Formula Factory
Use valencies to write correct chemical formulae.
What this lesson covers
Why it matters
Imagine atoms as puzzle pieces with specific connectors. You cannot force a square peg into a round hole; they only snap together in fixed, predictable patterns.
Predict first
If Hydrogen has a valency of 1 and Oxygen has a valency of 2, how many Hydrogen atoms are needed to satisfy one Oxygen atom?
Oxygen needs two bonds to be stable. Since each Hydrogen provides only one bond, you need two Hydrogens to fill Oxygen's two slots.
- One (HO)
- Two (H₂O) — correct
- Three (H₃O)
What you do
Use each element's combining power (valency) to build the formula — the valencies must balance or the pieces won't snap. Build H₂O, then NaCl, then MgCl₂.
Check yourself
Why is the formula for water written as H₂O and not HO?
Chemical formulae show the fixed ratio of atoms. Oxygen (valency 2) needs two Hydrogens (valency 1) to share electrons equally.
In the formula CO₂, what does the subscript '2' indicate?
Chemical formulae show the fixed ratio of atoms. Oxygen (valency 2) combines with two hydrogens (valency 1), so the valencies balance in H₂O.
- Because Hydrogen is lighter than Oxygen
- Because Oxygen has a valency of 2 and requires two Hydrogens (valency 1) to balance — correct
- Because water is a liquid
- There are two Carbon atoms
- There are two Oxygen atoms bonded to one Carbon atom — correct
- The molecule weighs twice as much as Carbon