Ions form when atoms gain or lose electrons. A group−1 atom commonly loses one electron to form a+1 ion; group 2 commonly forms +2 ions. Group 17 commonly gains one electron to form −1 ions, while group 16 often forms −2 ions. Proton counts remain unchanged. These rules apply to common main-group ions; other elements can have different or variable charges.
An ionic compound contains positive and negative ions in a ratio giving zero total charge. Sodium ions, Na+, and chloride ions, Cl−, combine 1:1 as NaCl. Magnesium ions, Mg2+, require two chloride ions, giving MgCl2. The formula reports the simplest whole-number ion ratio rather than an isolated molecule. Use the ion charges supplied in a question and check that their positive and negative totals cancel.
Example 1
Combine Al3+ and O2− into a neutral formula.
Two Al3+ ions give +6; three O2− ions give −6. The simplest neutral ratio is 2:3, so the formula is Al2O3.
Example 2
A neutral magnesium atom has 12 electrons. How many are in Mg2+?
A+2 ion has lost two electrons: 12−2=10. The nucleus still has 12 protons.
Watch out: It has usually lost two electrons. Its unchanged proton count means it is still the same element.