An atom of lithium contains 3 electrons, 3 protons and 4 neutrons. Its mass number is:

7

Reason — The mass number of an atom is the total number of protons and neutrons present in its nucleus. In a lithium atom, there are 3 protons and 4 neutrons, so the mass number is 3 + 4 = 7.

Therefore, the mass number of lithium is 7.