Class – 7
Sub. – Chemistry
Chapter – 4
Atoms, Molecules and Radicals
Short answer questions:
Q.1 – Define atom and molecule.
Ans. – Atom – An Atom is the smallest part of an
element that can take part in a chemical reaction.
Molecule – A molecule is the smallest part of an
element or a compound that is capable of
independent existence.
Q.2 – What do you mean by the atomicity of an
element?
Ans. - The number of atoms in a molecule of an
element or a compound is known as it’s atomicity.
Q.3 – What are atomicities of neon, Oxygen, ozone,
phosphorus and sulphur ?
Ans.- Neon – 1
Oxygen – 2
Ozone – 3
Phosphorus – 4
Sulphur – 8
Q.4 – What do you mean by the valency of an
element ? How is it expressed ? Name two divalent
elements ?
Ans.- The combining capacity of an element with
other elements is known as its valency.
The valency of an element is expressed by the
number of hydrogen atoms an atom of the element
combines with or displaces from a compound.
Example of divalent elements – Oxygen and Calcium.
Q.5 – What do you mean by the groups and periods
of the Periodic table ?
Ans. - Groups – The periodic table contains 18
vertical columns, called groups.
Periods – The periodic table contains 7 horizontal
rows, called periods.
Q.6 – What is the correlation between the valency of
an element and it’s position in the periodic table ?
Ans. - Elements present in same group show valency
equal to the group number.
Q7 – What is the valency of the underlined element
or radical in each of the following compounds ?
Ans. - a.3
b.2
c.2
d.3
e.2
f.1
g.2
h.1
Long answer questions:
Q.1 – Discuss how atoms, molecules and radicals differ
from each other. Give examples.
Ans. -
Property Atom Molecule Radical
1. Independent Not Capable Capable
existence capable
2. Charge Not Not Charged
charged charged
3. Valency Has a Has no Has a
valency valency valency
4. Number of Only one One or One or
constituent more than more than
elements one one
5. How it takes Directly Breaks into Takes part
part in a atoms, in a
chemical which take reaction as
reaction part in a a unit
reaction
Q.2 – Describe giving three examples, how atoms form
molecules.
Ans. – An atom of hydrogen combines with another atom
of hydrogen to form molecule of element of hydrogen.
The molecules of oxygen, nitrogen and chlorine are also
made up of two atoms each of the corresponding
element and therefore represented by the formula O2,
N2, Cl2 respectively.
Q.3 – Can valency of an element be variable ? If it can,
mention two such elements and show how they form
compounds. How are compounds named ?
Ans. – Yes, valency of an element can be variable.
Example – Copper (1 and 2 )and iron ( 2 and 3 ) show
variable valency.
Compounds of copper – CuCl →Copper(l) chloride
CuCl₂ → Copper (ll) chloride
Compounds of iron – FeO → Iron(ll) oxide
Fe₂O₃→ Iron (lll) oxide
Bookwork
Objective Questions:
Choose the correct option:
1. Which of the following species can exist
independently?
Ans. Option d.- Ne
2. Which of the following species cannot exist
independently ?
Ans. Option c. – N
3. Which of the following has an atomicity of 3 ?
Ans. Option d. – ozone
4. If the valency of oxygen is 2, what is the valency of
copper in CuO ?
Ans. Option b. – 2
Fill in the blanks :
1. An atom is the smallest part of an element that
takes part in a chemical reaction.
2. A molecule is the smallest part of an element or a
compound that can exist independently.
3. A molecule of a nobel gas element contains one
atom.
4. Complete the following table :
Positive Negative Formula of Name of the
radical radical the compound
compound
Na⁺ O²⁻ Na₂O Sodium
oxide
Ca²⁺ S²⁻ CaS Calcium
sulphide
Ca²⁺ OH⁻ Ca(OH)₂ Calcium
hydroxide
Zn²⁺ SO₄²⁻ ZnSO₄ Zinc
sulphate
Al³⁺ O²⁻ Al₂O₃ Aluminum
oxide
Cu²⁺ CO₃²⁻ CuCO₃ Copper
carbonate
Na⁺ HCO₃⁻ NaHCO₃ Sodium
hydrogen
carbonate or
sodium
bicarbonate
Write T for true and F for false for the following
statements:
1. HCl is a diatomic molecule . T
2. The valency of a radical is the same as the charge
over it. T
3. The valency of a group – 14 element is 4. T
4. Calcium, an element of group 2 of the periodic
table, forms Ca2H. F
