What can be determined if only the atomic number of an atom is known? 1) the total number of neutrons
2) total number of protons
3) sum of protons and neutrons
4) sum of neutrons and electrons

2) total number of protons

Helium has an atomic number of one, and has only one proton in its nucleus.

Oxygen has an atomic number of 8 and an atomic mass of 16. From this we know that the atomic number measures the number of protons, and the atomic mass is the overal sum of protons and neutrons.

2.) The total number of protons

Explanation:

The definition of atomic number is the total number of protons in the nucleus of an atom.

Hope I helped

a is the correct answer