When an atom gains an electron, what is it called?

When an atom gains an electron, it becomes a negative ion, also known as an anion. This occurs because the addition of an electron results in an increase in the overall negative charge of the atom, which was initially neutral with equal numbers of protons (positively charged) and electrons (negatively charged).

As the atom gains an electron, the balance of charges is altered, leading to a net negative charge. This transformation changes the charging state of the atom, which is fundamental in various chemical interactions and reactions. Understanding this concept is crucial in fields such as chemistry and physics, as it helps explain ion formation and the behavior of substances in solution, among other phenomena.

The other terms provided do not correctly describe the situation where an atom gains an electron. For example, a positive ion results from the loss of electrons and is not relevant here. Isotopes refer to atoms of the same element that contain different numbers of neutrons, which also does not pertain to the charge state upon gaining electrons.