Learning Objectives Describe how an ion is formed. Distinguish the difference between the two types of ions. Ions As introduced previously, atoms contain a nucleus with neutrons and positively charged protons, surrounded by negatively charged electrons. In an atom, the total number of electrons, negative charge, equals the total number of protons, positive charge, and therefore, atoms are electrically neutral or uncharged. If an atom loses or gains electrons, it will become a positively or negatively charged particle, called an ion . The loss of one or more electrons results in more protons than electrons and an overall positively charged ion , called a cation . For example, a sodium atom with one less electron is a cation, Na + , with a +1 charge (Figure \(\PageIndex{1}\)). When an atom gains one or more electrons, it becomes a negatively charged a nion , because there are more electrons than protons. When chlorine gains one electron it forms a chloride ion, Cl – , with a –1 charge (Figures \(\PageIndex{2}\)) The names for positive and negative ions are pronounced CAT-eye-ons (cations) and ANN-eye-ons (anions), respectively. Note Naming Ions Cations are named using the element name plus " ion " to indicate it is charged. Anions are named by changing the element name ending to " ide ". For example, a magnesium ion is formed when neutral magnesium loses electrons and a fluor ide ion is formed when neutral fluorine gains electrons. Example \(\PageIndex{1}\) A calcium (Ca) atom loses two electrons and a sulfur (S) atom gains two electrons. Determine if the resulting ions are cations or anions? Write the ion symbols for each. Solution When calcium ( Z = 20) gains two electrons the resulting ion will have 18 electrons and 20 protons and therefore a charge of +2 (there are two more positive protons than negative electrons), it is a cation. The symbol for a calcium ion is Ca 2 + . When sulfur ( Z = 16) gains two electrons the resulting ion will have 18 electrons and 16 protons and therefore a charge of –2 (there are two more negative electrons than positive protons), it is an anion. The symbol for a sulf ide ion is S 2 – . Key Takeaways Ions are formed when atoms gain or lose electrons. Ions can be positively charged (cations) or negatively charged (anions). Contributors Lisa Sharpe Elles, University of Kansas