The Combining Power
Understand valency through hydrogen and oxygen
Setting up the bench…
What this lesson covers
Why it matters
Hydrogen holds out one hand; oxygen holds out two. A single hydrogen can never satisfy oxygen on its own — so two of them must join in. That 'number of hands' is valency.
Predict first
If one hydrogen atom links to one oxygen atom, what is the result?
The valencies criss-cross so every hand is held: H (1) with O (2) gives H₂O; Mg (2) with O (2) gives MgO; Al (3) with Cl (1) gives AlCl₃.
- Unstable — oxygen still has one hand free, so it needs a second hydrogen (H₂O) — correct
- Stable HO, the formula of water
- Nothing — hydrogen and oxygen cannot combine
What you do
Use combining power to build H₂O, then MgO, then AlCl₃. Notice how the valencies criss-cross into the subscripts.
Check yourself
Why is water written H₂O and not HO?
One O (valency 2) is satisfied by two H atoms (valency 1 each) → H₂O.
Aluminium (valency 3) combines with chlorine (valency 1) to form…
Al's three hands each hold one Cl → AlCl₃.
- Oxygen's valency is 2, so it needs two hydrogens (valency 1 each) — correct
- Hydrogen is twice as heavy as oxygen
- Two waters are always written together
- AlCl₃ — correct
- AlCl
- Al₃Cl