Why does an atom become negative when it gains an electron?

An atom becomes negative when it gains an electron because, normally, the number of protons (positively charged particles) and electrons (negatively charge particles) is even, making the atom neutrally charged. However, when an electron is gained, the balance of positively and negatively charged particles is disrupted. There is now more negatively charged particles than positively charge, so the atom becomes on anion, or negatively charged ion.