Question

06
SCIENCE FOR NINTH CLASS : CHEMISTRY
Sample Problem 1. The number of protons, neutrons and electrons in species A to E are given in the
following table :
Species
Electrons
4
Protons
6
18
17
A
B
C С
D
Neutrons
6
22
20
18
17
9
10
11
E
17
18
17
Indicate from the above table the species that represent a pair of isotopes.
Solution. Those species which contain the same number of protons but different number of neutr
will be a pair of isotopes. In the above table only two species C and E have the same number of protons
each) but different number of neutrons (20 and 18 respectively). Thus, C and E are a pair of isoto
(Please note that the atomic number 17 is of chlorine, so C and E are actually the two isotopes of chlori
Sample Problem 2. Composition of the nuclei of two atomic species X and Y is given as under:
Х Y
Protons : 6
6
Neutrons : 6 8
Give the mass numbers of X and Y. What is the relation between the two species and which elem
elements they represent ?
(NCERT Book Que
Solution. We know that:
Mass number = No. of protons + No. of neutrons
So, Mass number of X = 6 + 6
= 12
Mass number of Y = 6 + 8​

Answer 1

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