You have an atom with 3 protons, 4 neutrons, and 3 electrons. adding a neutron will ...

Answer: Add to your atomic mass

Explanation: Atomic mass is made up of the protons and neutrons, so your atomic mass right now is 7. Adding a neutron will make the atomic mass 8.

Convert into an atom of lithium-8.

Explanation:

Your nucleus has three protons and four neutrons, so you have an atom of lithium-7.

If you add a neutron, you are creating an isotope of lithium with one more unit of mass.

That is, you are forming an atom of lithium-8.