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History of Atoms
The idea of divisibility of matter was
considered long back in India, around 500
BC. An Indian philosopher Maharishi Kanad,
postulated that if we go on dividing matter
(padarth), we shall get smaller and smaller
elaborated this doctrine
and said that these particles
normally exist in a combined
form which gives us various
forms of matter.
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
Father of Modern Chemistry: Antoine L.
Lavoisier (26 August 1743 –
8 May1794)was a French
nobleman and chemist centr
al to the 18thcentury Chemical
Revolution and a large
influence on both
the histories of chemistry
and biology. Antoine L.
Lavoiser laid the foundation
of chemical sciences by
establishing two important
laws of chemical
Laws of Chemical
There are two laws of chemical
Law of Conservation of Mass
Law of Constant Proportions
Law of Conservation of Mass
Antoine L. Lavoisier, a French scientist,
established the theory of Law of
Conservation of Mass.
The law of conservation of mass states,
“Mass can neither be created nor
destroyed in a chemical reaction”.
Explanation of the Law:
According to this law mass of an isolated
system will remain constant over time.
This means when mass is enclosed in a
system and none is allowed in or out, its
quantity will never change. That is mass
will be conserved, and hence this is
called Law of Conservation of Mass. This
means total mass of products is always
equal to the total mass of reactants. As
there is no loss of mass of substances,
i.e. mass is conserved, that‟s why
Lavoisier called this the law of
conservation of mass.
Reactants and Products:
In a chemical reaction
The substances that combine or react are
known as reactants.
The new substance/substances formed
are called product or products.
When calcium oxide is dissolved in water
calcium hydroxide is formed. The reaction
involved in this can be written as:
In this reaction calcium oxide and water
are reactants while calcium hydroxide is
In this reaction 74 g of calcium hydroxide
is obtained when 56 g of calcium oxide
reacts with 18 g of water, which is proved
Here the total mass of reactants, i.e.
calcium oxide and water is equal to 74
g. And the mass of product, i.e.
calcium hydroxide is also equal to
74g. This proves that the total mass of
reactants is always equal to the total
mass of product, which proves the
Law of Conservation of Mass.
Law of Constant Proportions
Law of Constant Proportion states that a
chemical compound always contains
exactly the same proportion of
elements by mass.
This law is also known as Law of definite
proportions. Joseph Louis Proust gave this
law hence, this law is also known as
Explanation of the law
Compounds are formed by the
combination of two or more elements.
In a compound the ratio of the atoms
or element by mass remains always
same irrespective of the source of
compound. This means a certain
compound always formed by the
combination of atoms in same ratio by
mass. If the ratio of mass of
constituent atoms will be altered the
new compound is formed.
In a water molecule (H20)
Hydrogen : Oxygen= 1:8
In Ammonia ( NH3)
Nitrogen : Oxygen = 14:3
In Common Salt (NaCl)
Sodium: Chlorine= 23: 35.5
In Carbon dioxide (CO2)
Carbon: Oxygen = 3:8
In Calcium Oxide (CaO)
Calcium: Oxygen= 5:2
In Magnesium Sulphide (MgS)
Magnesium: Sulphur = 3:4
John Dalton (6
September 1766 – 27
July 1844) was an
English chemist, mete
orologist and physicist
. He is best known for
his pioneering work in
the development of
modern atomic theory,
and his research
into colour blindness.
Dalton’s Atomic Theory
John Dalton, a British Chemists and
scientists gave the Atomic Theory in
1808. This theory is popularly known
as Dalton‟s Atomic Theory in the
honor of John Dalton. He gave the
theory on the basis of Laws of
Chemical Combination and explains
them properly. In his theory he
explains about atom.
Main postulates of Dalton's
Elements are made of extremely small
particles called atoms.
Atoms of a given element are identical in
size, mass, and other properties;
Atoms of different elements differ in size,
mass, and other properties.
Atoms cannot be subdivided, created, or
Atoms of different elements combine in
simple whole-number ratios to form chemical
In chemical reactions, atoms are combined,
separated, or rearranged.
What is an Atom?
Atom can be defined as the
smallest particle of matter that
takes part in a chemical
How big are Atoms?
Atoms are very small, they are
smaller than anything that we
can imagine or compare with.
Characteristics of atoms:
Atom is the smallest particle of matter.
All elements are made of tiny particles called
Atoms are very small in size and cannot be seen
through naked eyes.
Atom does not exist in free-state in nature. But
atom takes part in a chemical reaction.
The properties of a matter depend upon the
characteristics of atoms.
Atoms are the building block of an element
similar to a brick which combine together to
make a building.
The size of atoms is indicated by its radius.
In ancient time atoms was considered indivisible.
Symbols of Atoms of Different
Symbols of some elements are formed
from the first letter of the name and a
appearing later in the name.
E.g.: (i) chlorine: Cl, (ii) zinc: Zn etc
Some have been taken from the
names of elements in Latin, German or
E.g.: s Iron is Fe from ferrum,
Sodium is Na from natrium,
Potassium is K from kalium
Mass of atom is called atomic mass. Since,
atoms are very small consequently actual
mass of an atom is very small. For example
the actual mass of one atom of hydrogen is
equal to 1.673 x 10-24 g. This is equal to
To deal with such small number is very
difficult. Thus for convenience relative
atomic mass is used.
Existence of Atoms
Atoms of most of the elements exist in
the form of molecule or ion, since they
are most reactive. For example,
hydrogen, oxygen, chlorine, etc.
However, atoms of some elements,
which are non-reactive, exist in free-state
in nature. For example helium, neon,
Usually atoms are exist in following two
forms 1. In the form of molecules
2. In the form of ions
A molecule is a group of two or more
atoms that are chemically bonded
together, that is, tightly held together by
Mono-atomic: When molecule is formed by
single atom only, it is called mono-atomic
molecule. Generally noble gases form monoatomic molecules. For example: Helium (He),
Di-atomic: When molecule is formed by the
combination of two atoms of it is called diatomic
molecule. For example: Hydrogen (H2), Oxygen
(O2), Nitrogen (N2), Chlorine (Cl2), etc.
Tri-atomic: When molecule is formed by the
combination of three atoms it is called tri-atomic
molecule. For example: molecule of ozone (O3)
Tetra-atomic:- When molecule is formed
by the combination of four atoms it is
called tetra-atomic molecule. For example:
Phosphorous molecule (P4)
Polyatomic:- When molecule is formed by
the combination of more than two atoms, it
is called polyatomic molecule. For
example: Sulphur molecule (S8)
All metals are mono-atomic.
All gases are di-atomic.
Molecules of Elements
The molecules of an element are
constituted by the same type of atoms.
e.g.: O + O→ O2 (Oxygen Gas) „diatomic‟
O + O + O→ O3 (Ozone) „tri-atomic‟
4P→ P4 (Phosphorous) „tetraatomic‟
8S→ S8 (Sulphur) „poly-atomic‟
Molecules of Compound
Atoms of different elements join together in
definite proportions to form molecules of
e.g.: Ca + O → CaO (Calcium Oxide) „diatomic‟
2H + O → H20 (Water) „tri-atomic‟
N + 3H → NH3 (Ammonia) „tetra-atomic‟
2K + S + 4O → K2SO4 (Potassium
Particles carrying positive or negative
charges are called ions.
Types of ions:
1. Cations: Positively charged ions E.g.:
2. Anions: Negatively charged ions E.g.:
All metals form cations by loosing
All non-metals form anions by gaining
The chemical formula of a compound is a
symbolic representation of its
e.g.: H20 (Water), CaO (Calcium Oxide),
NH3 (Ammonia) , K2SO4 (Potassium
Rules of writing chemical
• The valencies or charges on the ion must
• 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 (CuO) etc., where
oxygen, chlorine, sulphur are non-metals and
are written on the right, whereas calcium,
sodium, iron and copper are metals, and are
written on the left.
• In compounds formed with polyatomic ions,
the ion is enclosed in a bracket before writing
the number to indicate the ratio.
Formulae of Compounds
One mole of any species (atoms,
molecules, ions or particles) is that
quantity in number having a mass
equal to its atomic or molecular mass
1 mole (of anything) = 6.022×1023 in
Mass of 1 mole of a particular
substance is always fixed.
It was named after the Italian scientist
named Amedeo Avagadro.
It is denoted by.
Its value is 6.022×1023
Ao is equal to 1 mole.