The Combining Power

Why it matters

Why is table salt NaCl but magnesium chloride MgCl₂? Atoms have a combining power — valency — and formulae only click together when the powers match.

Predict first

Magnesium's valency is 2 and chlorine's is 1. The formula of magnesium chloride must be…

Mg²⁺ needs two Cl⁻ to cancel its charge — one chloride is only half the job. The criss-cross rule does this automatically.

• MgCl
• MgCl₂ — correct
• Mg₂Cl
• Mg₂Cl₂

What you do

Build all three: NaCl, MgCl₂, AlCl₃. Watch the charges criss-cross into subscripts — and notice you cannot build a wrong formula.

Check yourself

Valency is best described as…

Valency counts how many bonds/charges an atom brings to a partnership — Na brings 1, Mg brings 2, Al brings 3.

The formula of calcium chloride is…

Ca is 2+, Cl is 1− — criss-cross gives CaCl₂. Two chlorides cancel one calcium.

Why is the compound MgO and not Mg₂O₂?

2+ and 2− criss-cross to Mg₂O₂, then divide by the common factor 2 → MgO. A formula is a ratio, not a head-count.

• the charge on the nucleus
• an atom's combining power with other atoms — correct
• the number of shells in the atom
• the atom's mass number
• CaCl
• Ca₂Cl
• CaCl₂ — correct
• Ca₂Cl₂
• Mg₂O₂ would be radioactive
• Formulae always use the simplest whole-number ratio — correct
• Oxygen can only ever bond once
• It is just tradition