Q 6 : Which of the following supports
Dalton's atomic theory of matter?
(a) Formation of isotopes
(b) Formation of isobars
(c) Law of multiple proportion
(d) Graphite and diamond though
made from carbon, show different
properties.

105. Answer :
Option :
(c) Law of Multiple Proportion

106.  Q 7 : Choose the odd molecule
(a) Argon molecule
(b) Chlorine molecule
(c) Oxygen Molecule
(d) Fluorine Molecule

107. Answer :
Option :
(a) Argon Molecule

108. • Q 8 : choose the element which will
not have a charged valency
(a) Calcium
(b) Argon
(c) Silicon
(d) Aluminium

109. Answer :
Option :
(b) Argon

110. Q 9 : Take out the odd concept
(a) Atomic mass unit
(b) Mole
(c) Avogadro
(d) Gram atomic mass

111. Answer :
Option :
(a) Atomic mass unit

112.  Q 10 : Which one of the following
elements give polyatomic molecules?
(a) Ne
(b) P
(c) Ni
(d) Si

113. Answer :
Option :
(b) P

114. Some Interesting Pictures

150. Thank You !!!
Compiled By :
Sudarshan.S.K.
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