Chemical Bonds

Science — First Preparatory

Writing the formula of an ionic compound

We write the formula so that the total positive charge equals the total negative charge:

Comparison aspect	Main idea	Keywords
Magnesium Mg2+ and chloride Cl−	two chloride ions are needed for each magnesium ion so the formula is MgCl₂. Total charge: 2 + (−1 × 2) = 0	ionsneededformulaMgCl₂
Aluminium Al3+ and oxide O2−	the least common multiple of 3 and 2 is 6 so we need 2 aluminium ions and 3 oxide ions. The formula is Al₂O₃. Total charge: 2 × 3 = 6 and 3 × 2 = 6	leastcommonmultipleneed

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