Valency and Radicals — The Criss-Cross Method That Writes Any Chemical Formula

One Rule Writes Every Formula

Ask a Class 8 student to write the formula for aluminium oxide and most reach for memory. There is no need to memorise — there is one rule.

Valency is the combining capacity of an element or radical: how many hydrogen atoms (or their equivalent) one atom of it can combine with. It comes directly from the electrons in the outermost shell — the same shell that decides where an element sits in the periodic table and how badly it wants to complete its octet of eight.

A metal loses electrons to empty its outer shell, so its valency equals the electrons it gives away. A non-metal gains electrons to fill its outer shell, so its valency equals the electrons it takes. Once you know an atom's valency, you don't calculate a formula — you write it, in seconds, every time.

1. What is the formula for calcium phosphate, given calcium's valency is +2 and phosphate's is -3?

2. What is the formula for aluminium sulphate, given aluminium's valency is +3 and sulphate's is -2?

3. Which pair correctly matches the element with its fixed valency?

4. 'Ferrous' compounds correspond to which oxidation state of iron?

5. Which radical carries a -3 charge?

6. What is the correctly simplified formula for calcium oxide, given calcium is +2 and oxygen is -2?

7. What is the formula for ammonium carbonate, given ammonium (NH4) is +1 and carbonate (CO3) is -2?

8. Which suffix marks the radical with MORE oxygen atoms than its partner (e.g. sulphate vs sulphite)?