3.2.2—Ionic bonding, e.g. sodium chloride, magnesium

Use a fixed charge-balance method: identify the metal and non-metal, infer the ions formed from their outer electrons or group positions, write each ion charge, and choose the smallest whole-number combination whose total charge is zero.

For sodium chloride, Na⁺ and Cl⁻ combine 1:1 to give NaCl. For magnesium oxide, Mg²⁺ and O²⁻ combine 1:1 to give MgO. For calcium fluoride, Ca²⁺ requires two F⁻ ions, giving CaF₂. In every case, electron transfer creates the ions and electrostatic attraction holds the lattice together.

Check the formula by adding the ionic charges after writing the subscripts: the total must be zero. The subscript counts ions in the formula unit; it does not change the charge of an individual ion. The same method supports the SME examples Li₃N and Al₂O₃.

Do not write subscripts before balancing charge, confuse charge magnitude with the number of atoms, or treat the formula as a discrete molecule when it represents an ionic lattice. Detailed lattice-energy calculations and broader property explanations are outside this card’s objective.