Combining Power
What is valency and why it matters
What this lesson covers
Why it matters
Imagine hydrogen is a single-handed handshake. Some elements need two hands, some three, to hold on tight.
Predict first
Why does oxygen need two hydrogen atoms to form water (H₂O)?
Valency is the number of hydrogen atoms an element can combine with. Oxygen grabs two.
- Oxygen is heavier than hydrogen
- Oxygen has a valency of 2, needing two bonds to be stable — correct
- Hydrogen atoms are too small to fill oxygen alone
What you do
Match atoms by their valency (combining power) to build each target — the valencies decide the subscripts (H=1, O=2, Cl=1, Mg=2). Build HCl, H₂O, then MgCl₂.
Check yourself
If Magnesium (Mg) has a valency of 2 and Chlorine (Cl) has a valency of 1, what is the formula?
Mg needs 2 bonds; each Cl provides 1. You need two Cl atoms to satisfy one Mg atom.
Why is the valency of Hydrogen defined as 1?
Valency is a reference standard. Since H consistently forms one bond, it is the unit of measurement.
- MgCl
- Mg₂Cl
- MgCl₂ — correct
- It is the lightest element
- It forms only one bond in stable compounds like H₂O and HCl — correct
- It has only one proton