Unleash Your Inner Scientist: NOVA Hunting the Elements Practice Test 2026 – Ready, Set, Discover!

When atoms gain or lose electrons to reach a stable electron arrangement, they form ions. Chlorine has seven valence electrons, so it only needs one more to complete its outer shell of eight. Gaining that electron adds a negative charge, producing a negatively charged ion known as an anion (specifically, chloride, Cl−). This is a common pattern for halogens: they tend to gain one electron to form -1 ions.

A cation would come from losing electrons, giving a positive charge, which isn’t what chlorine does when it gains one electron. A neutral atom would have the same number of protons and electrons, with no net charge, which isn’t the result here. A radical would have an unpaired electron, which isn’t the usual outcome when chlorine gains a single electron to fill its octet.