An atom that gains electrons will have a negative charge.
Atoms are composed of three types of particles of different charges: protons (positive), neutrons (neutral), and electrons (negative).
When an atom has the same number of protons and electrons, the positive and negative charges cancel out and the atom has a net charge of zero. Many atoms have zero charge in their base state. But energetic reactions can knock this balance out of whack, and some atoms are especially prone to becoming unbalanced (ionized) due to the way electrons sort themselves into valence shells. When this happens, the atom will carry a charge—a positive charge if the ion doesn't have enough electrons to balance the protons in its nucleus, and a negative charge if it has excess electrons beyond its atomic number.