Composition of Matter: Atom, Molecule

Chapter-1

Answer to the Short Questions

(a) What is an element? Give examples of solid, liquid and gaseous elements.

Ans.: Element : A substance which can not be split up or decomposed into

different chemical substances and out of which no new substance other than the
original one is obtained is called an element.

The examples of solid, liquid and gaseous elements are as follows:

Solid elements: Iron, copper, gold, silver etc.
Liquid elements: Mercury, bromine etc.
Gaseous elements: Hydrogen, oxygen etc.

(b) Name four elements which are available in nature in Free State.

Ans.: Name of elements : Oxygen, nitrogen, carbon, copper.

(c) Define the compound and write down their characteristics.

Ans.: Compound: The substances which can be split up into more than one
element are called compound.
In other words, compound can be defined as the substances which are composed
of two or more elements of different characteristics. For example water, sugar,
soap etc.

Their characteristics are as follows:

1. It is formed by the union of two or more elements of different properties.
 General Science: Physics and Chemistry 2

2. The numbers of compounds are unlimited in nature.

3. The smallest particles of compound are called molecule.
4. Every day new compounds are formed. As a result the number of
compound is increasing everyday.
5. compounds are of two types :
(i) Organic compound and (ii) Inorganic compound.

(d) Classify the following substances into element and compound:

Edible salt, Gold, Sulphur, Tin, Sugar, Water, Copper, Coal, Urea,
Mercury, Nitrogen.

Ans.:
Element Compound
1. Gold 1. Edible salt
2. Sulphur 2. Sugar
3. Tin 3. Water
4. Cupper 4. Fertilizer
5. Mercury
7. Nitrogen

(e) What is intermolecular space?

Ans.: Intermolecular space :

Substances are the combination of molecules.
Molecules are arranged side by side in the
substance. There are certain spaces in
between the molecules. The space between
two molecules is called intermolecular space.
Because of intermolecular spaces a spike can
be penetrated into the wood. Fig.: Aggregation of solid molecules

(f) What happens when a liquid is heated in a closed condition?

Ans.: The molecules of a liquid start vibrating when it

is heated in closed condition and thus molecules
obtained kinetic energy. As a result inter molecular
energy decreases and intermolecular space increases.

Fig.: Molecules in the liquid state

 Structure of Matter: Atom, Molecule 3

But the volume of the container is constant, so the molecules struck the surface
of the container with a very high speed. As a result the container can be broken
due the extra heat.

Answer to the Essay type Questions

(a) What are atoms and molecules? Define them.

Ans.: Atoms: The smallest particle of the elements which participate in the
chemical reaction and on further division loses its own identity totally is called
atom.
But there are atoms of some elements which can exist freely. For example
helium, neon, argon etc. As 109 elements have been so far invented, there are
109 types of atoms remain in this world.

Example : Iron, hydrogen etc. are composed with their very small particles.

Molecules: Molecule is the smallest particle of element or compound. Atoms are

obtained by split up molecule. That is, molecules are the smallest particles of
substance which can exist freely, can be divided into atoms and which can not
participate into chemical reaction without dividing into atoms, is called
molecules. Molecules are composed with two or more atoms.

Example : O2 – is a molecule of oxygen which contains two atoms of oxygen.

N2 – is a molecule of nitrogen which contains two atoms of nitrogen.
CO2 – is a molecule of carbon dioxide which contains two atoms of oxygen and
one atom of carbon.

(b) How many types of molecules are there? What is the difference between
the molecule of hydrogen and water?

Ans.: There are two types of molecules. These are as follows:

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(1) Elementary molecules: When the atoms of the same elements combine
together to form a molecule then it is called elementary or simple molecule. For
example----

H + H = H2
Hydrogen atom hydrogen atom hydrogen molecule
O + O = O2
Oxygen atom Oxygen atom Oxygen molecule

But there are some elementary molecules which are composed of only one atom.
For example the inert gases are elementary molecules.

(2) Compound molecules: When two or more atoms of different element

combine together to form a molecule, it is then called compound molecule. For
example---

2Na + 2Cl = 2NaCl

Sodium atom chlorine atom sodium chloride molecule

2H + O = H2 O
Hydrogen atom oxygen atom water molecule

Difference between the molecule of hydrogen and water are as follows:

Hydrogen molecule Water molecule

1.The formula of hydrogen is H2 1. The formula of water is H2O
2. Two atoms of hydrogen remain in 2. There are two hydrogen and one
hydrogen molecule oxygen atoms in water molecule
3. Two hydrogen atoms will be 3. Two hydrogen atoms and one
obtained by splitting up a hydrogen oxygen atoms will be obtained by
molecule splitting up a water molecule
4. One molecule of hydrogen is 4. One molecule of water is composed
composed of two atoms of hydrogen of one atom of oxygen and two atoms
of hydrogen

5.Hydrogen is an elementary molecule 5. Water is compound molecule

 Structure of Matter: Atom, Molecule 5

(c) What are the ideas of Dalton about atom?

Ans.: In 1880 British Mathematician and natural scientist John Dalton gave new
idea of atom. The idea of Dalton about atom is known as atomic theory. The
theory can be described as follows:
(i) Element is made up of indivisible particles called atom.
(ii) The atoms of the same element are same in nature, property and mass. So
the atoms of hydrogen are all alike in mass and property and so the case with
the atoms of all elements.
(iii) The atoms of the different elements are different from one another in
mass and properties. So the atoms of hydrogen, oxygen, carbon, sulphur etc
are different from each other in mass and properties.
(iv) The atoms of different elements combine with one another in simple
numerical proportions to form a compound.

(d) What are the states of matter? What influences these changes? Describer
how these changes occur in case of water.

Ans.: There are three kinds of the states of matter. These are as follows:
(i) Solid (ii) Liquid (iii) Gas
Heat influences these changes.

How these changes occur in case of water:

Generally water remains in three physical states. These are –ice, water and
vapour.
If we heat solid ice slightly, the vibration of the molecules of ice increases. As a
result the molecules move away from one another. On the application of more
heat intermolecular spaces increase further resulting in reduction of
intermolecular forces and allowing free movement of molecules. At this stage ice
transforms into water. If more heat is applied to liquid the kinetic energy of the
molecules increase to a great extent and some of the molecules leave the liquid
surface after overcoming the intermolecular force of attraction. Thus the water is
converted to a vapour state.
Again when the gaseous substances are cooled the velocity of the molecules
decrease and the molecules come closer to each other. As a result intermolecular
space decreases and intermolecular force increases. If cooling is continued a
stage is reached when the intermolecular force brings the molecules together and
converts them into water. When the liquid is cooled further the molecules come
 General Science: Physics and Chemistry 6

closer to each other and the intermolecular space decreases to a great extent. As
the temperature falls down the kinetic energy of the molecules also decreases
resulting in the increase of inter molecular force of attraction and the water is
finally converted to the solid state. Thus with the change of temperature, state of
substance is changed.

Increased temperature Rising temperature

Solid Liquid Gas
Decreased temperature Lowering temperature

(e) How the molecules of solid, liquid and gas exist? Describe with figure.

Ans.: Arrangements of the molecules of solid substances :

(i) In the case of solid their have great intermolecular attraction force between the
molecules. The force holds together the molecules of the solid substances.
So these make solid very strong. The solid substances like brick, wood, chair,
table etc have fixed size, shape and volume.

Fig.: Molecules of solid

Arrangements of the molecules of liquid substances :

(ii) In the case of liquid the molecules are

loosely held together leaving some space
between the molecules. For that reason
liquid possesses the definite shape and
volume. But they have lack of definite size.

Fig.: Molecules of liquid

 Structure of Matter: Atom, Molecule 7

Arrangements of the molecules of gaseous substances :

(iii) In case of gas the

intermolecular force are
negligibly small. As a result they
move freely. The molecules
remain far apart. So gas has no
definite size, shape and volume.
Fig.: Molecules of gases

(f) Why the condition of substance changes due to the application of heat?

Ans.: At normal temperature the molecules of a solid continue to vibrate in their

own position. After application of heat it gains additional energy causing rapid
vibration among the molecules. As a result the molecules move away from one
another. On application of more heat intermolecular spaces increase further
resulting in reduction of the intermolecular forces and allowing free movement
of molecules. At this stage solid transforms into liquid. If more heat is applied to
the liquid the kinetic energy of the molecules increase to a great extent and some
of the molecules leave liquid surface after overcoming the intermolecular force
of attraction. Thus a liquid is converted to a vapour state.

Conversely, when the gaseous substances are cooled, the velocity of the
molecules decrease and the molecules come closer to each other. As a result
intermolecular space decreases and intermolecular force increases. If cooling is
continued a stage is reached when the intermolecular forces brings the molecules
together and convert them into liquid. When the liquid is cooled further the
molecules come closer to each other and the intermolecular space decreases to a
great extent. As a result the liquid finally is converted to solid.

(g) What happens when equal pressure is applied to solid, liquid and gas?

Ans.: Solid substances : Every substance is made with molecules of its own.
These molecules are held together in the solid substances very strongly for their
attraction force. There is no space between the molecules. For that reason,
change of volume is not occurred applying strong force on solid substances.
 General Science: Physics and Chemistry 8

Liquid substances : Like solid substances the molecules of liquid substances are
also combined together. But in this case the molecules are held together loosely.
That is, there are spaces between the molecules of liquid substances. For that
reason there is short of attraction force between the molecules of liquid
substances. So change of volume is occurred in the case of liquid substance, if
the equal pressure is applied in it.

Gaseous substances : Gaseous substances are also composed of molecules. In

case of gas the intermolecular force is negligibly small. As a result they move
freely. The molecules remain far apart. The molecules of gas scatter all around.
For that reason gaseous substances are kept in container by closing the conver. If
the cover is opened a little, all the gases are escaped away. But the molecules of
gaseous substances come closer, if pressure is applied on these. So, the
conditions are changed. Therefore, if pressure is increased successively of
gaseous substances, the substances will be converted to liquids and solid
respectively.

Answer to the Extra Questions

1. What is substance? How many types of substances are there and what are
those? What are the differences between element and compound?

Ans.: Substance : The matter which has weight, occupies space and opposes
when any force is applied on it is called substance. Substances are of two types;
(1) Element (2) Compound.

The differences between element and compound are as follows :

Subject of differences Element Compound

1. Definition 1. A substance which is 1. A substance which is
not be split up or composed of different
decomposed into substances is called
different chemical compound.
substance is called
element.
 Structure of Matter: Atom, Molecule 9

2. Numbers 2. The numbers of 2. The numbers of

element are 109 in this compound are unlimited.
world.
3. Characteristics 3. After dividing an 3. The properties of
element all the atoms compound are
found are same. completely different than
its constituent elements.
4. Name of smallest 4. The smallest particles 4. The smallest particles
particles of element are called of compound are called
atoms. molecules.

2. Write down the differences between atoms and molecules.

Ans.: The differences between atoms and molecules are as follows :

Subjects of Atoms Molecules

differences
1. structure 1. Atoms are composed of 1. Molecules are composed
the smallest particles of of the smallest particles of
element. element or compound.
2. Independence 2. Maximum atom can not 2. All molecules can exist
exist freely. freely.
3.Chemical 3. Atoms can participate in 3. Before participating in the
reaction the chemical reaction chemical reaction the
directly. molecules have to be divided
into atoms.
4. Number 4. The number of atoms is 4. Unlimited amount of
limited; so far 109 atoms molecules have been
have been discovered. discovered, still discovering.
5. Decomposition 5. Electrons and protons are 5. Atoms are found by
found by decomposing decomposing molecules.
atoms.
6.Dividing 6. The characteristics of 6. Atoms are obtained by
character atoms do not exist if these dividing the molecules.
are further divided.
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3. What is elementary molecule and compound molecule?

Ans.: Elementary molecules: When the atoms of the same elements combine
together to form a molecule, it is called elementary or simple molecule. For
example----

H + H = H2
Hydrogen atom hydrogen atom hydrogen molecule

O + O = O2
Oxygen atom Oxygen atom Oxygen molecule

But there are some elementary molecules which are composed with only one
atom. For example the inert gases are elementary molecules.

Compound molecules: When two or more atoms of different elements combine

together to form a molecule, it is called compound molecule. For example---

2Na + 2Cl = 2NaCl

Sodium atom chlorine atom sodium chloride molecule
2H + O = H2 O
Hydrogen atom oxygen atom water molecule

4. What is intermolecular force? Why different conditions are shown by an

object at different temperatures?

Ans.: Intermolecular force : There is an

attractive force within the molecules of a
substance whereby the molecules are held
together closely; at the same time there exists
a repulsive force within the molecules due to
which they tend to move apart. The resultant
of these two opposing forces is called the
intermolecular force.

Fig.: Intermolecular force

 Structure of Matter: Atom, Molecule 11

Causes of different conditions of the same substances at different

temperatures :
At normal temperature the molecules of a solid
continue to vibrate in their own position. After
application of heat it gains additional energy
causing rapid vibration among the molecules. Fig.: Solid condition

As a result the molecules move away from one

another. On application of more heat intermolecular
spaces increase further resulting in reduction of the
intermolecular forces and allowing free movement
of molecules. At this stage solid transforms into
liquid. Liquid condition Gaseous condition

If more heat is applied to the liquid the kinetic

energy of the molecules increase to a great extent
and some molecules leave the liquid surface after
overcoming the intermolecular force of attraction.
Thus a liquid is converted to a vapour state.

5. Why solid substances have definite size and shape?

Ans.: Causes for having definite size

and shape of solid substances : In the
case of solid there is great
intermolecular attraction force between
the molecules. The force holds the
molecules of the solid substances
together. At normal temperature the
molecules of a solid continue to vibrate
in their own position. So these make
solid very strong.
For this reason, the solid substances
like brick, wood chair, table etc have
fixed size, shape and volume.
Fig.: Molecules of solid
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6. Why liquid substances have no definite size?

Ans.: Causes for having no definite size of liquid

substances : In the case of liquid the molecules are loosely
held together leaving some space between the molecules.
For that reason they have lack of definite size.

Fig.: Molecules of liquid

7. Why gaseous substances have no definite size, shape and volume?

Ans.: Causes for having no definite size of

gaseous substances : In case of gas the
intermolecular force are negligibly small.
As a result they move freely. The molecules
remain far apart.
So, gaseous substances have no definite
size, shape and volume. Fig.: Molecules of gases

Answer to the Objective Questions

Multiple Choice Questions :

1. Tick (√) the correct Answer :

(1) What is the number of elementary gases in the universe?

(a) 13 (b) 12
√ (b) 11 (d) 9
(2) What are there in one molecule of carbon dioxide molecule?
(a) One Carbon and two Oxygen molecules
(b) One Carbon atom and one Oxygen molecule
√ (c) One Carbon atom and two Oxygen atoms
(d) Carbon dioxide has two atoms.
 Structure of Matter: Atom, Molecule 13

(3) What is the distinction between oxygen and carbon dioxide molecule?
√ (a) The molecule of oxygen is elementary molecule and the molecule of
carbon dioxide is a compound molecule
(b) Both the molecule of oxygen and carbon dioxide are compound
molecule
(c) Both the molecules of oxygen and carbon dioxide are elementary
molecule.
(d) The molecule of oxygen is compound molecule and the molecule of
carbon dioxide is elementary molecule.
(4) Why it is easy to push spike into wood?
√ (a) Because of space in between the molecules
(b) Because of rapid vibration of molecules
(c) Because of Intermolecular force
(d) Because of attractive forces of the molecules.
(5) Which substance has least intermolecular force?
(a) Solid (b) Liquid
√ (c) Gas (d) Ice
(6) What is the reason of change in a substance into three states?
√ (a) By the influence of heat the speed of molecules changes and thus the
state of the substance is changed.
(b) Due to rearrangement of molecules
(c) Due to rearrangement of atoms
(d) Due to chemical change
(7) Why the tube of a cycle is punctured some times in the summer?
(a) The volume of the rubber tube increases
(b) The number of molecules of air inside the tube increases
√ (c) The molecules of air inside tube move faster due to heat and strike
wall of the tube strongly.
(d) The molecules of the air inside become heavier.
(8) What happens when steam in a closed air container is cooled and
transformed into water?
(a) The speed of the molecules increase
√ (b) The speed of the molecules reduce
(c) The number of molecules decreases
(d) The total weight of the container increases.
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2. Fill in the blanks :

(a) Carbon dioxide is ……………molecule

(b) Hydrogen consists of ………atoms
(c) Solid substance has fixed …………..
(d) Liquid has only fixed………….
(e) The idea of the molecules was put forward by …..
(f) …………is liquid
(g) The space between two molecules is called…….
(h) Intermolecular force is minimum ……….
(i) When a liquid is cooled the speed of the molecules……….

Ans.: (a) a compound (b) the combination of two (c) shape, size and volume
(d)volume (e) Avogadro (f) Mercury (g) intermolecular space (h) in gaseous
substances (i)decreases.

3. Match the left side sentence with that on the right side

Left Right
(a) Modern idea about atom was (a) Solid
given by
(b) Gaseous elements (b) Atoms
(c) Intermolecular force is (c) Scientist Dalton
greatest in
(d) Take part in a chemical (d) Consist of elementary
reaction molecule
(e) Element loses its identity (e) when atoms are split up
(f) In liquid.

Ans.:
1. Modern idea about atom was given by Scientist Dalton.
2. Gaseous elements consist of elementary molecule.
3. Intermolecular force is greatest in Solid.
4. Atoms take part in a chemical reaction.
5. Element loses its identity when atoms are split up
Structure of Matter: Atom, Molecule 15