Question

Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and iv) Isobars. Give any two uses of isotopes.

NCERT Class IX
Sciences - Exemplar Problems

Chapter 4. Structure of the Atom

Answer 1

ANSWER:-

◆ ATOMIC NUMBER

● the total no. of protons lying in the nucleus of any atom is called ATOMIC NUMBER

● An atomic no. is identify of an atom, changing atomic no. means changing the atom.

● It is denoted by 'Z'

● Atomic no. = no. of protons or no. of neutral atom, no. of protons and electrons are equal.

● Example -
1. Hydrogen
ATOMIC NUMBER = 1 (1 protons)

2. Helium
ATOMIC NUMBER = 2 (2 protons)

◆ MASS NUMBER

● It's defined as the sum of the number of protons and electrons present in the nucleus of an atom.

● It is denoted by 'Z'

● Mass no. = no. of protons + no. of electrons

Example -
1. Carbon
Mass no. = 12
6 protons + 6 electrons = 12
so, mass = 12u

2. Aluminum
Mass no. = 27
13 protons + 14 electrons = 27
so, mass = 27u

◆ ISOTOPES

● Isotopes are the atoms of same elements having same atomic no. and different mass no.

● Example - Chlorine has two isotopes of mass no. 35 and 37

17Cl35 , 17Cl37

● Uses
# Uranium isotopes is used as fuel in nuclear rector.

# Isotopes of cobalt is useful in treatment of Cancer.

# An isotopes of iodine is used in treatment of goiter.

◆ Isobars

● Isobars are the atoms of those elements which have the same mass no. but different atomic no. is called Isobars.

● 20Ca40 and 18Ar40 have same mass no. and different atomic no.

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Answer 2

ANSWER:-

◆ ATOMIC NUMBER

● the total no. of protons lying in the nucleus of any atom is called ATOMIC NUMBER

● An atomic no. is identify of an atom, changing atomic no. means changing the atom.

● It is denoted by 'Z'

● Atomic no. = no. of protons or no. of neutral atom, no. of protons and electrons are equal.

● Example -

1. Hydrogen

ATOMIC NUMBER = 1 (1 protons)

2. Helium

ATOMIC NUMBER = 2 (2 protons)

◆ MASS NUMBER

● It's defined as the sum of the number of protons and electrons present in the nucleus of an atom.

● It is denoted by 'Z'

● Mass no. = no. of protons + no. of electrons

Example -

1. Carbon

Mass no. = 12

6 protons + 6 electrons = 12

so, mass = 12u

2. Aluminum

Mass no. = 27

13 protons + 14 electrons = 27

so, mass = 27u

◆ ISOTOPES

● Isotopes are the atoms of same elements having same atomic no. and different mass no.

● Example - Chlorine has two isotopes of mass no. 35 and 37

17Cl35 , 17Cl37

● Uses

# Uranium isotopes is used as fuel in nuclear rector.

# Isotopes of cobalt is useful in treatment of Cancer.

# An isotopes of iodine is used in treatment of goiter.

◆ Isobars

● Isobars are the atoms of those elements which have the same mass no. but different atomic no. is called Isobars.

● 20Ca40 and 18Ar40 have same mass no. and different atomic no.