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NCERT · Class 9 · Science

Chapter 4: Structure of the Atom

Practice each question, then open the answer or worked solution.

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Exercise-4.5

Textbook page 52 · 2 questions
Q1Written answer

If the number of electrons in an atom is 8 and the number of protons is also 8, then

(i) What is the atomic number of the atom? and

(ii) What is the charge on the atom?

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Worked solution

Given: Number of electrons = 8

Number of protons = 8

(i) The atomic number of an atom is the same as the number of protons in that atom; hence, its atomic number is 8.

(ii) In an atom, the number of protons is equal to the number of electrons. Hence, both the charges – positive and negative – neutralise each other. Therefore, the atom does not possess any charge.

Q2Written answer

With the help of the given table, find out the mass number of oxygen and sulphur atom.

Table: Composition of Atoms of the First Eighteen Elements with Electron Distribution in Various Shells.

Name of Element Symbol Atomic number Number of Protons Number of Neutrons Number of electrons Distribution of electrons

K L   M   N

 Valency
Hydrogen

Helium

Lithium

Beryllium

Boron

Carbon

Nitrogen

Oxygen

Fluorine

Neon

Sodium

Magnesium

 H

He

Li

Be

B

C

N

O

F

Ne

Na

Mg

 1

2

3

4

5

6

7

8

9

10

11

12

 1

2

3

4

5

6

7

8

9

10

11

12

 –

2

4

5

6

6

7

8

10

10

12

12

 1

2

3

4

5

6

7

8

9

10

11

12

 1

2

2

2

2

2

2

2

2

2

2

2

 –

–

1

2

3

4

5

6

7

8

8

8

 –

–

–

–

–

–

–

–

–

–

1

2

 –

–

–

–

–

–

–

–

–

—

–

–

 1

0

1

2

3

4

3

2

1

0

1

2
Aluminium

Silicon

Phosphorus

Sulphur

Chlorine

Argon

 Al

Si

P

S

Cl

Ar

 13

14

15

16

17

18

 13

14

15

16

17

18

 14

14

16

16

18

22

 13

14

15

16

17

18

 2

2

2

2

2

2

 8

8

8

8

8

8

 3

4

5

6

7

8

 –

–

–

–

–

 3

4

3,5

2

1

0
Show solutionHide solution
Worked solution

(a) To find the mass number of Oxygen,

Number of protons = 8

Number of neutrons = 8

Atomic number = 8

Atomic mass number = Number of protons + number of neutrons = 8 + 8 = 16

Therefore, the mass number of oxygen = 16

(b) To find the mass number of Sulphur,

Number of protons = 16

Number of neutrons = 16

Atomic number = 16

Atomic mass number = Number of protons + number of neutrons = 16 + 16 = 32