Question 10
The carbonate ion is CO32−. What is its valency?
The right answer is the one marked above.
2 — read straight off the charge in the top right of the formula.
That rule holds for every polyatomic ion in the table. A single charge means valency 1: ammonium NH4+, nitrate NO3−, hydroxide OH−. A 2− charge means valency 2: sulphate, chromate, carbonate. And phosphate PO43− has a valency of 3.