Valency and the outer shell

The outer shell decides

When two atoms meet, the electrons in the outermost shell come close to each other. So these electrons decide how an atom behaves. They are the valence electrons, and the outermost shell is the valence shell.

Stable atoms

The atom of helium has $2$ electrons in its only shell, which is full. The atom of neon has $2, 8$, and argon has $2, 8, 8$. In each of these, the outer shell is full. These gases are very unreactive. They are the noble gases (or inert gases), and their valency is zero.

• A full K shell, with $2$ electrons, is called a duet.
• An outer shell with $8$ electrons is called an octet.

Other atoms are not stable. They try to reach a full outer shell, like the nearest noble gas. They do this by losing, gaining or sharing electrons.

Valency from the outer shell

The valency of an atom is the number of electrons that it loses, gains or shares to get a full outer shell.

• If the outer shell has 1, 2 or 3 electrons, the atom usually loses them. The valency is the number of electrons in the outer shell. Example: sodium $2, 8, 1$ has a valency of $1$.
• If the outer shell has 5, 6 or 7 electrons, the atom needs a few more to make $8$. The valency is 8 minus the number of electrons. Example: chlorine $2, 8, 7$ has a valency of $8 - 7 = 1$.
• If the outer shell has 4 electrons, the valency is $4$. Example: carbon $2, 4$. It shares electrons.
• If the outer shell is full, the valency is zero.

You should find that the valency climbs to $4$ and falls back to $0$ along each row, ending at a noble gas.

Metals and non-metals

The atoms of metals usually have few valence electrons, and they lose them to become positive ions. The atoms of non-metals usually have more valence electrons, and they gain or share electrons. This is how the ions of the last chapter are formed. Sodium loses one electron and becomes $\text{Na}^+$ with the arrangement of neon. Chlorine gains one electron and becomes $\text{Cl}^-$ with the arrangement of argon.

Practise with new numbers

Try it in Python

This program finds the valency from the arrangement.

Remember these

• Valence electrons are the electrons in the outermost shell.
• A full outer shell ($2$ or $8$) is stable. The valency is zero.
• Valency is the number of valence electrons if it is $4$ or less, and $8$ minus that number if it is more than $4$.

Check yourself

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

1. What is the valency of an atom with the arrangement $2, 8, 5$?

2. Why are the noble gases unreactive?

3. What ion does lithium, $2, 1$, form?

4. Carbon has the arrangement $2, 4$. Why does it share electrons instead of gaining or losing them?