In which situation would an atom most likely become an anion?

An atom most likely becomes an anion when it gains electrons. An anion is defined as a negatively charged ion, which forms when an atom accumulates extra electrons beyond its neutral state.

In a neutral atom, the number of protons is equal to the number of electrons, which balances the positive and negative charges. When an atom gains electrons, it increases its negative charge, leading to the formation of an anion. This process often occurs with nonmetals, which have high electronegativity and a tendency to attract electrons.

The other scenarios presented do not lead to the formation of an anion. For instance, losing electrons would result in a positively charged ion, known as a cation. Having equal numbers of protons and electrons indicates that the atom remains neutral, and having a full outer shell generally refers to stability and does not imply any charge change on its own. Thus, gaining electrons is the definitive action that transforms an atom into an anion.