Often atoms join so that each atom will have a. an even number of electrons. b. an outermost energy level that is full of electrons. c. an equal number of protons and neutrons d. more electrons than either protons or neutrons

b. an outermost energy level that is full of electrons.

Explanation:

The reason why atoms combines is to have a complete outermost shell. When the outermost shell or valence shell of atoms are completely filled withe electrons, they become very stable. Noble gases for example, shows no tendency to react with other atoms because their outermost energy level is completely filled.

The tendency to have a completely filled outer energy level is what drives most atoms into bonding.