Ionic bonding
An ionic bond forms when a metal transfers electrons to a nonmetal. The metal becomes a cation, the nonmetal becomes an anion, and the opposite charges attract.
Ionic compounds are not separate molecules. The ions pack into a repeating pattern called a crystal lattice. The formula gives the simplest ratio of ions, called a formula unit.
Typical properties: hard and brittle solids with high melting points. They conduct electricity when melted or dissolved, because then the ions can move.
To write the formula, make the total positive charge equal the total negative charge.
Worked example: sodium and chlorine
Sodium loses 1 electron to form Na+. Chlorine gains 1 to form Cl−. One of each balances.
NaCl
Worked example: calcium and chlorine
Calcium forms Ca2+ and chlorine forms Cl−. It takes two 1- charges to balance one 2+.
CaCl2
A shortcut is to cross the charges: the size of each ion’s charge becomes the other ion’s subscript. For Al3+ and O2− this gives Al2O3. Reduce the subscripts if they share a factor.
Common mistake
Writing charges in the final formula, or forgetting to reduce. Mg2+ and O2− give MgO, not Mg2O2.