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Question:

Composition of the nuclei of two atomic species X and Y are given as under:
X: Protons=6, Neutrons=6
Y: Protons=6, Neutrons=8
Given the mass numbers of X and Y. What is the relation between the two species?

Solution:

Atomic mass = number of protons + number of neutrons
Atomic mass of X = 6 + 6 = 12
Atomic mass of Y = 6 + 8 = 14
Both X and Y have the same atomic number (number of protons) but different atomic mass numbers (number of protons + number of neutrons). Therefore, they are isotopes.