If an atom has a mass number of 23, which of the following is always true? a. the number of protons in the nucleus does not equal the number of neutrons. b. the atom is not an isotope c. the atom has an odd number d. the atomic number is less than 11

The correct answer will be A;

A) The number of protons in the nucleus does not equal the number of neutrons.

A=mass number:
Z=atomic number (= number of protons)
N=number of neutrons:

A=Z+N

If the number of protons in the nucleus is equal the number of neutrons , we would have an even mass number; because Z=N=x; then A=x+x=2x  (this is always an even number) but  23 is an odd number, therefore the number of protons in the nucleus does not equal the number of neutrons.