Question

An element X has two isotopes. X–15 has 7 protons and 8 neutrons. Determine the atomic number and mass number of the second isotope, which has 10 neutrons in its nucleus.
a. atomic number = 8; mass number = 9
b. atomic number = 8; mass number = 17
c. atomic number = 7; mass number = 17
d. atomic number = 7; mass number = 15

Answer 1

c. Atomic number = 7; mass number = 17  

The atomic number of the first isotope is 7.

Isotopes of the same element have the same atomic number, so the atomic number of the second isotope must also be 7.

Mass number = no. of protons + no. of neutrons, or

A = p + n

∴ A = 7 + 10 = 17

Options “a.” and “b.” have the wrong atomic numbers, and option “d.” has the wrong mass number.