How ionic compounds form

Why do metals and non-metals join?

You know that atoms with a full outer shell, such as neon and argon, are stable. Other atoms try to reach this arrangement by gaining or losing electrons.

Sodium and chlorine

• Sodium, with the arrangement $2, 8, 1$, has one electron in its outer shell. It is easier to lose this one than to gain seven.
• Chlorine, with the arrangement $2, 8, 7$, has seven in its outer shell. It is easier to gain one than to lose seven.

When they meet, sodium gives one electron to chlorine.

• Sodium becomes the sodium ion, $\text{Na}^+$, with the arrangement $2, 8$ (like neon).
• Chlorine becomes the chloride ion, $\text{Cl}^-$, with the arrangement $2, 8, 8$ (like argon).

The two ions have opposite charges, so they attract each other with a strong force. This force is the ionic bond, or electrovalent bond.

More examples

Magnesium chloride. Magnesium ($2, 8, 2$) loses two electrons to become $\text{Mg}^{2+}$. Each chlorine takes one. So two chloride ions are needed for each magnesium ion: $\text{MgCl}_2$.

Magnesium oxide. Magnesium loses two electrons. Oxygen ($2, 6$) gains two. One of each: $\text{MgO}$.

Aluminium oxide. Aluminium ($2, 8, 3$) loses three, and oxygen gains two. For equal charges, $2$ aluminium ions ($+6$) go with $3$ oxide ions ($-6$): $\text{Al}_2\text{O}_3$.

You should find that the two bars are always equal: the formula is whatever makes the charges cancel.

An ionic compound is neutral

In every formula, the positive and the negative charges cancel. We can check: $\text{Al}_2\text{O}_3$ has $2 \times (+3) + 3 \times (-2) = 0$.

Ionic compounds come from metals and non-metals

The element that loses electrons is a metal, and the one that gains is a non-metal. So ionic compounds are formed between metals and non-metals. Compounds that are formed between non-metals share electrons instead, and they are not ionic.

Practise with new numbers

Try it in Python

This program works out the ionic compound from the atomic numbers of a metal and a non-metal.

Remember these

• A metal atom loses electrons and becomes a positive ion. A non-metal atom gains electrons and becomes a negative ion.
• The ionic bond is the attraction between the opposite ions. Each ion has the arrangement of a noble gas.
• The total positive charge equals the total negative charge.

Check yourself

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

1. How many electrons does an aluminium atom lose when it forms an ion?

2. What is the electron arrangement of the chloride ion?

3. Why is the formula of calcium fluoride $\text{CaF}_2$?

4. Why do two non-metals not form an ionic bond?