Question

what is the mass number? * the number of protons in the nucleus. the number of protons and neutrons in the nucleus. the number of neutrons in the nucleus. the number of protons and electrons in the atom.

Explanation:

The mass number of an atom is defined as the sum of the number of protons and neutrons in the nucleus. Electrons have negligible mass compared to protons and neutrons, so they are not included in the mass number calculation. The number of protons alone is the atomic number. The number of neutrons alone is just the neutron count.

Answer:

the number of protons and neutrons in the nucleus.