Full outer shells are calm

The idea

Atoms with a complete octet (8 electrons, or 2 for helium) in the valence shell are largely unreactive and stable; atoms with incomplete valence shells are more reactive and lose, gain or share electrons to complete their octet.

Some atoms keep to themselves

Some elements hardly take part in changes with other atoms. Others join in readily.

Which atom would you expect to be the most unreactive?

• One with a complete outermost shell
• One with a very small nucleus
• One with the most neutrons

Stable or reactive?

Sort each atom by its electronic configuration.

The octet

Eight electrons in the outermost shell make an octet. For helium, two electrons are enough. Atoms with incomplete valence shells lose, gain or share electrons to complete their octet.

An atom with a complete octet (8 electrons, or 2 for helium) in its valence shell is largely unreactive and stable. Atoms with incomplete valence shells are usually more reactive, and lose, gain or share electrons to complete their octet.

Notes

A complete octet (8 electrons, or 2 for helium) in the valence shell makes an atom stable and largely unreactive. Incomplete shells make atoms more reactive: they lose, gain or share electrons.

Check yourself

Which atom has a complete octet in its outermost shell?

Why is helium stable with only 2 electrons?

What do atoms with incomplete valence shells do?

Argon has the configuration 2, 8, 8. How reactive is it?

• Oxygen, 2, 6. Oxygen has 6 electrons in its outermost shell.
• Neon, 2, 8 — correct. Yes!
• Sodium, 2, 8, 1. Sodium has 1 electron in its outermost shell.
• It has 8 electrons in its outermost shell. Helium has just 2 electrons in total.
• It has no electrons at all. A helium atom has 2 electrons.
• Its first shell, the only shell it has, is complete with 2 electrons — correct. Yes!
• Lose, gain or share electrons to complete their octet — correct. Yes!
• Stay exactly as they are. Atoms with incomplete valence shells are usually more reactive.
• Lose neutrons from the nucleus. The change is in the electrons of the valence shell.
• Very reactive, since its M shell can hold 18. When the M shell is the outermost shell, 8 electrons already make a complete octet.
• Largely unreactive, since its outermost shell has 8 electrons — correct. Yes!
• Very reactive, since it has 18 electrons. Argon's outermost shell has a complete octet of 8, so it is largely unreactive.