Writing Ionic Formulas

Atoms are most stable when they have a full outer shell of electrons (the Octet Rule). To achieve this, atoms will either lose or gain electrons, becoming charged particles called ions. Metals on the left side of the Periodic Table tend to lose electrons, forming positive cations. Non-metals on the right side tend to gain electrons, forming negative anions. The charge of an ion is determined by its Group number. For example, Group 1 elements always lose one electron to become $+1$, while Group 17 elements gain one to become $-1$. When these opposites attract, they form an ionic bond.

The golden rule of ionic compounds is that the total charge must be zero. To find the correct ratio of atoms without doing complex math, we use the Criss-Cross Method. You simply take the numerical value of the cation's charge and make it the subscript of the anion, and take the numerical value of the anion's charge and make it the subscript of the cation. This automatically balances the positive and negative forces. Remember: we don't write the number '1' as a subscript, and we always simplify the ratio if both numbers are divisible by the same factor (like $2:2$ becoming $1:1$).

Naming binary ionic compounds follows a strict set of IUPAC rules. The name always has two parts. First, write the name of the cation (the metal) exactly as it appears on the Periodic Table. Second, write the name of the anion (the non-metal), but change its ending to -ide. For example, Oxygen becomes Oxide, Sulfur becomes Sulfide, and Nitrogen becomes Nitride. Unlike molecular compounds, we do not use prefixes like 'di-' or 'tri-' because the charges already tell us how many atoms are there!