What is the correct chemical formula for calcium phosphate, given that calcium has a valency of 2 and phosphate has a valency of 3?
Ca3(PO4)2
The formula of an ionic compound is written by the criss-cross method: the valency of each ion becomes the subscript of the other ion, and the subscripts are then reduced to their simplest whole-number ratio.
Calcium is the ion Ca2+ with valency 2, and phosphate is the polyatomic ion PO43– with valency 3. Crossing over, calcium takes the subscript 3 and the phosphate group takes the subscript 2.
The ratio 3 : 2 cannot be reduced further, and since phosphate is a polyatomic group with more than one unit, it is placed in brackets. So the formula is Ca3(PO4)2.
Check: total positive charge = 3 × (+2) = +6 and total negative charge = 2 × (–3) = –6, so the compound is electrically neutral. Any other combination of subscripts leaves a net charge, for example 2 × (+2) with 2 × (–3) gives –2.
Hence, Ca3(PO4)2.
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