Question

which of the choices is the unique, defining property of atoms of a particular element?
○ the number of protons
○ the number of neutrons
○ the number of electrons
○ the number of protons plus the number of neutrons

Explanation:

The number of protons (atomic number) uniquely identifies an element. Neutrons vary in isotopes, electrons change in ions, and proton-neutron sum (mass number) is not unique to an element. Thus, protons are the defining property.

Answer:

the number of protons