What is valency?
Valency is the combining capacity of an atom of an element. It is measured relative to hydrogen.
The definition
The valency of an element is the number of hydrogen atoms that can combine with one atom of that element, or be replaced by it.
The electrons in the last energy level of an atom are called the valence electrons, and the number of them is generally equal to the maximum valency of the element. Some elements can have more than one valency.
Three ways to count the same thing
In a chemical combination, the valency of an element equals:
- the number of electrons removed from one of its atoms, or
- the number of electrons gained by one of its atoms, or
- the number of electron pairs shared between the atoms.
Symbols and formulae
We already use chemical symbols to name elements briefly — C for carbon, K for potassium, Ca for calcium, S for sulphur. We use a matching scheme for compounds.
Water is made of two hydrogen atoms and one oxygen atom, and we write it H2O. That is the chemical formula of water.
Reading a formula
A small number written below and to the right of a symbol is the number of atoms of that element in one molecule of the compound. No number means there is one atom of it.
Glucose is C6H12O6 — 6 carbon, 12 hydrogen and 6 oxygen atoms in one molecule.
Be careful
Not every formula represents a molecule. Solid sodium chloride — table salt — has no separate molecules in it. It is an ionic lattice of alternating Na+ and Cl− ions, and because those ions are in a 1 : 1 ratio its formula is written NaCl.