Valency and writing chemical formulae

The combining power

An atom of hydrogen joins with one atom of chlorine in HCl. An atom of oxygen joins with two atoms of hydrogen in $\text{H}_2\text{O}$. Different atoms have different combining capacities. This capacity is called the valency.

The valency of an ion is the same as its charge, without the sign. So $\text{Na}^+$ has a valency of $1$, $\text{Mg}^{2+}$ has $2$, and $\text{Al}^{3+}$ has $3$. For non-metals, the valency is the number of electrons that the atom takes, or shares.

Some valencies

Some elements have more than one valency. Iron can have $2$ or $3$, and copper $1$ or $2$.

The criss-cross method

To write the formula of a compound:

1. Write the symbols of the two parts, with the positive one first.
2. Write the valency of each below its symbol.
3. Cross over the valencies: the valency of the first becomes the number of atoms of the second, and the other way round.
4. Simplify the numbers if they have a common factor, and do not write the number $1$.
5. A polyatomic ion goes in brackets if its number is more than $1$.

Example 1: aluminium oxide. Al has a valency of $3$, and O has $2$. Cross over: $\text{Al}_2\text{O}_3$.

Example 2: magnesium chloride. Mg has $2$, and Cl has $1$. Cross over: $\text{Mg}_1\text{Cl}_2$, which we write as $\text{MgCl}_2$.

Example 3: calcium carbonate. Ca has $2$, and $\text{CO}_3$ has $2$. Cross over: $\text{Ca}_2(\text{CO}_3)_2$. Simplify by $2$: $\text{CaCO}_3$.

Example 4: calcium hydroxide. Ca has $2$, and OH has $1$. Cross over: $\text{Ca(OH)}_2$. The OH is in brackets, because there are two groups of it.

Check the formula

The total of the charges must be zero. In $\text{Al}_2\text{O}_3$: $2 \times (+3) + 3 \times (-2) = +6 - 6 = 0$. Yes.

You should find that valencies $2$ and $2$ give MX, not $\text{M}_2\text{X}_2$: crossing over is always followed by simplifying.

Practise with new numbers

Try it in Python

This program writes formulas with the criss-cross method.

Remember these

• The valency is the combining capacity of an atom or an ion. It is equal to the charge without the sign.
• Criss-cross: write the symbols and valencies, cross the valencies over, simplify, and use brackets for a polyatomic ion.
• The total charge of the formula must be zero.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Write the formula of zinc chloride. Zinc has a valency of $2$ and chlorine $1$.

2. Write the formula of aluminium nitrate. Aluminium has a valency of $3$ and the nitrate group $1$.

3. Check the formula $\text{Na}_2\text{SO}_4$ by adding the charges.

4. In $\text{Fe}_2\text{O}_3$, what is the valency of iron?