An atom has an atomic number of 18 and a mass number of 38.Letting P = the number of protons and N = the number of neutrons, which of the following atoms is the isotope of this atom?
A)P = 18, N = 20
B)P = 18, N = 21
C)P = 20, N = 18
D)none of the above

Question

Quizplus · Accepted Answer

The atomic number (number of protons) is 18, so option A and C are already ruled out since they have a different number of protons. 
The mass number (sum of protons and neutrons) is 38, so if we subtract the atomic number (18), we get the number of neutrons. Therefore, the number of neutrons for this atom is 20. 
Option B has the correct number of neutrons, which is 21 (18 + 21 = 38), so it is the correct isotope. Option D is incorrect since we have already identified an isotope in option B.

An Atom Has an Atomic Number of 18 and a Mass