Valency of elements
Valency is the number of chemical bonds that an atom of a given element can form with other atoms. It is written with Roman numerals: I, II, III, IV…
This is one of the first concepts in chemistry in grade 7 — and one of the most frequently memorized without understanding. Unnecessarily, because the principle is simple.
An atom is a block with connectors
Imagine that each atom is a block with a specific number of protrusions. Blocks connect only when all protrusions have a pair:
| Element | Valency | Which means a block with… |
|---|---|---|
| Hydrogen (H) | I | one connector |
| Chlorine (Cl), Fluorine (F) | I | one connector |
| Oxygen (O) | II | two connectors |
| Sulfur (S) | II (also IV and VI) | two connectors |
| Nitrogen (N) | III | three connectors |
| Phosphorus (P) | III (also V) | three connectors |
| Carbon (C) | IV | four connectors |
And metals that most often appear in problems:
| Element | Valency |
|---|---|
| Sodium (Na), Potassium (K) | I |
| Calcium (Ca), Magnesium (Mg), Zinc (Zn) | II |
| Aluminum (Al) | III |
| Iron (Fe) | II or III |
| Copper (Cu) | I or II |
Why is valency important at all?
Because it tells you in what proportions atoms combine into molecules. A few examples:
- Water H₂O — oxygen has two connectors, hydrogen has one each. Oxygen will therefore accept exactly two hydrogens. Not one, not three — two.
- Ammonia NH₃ — nitrogen has three connectors, so it combines with three hydrogens.
- Methane CH₄ — carbon has four connectors, so it takes four hydrogens.
- Carbon dioxide CO₂ — carbon (IV) combines with two oxygens (II each), and each of these bonds is double.
Common mistake: treating formulas as random combinations to memorize. Meanwhile, the formula always results from valency — if you know the number of connectors, the formula can be derived independently, even if you've never seen it before.
Where does valency come from?
From the atom's structure. Atoms strive to have their outer electron shell full. Hydrogen has one electron to "complete a pair" — hence one bond. Oxygen needs two electrons to complete its set — hence two bonds. Carbon can share four electrons — hence four. In grade 7, it's enough to remember the connector rule itself; the electronic explanation comes later and only confirms it.
How to solidify this without memorizing?
The fastest way — by building. On Cząsteczki.pl, each atom has visible empty spaces for bonds, exactly as many as its valency. If you try to connect atoms inconsistently with the rule, they simply won't connect. After a few molecules, the numbers I, II, III, IV are remembered automatically — because you've seen them in action.