Final answer:
After an atom gains an electron during bond formation, it acquires one negative charge, becoming an anion, like chloride which forms with a -1 charge.
Step-by-step explanation:
When an atom gains an electron during the formation of a bond, it becomes a negatively charged ion called an anion. The charge on an atom after it gains one electron is one negative charge. This is because the number of electrons has become greater than the number of protons in the atom. For example, when chlorine, which normally has seven electrons, gains an extra electron, it forms a chloride ion with a -1 charge (Cl-). These electrons are gained or lost through a process called electron transfer, which often leads to the formation of ionic bonds between positively (cation) and negatively charged ions (anion).