NCERT Textbook: Atoms & Molecules | Science Class 9 PDF Download

 Page 1


Ancient Indian and Greek philosophers have
always wondered about the unknown and
unseen form of matter. 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 particles. Ultimately, a stage will
come when we shall come across the smallest
particles beyond which further division will
not be possible.  He named these particles
Parmanu. Another Indian philosopher,
Pakudha Katyayama, elaborated this doctrine
and said that these particles normally exist
in a combined form which gives us various
forms of matter.
Around the same era, 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). All this was based on
philosophical considerations and not much
experimental work to validate these ideas
could be done till the eighteenth century.
By the end of the eighteenth century,
scientists recognised 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 L. Lavoisier laid the foundation
of chemical sciences by establishing two
important laws of chemical combination.
3.1 Laws of Chemical Combination
The following two laws of chemical
combination were established after
much experimentations by Lavoisier and
Joseph L. Proust.
3.1.1 LAW OF CONSERVATION OF MASS
Is there a change in mass when a chemical
change (chemical reaction) takes place?
Activity ______________3.1
• Take one of the following sets, X and Y
of chemicals—
X Y
(i) copper sulphate sodium carbonate
(ii) barium chloride sodium sulphate
(iii) lead nitrate sodium chloride
• Prepare separately a 5% solution of
any one pair of substances listed
under X and Y each in 10 mL in water.
• Take a little amount of solution of Y in
a conical flask and some solution of
X in an ignition tube.
• Hang the ignition tube in the flask
carefully; see that the solutions do not
get mixed. Put a cork on the flask
(see Fig. 3.1).
Fig. 3.1: Ignition tube containing solution of X, dipped
in a conical flask containing solution of Y
3
A A A A ATOMS TOMS TOMS TOMS TOMS     AND AND AND AND AND M M M M MOLECULES OLECULES OLECULES OLECULES OLECULES
Chapter
2024-25
Page 2


Ancient Indian and Greek philosophers have
always wondered about the unknown and
unseen form of matter. 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 particles. Ultimately, a stage will
come when we shall come across the smallest
particles beyond which further division will
not be possible.  He named these particles
Parmanu. Another Indian philosopher,
Pakudha Katyayama, elaborated this doctrine
and said that these particles normally exist
in a combined form which gives us various
forms of matter.
Around the same era, 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). All this was based on
philosophical considerations and not much
experimental work to validate these ideas
could be done till the eighteenth century.
By the end of the eighteenth century,
scientists recognised 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 L. Lavoisier laid the foundation
of chemical sciences by establishing two
important laws of chemical combination.
3.1 Laws of Chemical Combination
The following two laws of chemical
combination were established after
much experimentations by Lavoisier and
Joseph L. Proust.
3.1.1 LAW OF CONSERVATION OF MASS
Is there a change in mass when a chemical
change (chemical reaction) takes place?
Activity ______________3.1
• Take one of the following sets, X and Y
of chemicals—
X Y
(i) copper sulphate sodium carbonate
(ii) barium chloride sodium sulphate
(iii) lead nitrate sodium chloride
• Prepare separately a 5% solution of
any one pair of substances listed
under X and Y each in 10 mL in water.
• Take a little amount of solution of Y in
a conical flask and some solution of
X in an ignition tube.
• Hang the ignition tube in the flask
carefully; see that the solutions do not
get mixed. Put a cork on the flask
(see Fig. 3.1).
Fig. 3.1: Ignition tube containing solution of X, dipped
in a conical flask containing solution of Y
3
A A A A ATOMS TOMS TOMS TOMS TOMS     AND AND AND AND AND M M M M MOLECULES OLECULES OLECULES OLECULES OLECULES
Chapter
2024-25
ATOMS AND MOLECULES 27
• Weigh the flask with its contents
carefully.
• Now tilt and swirl the flask, so that the
solutions X and Y get mixed.
• Weigh again.
• What happens in the reaction flask?
• Do you think that a chemical reaction
has taken place?
• Why should we put a cork on the mouth
of the flask?
• Does the mass of the flask and its
contents change?
Law of conservation of mass states that
mass can neither be created nor destroyed in
a chemical reaction.
3.1.2 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.
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. Thus,
if 9 g of water is decomposed, 1 g of hydrogen
and 8 g of oxygen are always obtained.
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. This law was stated by Proust as
“In a chemical substance the elements are
always present in definite proportions by
mass”.
The next problem faced by scientists was
to give appropriate explanations of these laws.
British chemist John Dalton provided the
basic theory about the nature of matter.
Dalton picked up the idea of divisibility of
matter, which was till then just a philosophy.
He took the name ‘atoms’ as given by the
Greeks and said that the smallest particles of
matter are atoms. His theory was based on the
laws of chemical combination. Dalton’s atomic
theory provided an explanation for the law of
conservation of mass and the law of
definite proportions.
John Dalton was born in
a poor weaver’s family in
1766 in England. He
began his career as a
teacher at the age of
twelve. Seven years later
he became a school
principal. In 1793, Dalton
left for Manchester to
teach mathematics,
physics and chemistry in
a college. He spent most of his life there
teaching and researching. In 1808, he
presented his atomic theory which was a
turning point in the study of matter.
According to Dalton’s atomic theory, all
matter, whether an element, a compound or
a mixture is composed of small particles called
atoms. The postulates of this theory may be
stated as follows:
(i) All matter is made of very tiny particles
called atoms, which participate in
chemical reactions.
(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.
(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.
You will study in the next chapter that all
atoms are made up of still smaller particles.
 uestions
1. In a reaction, 5.3 g of sodium
carbonate reacted with 6 g of
acetic acid. The products were
2.2 g of carbon dioxide, 0.9 g
water and 8.2 g of sodium
acetate. Show that these
Q
John Dalton
2024-25
Page 3


Ancient Indian and Greek philosophers have
always wondered about the unknown and
unseen form of matter. 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 particles. Ultimately, a stage will
come when we shall come across the smallest
particles beyond which further division will
not be possible.  He named these particles
Parmanu. Another Indian philosopher,
Pakudha Katyayama, elaborated this doctrine
and said that these particles normally exist
in a combined form which gives us various
forms of matter.
Around the same era, 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). All this was based on
philosophical considerations and not much
experimental work to validate these ideas
could be done till the eighteenth century.
By the end of the eighteenth century,
scientists recognised 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 L. Lavoisier laid the foundation
of chemical sciences by establishing two
important laws of chemical combination.
3.1 Laws of Chemical Combination
The following two laws of chemical
combination were established after
much experimentations by Lavoisier and
Joseph L. Proust.
3.1.1 LAW OF CONSERVATION OF MASS
Is there a change in mass when a chemical
change (chemical reaction) takes place?
Activity ______________3.1
• Take one of the following sets, X and Y
of chemicals—
X Y
(i) copper sulphate sodium carbonate
(ii) barium chloride sodium sulphate
(iii) lead nitrate sodium chloride
• Prepare separately a 5% solution of
any one pair of substances listed
under X and Y each in 10 mL in water.
• Take a little amount of solution of Y in
a conical flask and some solution of
X in an ignition tube.
• Hang the ignition tube in the flask
carefully; see that the solutions do not
get mixed. Put a cork on the flask
(see Fig. 3.1).
Fig. 3.1: Ignition tube containing solution of X, dipped
in a conical flask containing solution of Y
3
A A A A ATOMS TOMS TOMS TOMS TOMS     AND AND AND AND AND M M M M MOLECULES OLECULES OLECULES OLECULES OLECULES
Chapter
2024-25
ATOMS AND MOLECULES 27
• Weigh the flask with its contents
carefully.
• Now tilt and swirl the flask, so that the
solutions X and Y get mixed.
• Weigh again.
• What happens in the reaction flask?
• Do you think that a chemical reaction
has taken place?
• Why should we put a cork on the mouth
of the flask?
• Does the mass of the flask and its
contents change?
Law of conservation of mass states that
mass can neither be created nor destroyed in
a chemical reaction.
3.1.2 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.
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. Thus,
if 9 g of water is decomposed, 1 g of hydrogen
and 8 g of oxygen are always obtained.
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. This law was stated by Proust as
“In a chemical substance the elements are
always present in definite proportions by
mass”.
The next problem faced by scientists was
to give appropriate explanations of these laws.
British chemist John Dalton provided the
basic theory about the nature of matter.
Dalton picked up the idea of divisibility of
matter, which was till then just a philosophy.
He took the name ‘atoms’ as given by the
Greeks and said that the smallest particles of
matter are atoms. His theory was based on the
laws of chemical combination. Dalton’s atomic
theory provided an explanation for the law of
conservation of mass and the law of
definite proportions.
John Dalton was born in
a poor weaver’s family in
1766 in England. He
began his career as a
teacher at the age of
twelve. Seven years later
he became a school
principal. In 1793, Dalton
left for Manchester to
teach mathematics,
physics and chemistry in
a college. He spent most of his life there
teaching and researching. In 1808, he
presented his atomic theory which was a
turning point in the study of matter.
According to Dalton’s atomic theory, all
matter, whether an element, a compound or
a mixture is composed of small particles called
atoms. The postulates of this theory may be
stated as follows:
(i) All matter is made of very tiny particles
called atoms, which participate in
chemical reactions.
(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.
(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.
You will study in the next chapter that all
atoms are made up of still smaller particles.
 uestions
1. In a reaction, 5.3 g of sodium
carbonate reacted with 6 g of
acetic acid. The products were
2.2 g of carbon dioxide, 0.9 g
water and 8.2 g of sodium
acetate. Show that these
Q
John Dalton
2024-25
SCIENCE 28
We might think that if atoms are so
insignificant in size, why should we care about
them? This is because our entire world is made
up of atoms. We may not be able to see them,
but they are there, and constantly affecting
whatever we do. Through modern techniques,
we can now produce magnified images of
surfaces of elements showing atoms.
observations are in agreement
with the law of conservation of
mass.
sodium carbonate + acetic acid
? sodium acetate + carbon
dioxide + water
2. Hydrogen and oxygen combine in
the ratio of 1:8 by mass to form
water. What mass of oxygen gas
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 building
walls, from these walls a room and then a
collection of rooms to form a building? What
is the building block of the huge building?
What about the building block of an ant-hill?
It is a small grain of sand. Similarly, the
building blocks of all matter are atoms.
How big are atoms?
Atoms are very small, they are smaller than
anything that we can imagine or compare
with. More than millions of atoms when
stacked would make a layer barely as thick
as this sheet of paper.
Atomic radius is measured in nanometres.
1/10
 9 
m = 1 nm
1 m = 10
9
 nm
Relative Sizes
Radii (in m) Example
10
–10
Atom of hydrogen
10
–9
Molecule of water
10
–8
Molecule of haemoglobin
10
–4
Grain of sand
10
–3
Ant
10
–1
Apple
Fig. 3.2: An image of the surface of silicon
3.2.1 WHAT ARE THE MODERN DAY
SYMBOLS OF ATOMS OF DIFFERENT
ELEMENTS?
Dalton was the first scientist to use the
symbols for elements in a very specific sense.
When he used a symbol for an element he
also meant a definite quantity of that element,
that is, one atom of that element. Berzilius
suggested that the symbols of elements be
made from one or two letters of the name of
the element.
Fig. 3.3: Symbols for some elements as proposed by
Dalton
2024-25
Page 4


Ancient Indian and Greek philosophers have
always wondered about the unknown and
unseen form of matter. 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 particles. Ultimately, a stage will
come when we shall come across the smallest
particles beyond which further division will
not be possible.  He named these particles
Parmanu. Another Indian philosopher,
Pakudha Katyayama, elaborated this doctrine
and said that these particles normally exist
in a combined form which gives us various
forms of matter.
Around the same era, 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). All this was based on
philosophical considerations and not much
experimental work to validate these ideas
could be done till the eighteenth century.
By the end of the eighteenth century,
scientists recognised 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 L. Lavoisier laid the foundation
of chemical sciences by establishing two
important laws of chemical combination.
3.1 Laws of Chemical Combination
The following two laws of chemical
combination were established after
much experimentations by Lavoisier and
Joseph L. Proust.
3.1.1 LAW OF CONSERVATION OF MASS
Is there a change in mass when a chemical
change (chemical reaction) takes place?
Activity ______________3.1
• Take one of the following sets, X and Y
of chemicals—
X Y
(i) copper sulphate sodium carbonate
(ii) barium chloride sodium sulphate
(iii) lead nitrate sodium chloride
• Prepare separately a 5% solution of
any one pair of substances listed
under X and Y each in 10 mL in water.
• Take a little amount of solution of Y in
a conical flask and some solution of
X in an ignition tube.
• Hang the ignition tube in the flask
carefully; see that the solutions do not
get mixed. Put a cork on the flask
(see Fig. 3.1).
Fig. 3.1: Ignition tube containing solution of X, dipped
in a conical flask containing solution of Y
3
A A A A ATOMS TOMS TOMS TOMS TOMS     AND AND AND AND AND M M M M MOLECULES OLECULES OLECULES OLECULES OLECULES
Chapter
2024-25
ATOMS AND MOLECULES 27
• Weigh the flask with its contents
carefully.
• Now tilt and swirl the flask, so that the
solutions X and Y get mixed.
• Weigh again.
• What happens in the reaction flask?
• Do you think that a chemical reaction
has taken place?
• Why should we put a cork on the mouth
of the flask?
• Does the mass of the flask and its
contents change?
Law of conservation of mass states that
mass can neither be created nor destroyed in
a chemical reaction.
3.1.2 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.
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. Thus,
if 9 g of water is decomposed, 1 g of hydrogen
and 8 g of oxygen are always obtained.
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. This law was stated by Proust as
“In a chemical substance the elements are
always present in definite proportions by
mass”.
The next problem faced by scientists was
to give appropriate explanations of these laws.
British chemist John Dalton provided the
basic theory about the nature of matter.
Dalton picked up the idea of divisibility of
matter, which was till then just a philosophy.
He took the name ‘atoms’ as given by the
Greeks and said that the smallest particles of
matter are atoms. His theory was based on the
laws of chemical combination. Dalton’s atomic
theory provided an explanation for the law of
conservation of mass and the law of
definite proportions.
John Dalton was born in
a poor weaver’s family in
1766 in England. He
began his career as a
teacher at the age of
twelve. Seven years later
he became a school
principal. In 1793, Dalton
left for Manchester to
teach mathematics,
physics and chemistry in
a college. He spent most of his life there
teaching and researching. In 1808, he
presented his atomic theory which was a
turning point in the study of matter.
According to Dalton’s atomic theory, all
matter, whether an element, a compound or
a mixture is composed of small particles called
atoms. The postulates of this theory may be
stated as follows:
(i) All matter is made of very tiny particles
called atoms, which participate in
chemical reactions.
(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.
(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.
You will study in the next chapter that all
atoms are made up of still smaller particles.
 uestions
1. In a reaction, 5.3 g of sodium
carbonate reacted with 6 g of
acetic acid. The products were
2.2 g of carbon dioxide, 0.9 g
water and 8.2 g of sodium
acetate. Show that these
Q
John Dalton
2024-25
SCIENCE 28
We might think that if atoms are so
insignificant in size, why should we care about
them? This is because our entire world is made
up of atoms. We may not be able to see them,
but they are there, and constantly affecting
whatever we do. Through modern techniques,
we can now produce magnified images of
surfaces of elements showing atoms.
observations are in agreement
with the law of conservation of
mass.
sodium carbonate + acetic acid
? sodium acetate + carbon
dioxide + water
2. Hydrogen and oxygen combine in
the ratio of 1:8 by mass to form
water. What mass of oxygen gas
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 building
walls, from these walls a room and then a
collection of rooms to form a building? What
is the building block of the huge building?
What about the building block of an ant-hill?
It is a small grain of sand. Similarly, the
building blocks of all matter are atoms.
How big are atoms?
Atoms are very small, they are smaller than
anything that we can imagine or compare
with. More than millions of atoms when
stacked would make a layer barely as thick
as this sheet of paper.
Atomic radius is measured in nanometres.
1/10
 9 
m = 1 nm
1 m = 10
9
 nm
Relative Sizes
Radii (in m) Example
10
–10
Atom of hydrogen
10
–9
Molecule of water
10
–8
Molecule of haemoglobin
10
–4
Grain of sand
10
–3
Ant
10
–1
Apple
Fig. 3.2: An image of the surface of silicon
3.2.1 WHAT ARE THE MODERN DAY
SYMBOLS OF ATOMS OF DIFFERENT
ELEMENTS?
Dalton was the first scientist to use the
symbols for elements in a very specific sense.
When he used a symbol for an element he
also meant a definite quantity of that element,
that is, one atom of that element. Berzilius
suggested that the symbols of elements be
made from one or two letters of the name of
the element.
Fig. 3.3: Symbols for some elements as proposed by
Dalton
2024-25
ATOMS AND MOLECULES 29
In the beginning, the names of elements
were derived from the name of the place
where they were found for the first time. For
example, the name copper was taken from
Cyprus. Some names were taken from
specific colours. For example, gold was taken
from the English word meaning yellow.
Now-a-days, IUPAC (International Union of
Pure and Applied Chemistry) is an
international scientific organisation which
approves names of elements, symbols and
units. Many of the symbols are the first one
or two letters of the element’s name in
English. The first letter of a symbol is always
written as a capital letter (uppercase) and the
second letter as a small letter (lowercase).
For example
(i) hydrogen, H
(ii) aluminium, Al and not AL
(iii) cobalt, Co and not CO.
Symbols of some elements are formed from
the first letter of the name and a letter,
appearing later in the name. Examples are: (i)
chlorine, Cl, (ii) zinc, Zn etc.
Other symbols have been taken from the
names of elements in Latin, German or Greek.
For example, the symbol of iron is Fe from its
Latin name ferrum, sodium is Na from natrium,
potassium is K from kalium. Therefore, each
element has a name and a unique
chemical symbol.
(The above table is given for you to refer to
whenever you study about elements. Do not
bother to memorise all in one go. With the
passage of time and repeated usage you will
automatically be able to reproduce
the symbols).
3.2.2 ATOMIC MASS
The most remarkable concept that Dalton’s
atomic theory proposed was that of the atomic
mass. According to him, each element had a
characteristic atomic mass. The theory could
explain the law of constant proportions so well
that scientists were prompted to measure the
atomic mass of an atom. Since determining the
mass of an individual atom was a relatively
difficult task, relative atomic masses were
determined using the laws of chemical
combinations and the compounds formed.
Let us take the example of a compound,
carbon monoxide (CO) formed by carbon and
oxygen. It was observed experimentally that 3
g of carbon combines with 4 g of oxygen to
form CO. In other words, carbon combines
with 4/3 times its mass of oxygen. Suppose
we define the atomic mass unit (earlier
abbreviated as ‘amu’, but according to the
latest IUPAC recommendations, it is now
written as ‘u’ – unified mass) as equal to the
mass of one carbon atom, then we would
assign carbon an atomic mass of 1.0 u and
oxygen an atomic mass of 1.33 u. However, it
is more convenient to have these numbers as
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
2024-25
Page 5


Ancient Indian and Greek philosophers have
always wondered about the unknown and
unseen form of matter. 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 particles. Ultimately, a stage will
come when we shall come across the smallest
particles beyond which further division will
not be possible.  He named these particles
Parmanu. Another Indian philosopher,
Pakudha Katyayama, elaborated this doctrine
and said that these particles normally exist
in a combined form which gives us various
forms of matter.
Around the same era, 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). All this was based on
philosophical considerations and not much
experimental work to validate these ideas
could be done till the eighteenth century.
By the end of the eighteenth century,
scientists recognised 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 L. Lavoisier laid the foundation
of chemical sciences by establishing two
important laws of chemical combination.
3.1 Laws of Chemical Combination
The following two laws of chemical
combination were established after
much experimentations by Lavoisier and
Joseph L. Proust.
3.1.1 LAW OF CONSERVATION OF MASS
Is there a change in mass when a chemical
change (chemical reaction) takes place?
Activity ______________3.1
• Take one of the following sets, X and Y
of chemicals—
X Y
(i) copper sulphate sodium carbonate
(ii) barium chloride sodium sulphate
(iii) lead nitrate sodium chloride
• Prepare separately a 5% solution of
any one pair of substances listed
under X and Y each in 10 mL in water.
• Take a little amount of solution of Y in
a conical flask and some solution of
X in an ignition tube.
• Hang the ignition tube in the flask
carefully; see that the solutions do not
get mixed. Put a cork on the flask
(see Fig. 3.1).
Fig. 3.1: Ignition tube containing solution of X, dipped
in a conical flask containing solution of Y
3
A A A A ATOMS TOMS TOMS TOMS TOMS     AND AND AND AND AND M M M M MOLECULES OLECULES OLECULES OLECULES OLECULES
Chapter
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ATOMS AND MOLECULES 27
• Weigh the flask with its contents
carefully.
• Now tilt and swirl the flask, so that the
solutions X and Y get mixed.
• Weigh again.
• What happens in the reaction flask?
• Do you think that a chemical reaction
has taken place?
• Why should we put a cork on the mouth
of the flask?
• Does the mass of the flask and its
contents change?
Law of conservation of mass states that
mass can neither be created nor destroyed in
a chemical reaction.
3.1.2 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.
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. Thus,
if 9 g of water is decomposed, 1 g of hydrogen
and 8 g of oxygen are always obtained.
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. This law was stated by Proust as
“In a chemical substance the elements are
always present in definite proportions by
mass”.
The next problem faced by scientists was
to give appropriate explanations of these laws.
British chemist John Dalton provided the
basic theory about the nature of matter.
Dalton picked up the idea of divisibility of
matter, which was till then just a philosophy.
He took the name ‘atoms’ as given by the
Greeks and said that the smallest particles of
matter are atoms. His theory was based on the
laws of chemical combination. Dalton’s atomic
theory provided an explanation for the law of
conservation of mass and the law of
definite proportions.
John Dalton was born in
a poor weaver’s family in
1766 in England. He
began his career as a
teacher at the age of
twelve. Seven years later
he became a school
principal. In 1793, Dalton
left for Manchester to
teach mathematics,
physics and chemistry in
a college. He spent most of his life there
teaching and researching. In 1808, he
presented his atomic theory which was a
turning point in the study of matter.
According to Dalton’s atomic theory, all
matter, whether an element, a compound or
a mixture is composed of small particles called
atoms. The postulates of this theory may be
stated as follows:
(i) All matter is made of very tiny particles
called atoms, which participate in
chemical reactions.
(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.
(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.
You will study in the next chapter that all
atoms are made up of still smaller particles.
 uestions
1. In a reaction, 5.3 g of sodium
carbonate reacted with 6 g of
acetic acid. The products were
2.2 g of carbon dioxide, 0.9 g
water and 8.2 g of sodium
acetate. Show that these
Q
John Dalton
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SCIENCE 28
We might think that if atoms are so
insignificant in size, why should we care about
them? This is because our entire world is made
up of atoms. We may not be able to see them,
but they are there, and constantly affecting
whatever we do. Through modern techniques,
we can now produce magnified images of
surfaces of elements showing atoms.
observations are in agreement
with the law of conservation of
mass.
sodium carbonate + acetic acid
? sodium acetate + carbon
dioxide + water
2. Hydrogen and oxygen combine in
the ratio of 1:8 by mass to form
water. What mass of oxygen gas
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 building
walls, from these walls a room and then a
collection of rooms to form a building? What
is the building block of the huge building?
What about the building block of an ant-hill?
It is a small grain of sand. Similarly, the
building blocks of all matter are atoms.
How big are atoms?
Atoms are very small, they are smaller than
anything that we can imagine or compare
with. More than millions of atoms when
stacked would make a layer barely as thick
as this sheet of paper.
Atomic radius is measured in nanometres.
1/10
 9 
m = 1 nm
1 m = 10
9
 nm
Relative Sizes
Radii (in m) Example
10
–10
Atom of hydrogen
10
–9
Molecule of water
10
–8
Molecule of haemoglobin
10
–4
Grain of sand
10
–3
Ant
10
–1
Apple
Fig. 3.2: An image of the surface of silicon
3.2.1 WHAT ARE THE MODERN DAY
SYMBOLS OF ATOMS OF DIFFERENT
ELEMENTS?
Dalton was the first scientist to use the
symbols for elements in a very specific sense.
When he used a symbol for an element he
also meant a definite quantity of that element,
that is, one atom of that element. Berzilius
suggested that the symbols of elements be
made from one or two letters of the name of
the element.
Fig. 3.3: Symbols for some elements as proposed by
Dalton
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ATOMS AND MOLECULES 29
In the beginning, the names of elements
were derived from the name of the place
where they were found for the first time. For
example, the name copper was taken from
Cyprus. Some names were taken from
specific colours. For example, gold was taken
from the English word meaning yellow.
Now-a-days, IUPAC (International Union of
Pure and Applied Chemistry) is an
international scientific organisation which
approves names of elements, symbols and
units. Many of the symbols are the first one
or two letters of the element’s name in
English. The first letter of a symbol is always
written as a capital letter (uppercase) and the
second letter as a small letter (lowercase).
For example
(i) hydrogen, H
(ii) aluminium, Al and not AL
(iii) cobalt, Co and not CO.
Symbols of some elements are formed from
the first letter of the name and a letter,
appearing later in the name. Examples are: (i)
chlorine, Cl, (ii) zinc, Zn etc.
Other symbols have been taken from the
names of elements in Latin, German or Greek.
For example, the symbol of iron is Fe from its
Latin name ferrum, sodium is Na from natrium,
potassium is K from kalium. Therefore, each
element has a name and a unique
chemical symbol.
(The above table is given for you to refer to
whenever you study about elements. Do not
bother to memorise all in one go. With the
passage of time and repeated usage you will
automatically be able to reproduce
the symbols).
3.2.2 ATOMIC MASS
The most remarkable concept that Dalton’s
atomic theory proposed was that of the atomic
mass. According to him, each element had a
characteristic atomic mass. The theory could
explain the law of constant proportions so well
that scientists were prompted to measure the
atomic mass of an atom. Since determining the
mass of an individual atom was a relatively
difficult task, relative atomic masses were
determined using the laws of chemical
combinations and the compounds formed.
Let us take the example of a compound,
carbon monoxide (CO) formed by carbon and
oxygen. It was observed experimentally that 3
g of carbon combines with 4 g of oxygen to
form CO. In other words, carbon combines
with 4/3 times its mass of oxygen. Suppose
we define the atomic mass unit (earlier
abbreviated as ‘amu’, but according to the
latest IUPAC recommendations, it is now
written as ‘u’ – unified mass) as equal to the
mass of one carbon atom, then we would
assign carbon an atomic mass of 1.0 u and
oxygen an atomic mass of 1.33 u. However, it
is more convenient to have these numbers as
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
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SCIENCE 30
whole numbers or as near to a whole numbers
as possible. While searching for various
atomic mass units, scientists initially took 1/
16 of the mass of an atom of naturally
occurring oxygen as the unit. This was
considered relevant due to two reasons:
• oxygen reacted with a large number of
elements and formed compounds.
• this atomic mass unit gave masses of
most of the elements as whole numbers.
However, in 1961 for a universally
accepted atomic mass unit, carbon-12 isotope
was chosen as the standard reference for
measuring atomic masses. One atomic mass
unit is a mass unit equal to exactly one-twelfth
(1/12
th
) the mass of one atom of carbon-12.
The relative atomic masses of all elements
have been found with respect to an atom of
carbon-12.
Imagine a fruit seller selling fruits without
any standard weight with him. He takes a
watermelon and says, “this has a mass equal
to 12 units” (12 watermelon units or 12 fruit
mass units). He makes twelve equal pieces of
the watermelon and finds the mass of each fruit
he is selling, relative to the mass of one piece
of the watermelon. Now he sells his fruits by
relative fruit mass unit (fmu), as in Fig. 3.4.
Fig. 3.4 : (a) Watermelon, (b) 12 pieces, (c) 1/12 of
watermelon, (d) how the fruit seller can
weigh the fruits using pieces of watermelon
Similarly, the relative atomic mass of the
atom of an element is defined as the average
mass of the atom, as compared to 1/12
th
 the
mass of one carbon-12 atom.
Table 3.2: Atomic masses of
a few elements
Element Atomic Mass (u)
Hydrogen 1
Carbon 12
Nitrogen 14
Oxygen 16
Sodium 23
Magnesium 24
Sulphur 32
Chlorine 35.5
Calcium 40
3.2.3 HOW DO ATOMS EXIST?
Atoms of most elements are not able to exist
independently. Atoms form molecules and
ions. These molecules or ions aggregate in
large numbers to form the matter that we can
see, feel or touch.
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 defined
as the smallest particle of an element or a
compound that is capable of an independent
existence and shows all the properties of that
substance. Atoms of the same element or of
different elements can join together to form
molecules.
Q
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