Question

Atom A has an atomic number of 19 and mass number of 40. Atom B has an atomic number of 20 and a mass number of 40. Which of these is an accurate statement?

A) Atom A has more protons than Atom B.
B) Atom A has one less proton and one more neutron than Atom B.
C) Atom A has an extra nucleon compared to Atom B.
D) Atom A has one more proton and one more electron than Atom B.

Answer 1

The atomic number (Z) is the number of protons. The mass number (A) is the number of nucleons. Hence, A is the sum of Z and the number of neutrons (N): A = N + Z and N = A - Z. So both atoms have 40 nucleons, atom A has 19 protons and 21 neutrons, atom B has 20 protons and 20 neutrons, and only statement B) is accurate.