What is an anion?

An anion is defined as a negatively charged ion that forms when an atom or molecule gains one or more electrons. This process occurs because atoms strive to achieve a more stable electronic configuration, typically resembling that of the nearest noble gas. When an atom gains electrons, it has more negatively charged particles than positively charged protons, resulting in a net negative charge. Common examples of anions include chloride (Cl⁻) and sulfate (SO₄²⁻).

Understanding what distinguishes anions from cations is important. Cations are positively charged ions formed by losing electrons, while anions counterbalance this by gaining electrons, leading to the formation of ionic compounds when they pair with cations. This distinction highlights the important role that electron gain or loss plays in defining the nature of ions in chemistry.