Lesson 7 of 11
Valency
Show on a big screenOxygen holds two hydrogen atoms in water. This number, 2, is its valency.
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The rule
Each element here has one usual combining number. This number is called its valency. Two elements' valencies decide a formula's counts.
Think of valency as hooks. An atom with valency 2 has 2 hooks. Each hook joins one hook of another atom.
Step 1 of 3
- Hydrogen has one hook. Oxygen has two hooks.
- Two hydrogen atoms fill oxygen's two hooks.
- Every hook is used. The molecule is H₂O.
Valencies to know
| Element | Symbol | Valency |
|---|---|---|
| Hydrogen | H | 1 |
| Sodium | Na | 1 |
| Chlorine | Cl | 1 |
| Potassium | K | 1 |
| Oxygen | O | 2 |
| Magnesium | Mg | 2 |
| Calcium | Ca | 2 |
| Aluminium | Al | 3 |
| Nitrogen | N | 3 |
| Carbon | C | 4 |
For an ion, the valency is the number in its charge. Na⁺ has valency 1. Mg²⁺ has valency 2. Cl⁻ has valency 1. O²⁻ has valency 2.
A few elements, such as iron and copper, have two valencies.
Sodium has one hook. Chlorine has one hook. One of each fills every hook. That gives NaCl.
Carbon has four hooks. Each hydrogen has one hook, so four hydrogen atoms fill them. That gives methane, CH₄.
Try the lab to finish this lesson.