What is the mass number of an atom of lithium, which has 3 protons, 4 neutrons, and 3 electrons?

The mass number of an atom is calculated by adding the number of protons and neutrons in its nucleus. In the case of lithium, it has 3 protons and 4 neutrons. Therefore, the calculation for the mass number would be:

Mass number = Number of protons + Number of neutrons

Mass number = 3 protons + 4 neutrons

Mass number = 7

This indicates that the correct answer is 7, which represents the total mass contribution from both protons and neutrons. It's important to note that the number of electrons does not affect the mass number, as mass number is solely determined by protons and neutrons. Thus, the mass number providing a definitive identity of the lithium atom in this scenario is indeed 7.