Group 18: noble gases and group comparisons

Neon and chlorine are both gases, but their reactivities differ greatly. Explain why a full outer shell matters, then compare the electron-loss, electron-gain and unreactive patterns across Groups 1, 17 and 18.

Use the electron arrangements from Periodic Table position and the trends from Group 1 and Group 17.

Group 18: the noble gases

Helium, neon and argon are the first three Group 18 elements. They are colourless gases, and they are very unreactive. The reason is their electron arrangement.

For the simple substances studied here, losing, gaining or sharing electrons often gives atoms a full outer shell. Noble gas atoms already have one, so they have almost no tendency to lose, gain or share electrons. They do not even bond to each other: a noble gas is made of single atoms.

Here is evidence of how unreactive they are. Argon makes up almost 1% of air, yet it was not identified until 1894. It was found by removing the oxygen and nitrogen from a sample of air with substances that react with them. A small volume of gas was always left over, because argon did not react with anything used.

Very unreactive does not mean that no compound can ever form. Chemists have made compounds of heavier noble gases, such as xenon, under suitable conditions. Helium, neon and argon form no compounds in ordinary conditions.

Guided practice 1

Why is neon unreactive?

About 4 min

Problem

A learner says neon is unreactive “because it is a gas”. Chlorine is also a gas. First compare their outer shells. In the next activity, use the electron arrangements to give the complete reason for their different reactivities.

Compare the two outer shells

Neon (2,8) has

Full outer shellCorrectIncorrectTwo electrons shortCorrectIncorrectOne electron shortCorrectIncorrect

Chlorine (2,8,7) has

Full outer shellCorrectIncorrectOne electron shortCorrectIncorrectTwo electrons shortCorrectIncorrect

Check answer

Hints

Hint 1

Test the claim

If being a gas made an element unreactive, chlorine would be unreactive too.

Hint 2

Why atoms react

Atoms react by losing, gaining or sharing electrons to reach a full outer shell.

Show solution step by step

1. Reject the state explanation

   Method

   Being a gas does not decide reactivity.

   Reason

   Chlorine is a gas and is very reactive.

   Working

   Answer

   Same state, opposite behaviour.

2. Use the electron arrangement

   Method

   Neon’s outer shell is already full.

   Reason

   It has no tendency to lose, gain or share electrons, so it is very unreactive.

   Working

   Answer

   Neon 2,8: full outer shell → very unreactive. Chlorine 2,8,7: one short → reactive.

Quick check

For helium, the full shell holds two electrons, not eight. The rule is “full outer shell”, not “eight electrons”.

Check your understanding 1 Give the reason beyond the choices

Neon (2,8) and chlorine (2,8,7) are both gases. Explain why neon is very unreactive while chlorine reacts readily. Work on paper or in the optional notes.

Optional working or comparison notes

Show answer

Neon already has a full outer shell, so it has almost no tendency to lose, gain or share electrons under ordinary conditions. Chlorine is one electron short of a full outer shell and can gain or share an electron in reactions. Being a gas cannot be the explanation: the gases have different reactivities because their electron arrangements differ.

Check your understanding 2 Two outer electrons are not always a full shell

Helium has arrangement 2; magnesium has 2,8,2. State whether each outer shell is full and predict the signed charge on the usual magnesium ion.

Helium’s outer shell

Not fullCorrectIncorrectFullCorrectIncorrect

Magnesium’s outer shell

Not fullCorrectIncorrectFullCorrectIncorrect

Signed magnesium ion charge

Check answer

Show answer

Helium’s first shell is full with two electrons. Magnesium’s third shell is not full; losing its two outer electrons gives Mg²⁺ with arrangement 2,8. The next activity asks for the complete comparison.

Check your understanding 3 Two outer electrons, different behaviour

Helium has electron arrangement 2; magnesium has 2,8,2. Both have two outer electrons. Should they have the same tendency to form ions? Explain using the shells rather than the outer-electron count alone.

Write your explanation on paper or in the optional notes and compare the shells, electron change and ion charge.

Optional working or comparison notes

Show answer

No. Helium’s first and only shell is already full with two electrons, so it has almost no tendency to form ions. Magnesium’s third shell is not full: it can lose its two outer electrons to give Mg²⁺, with arrangement 2,8. The same outer-electron count does not mean the shells are equally full.

Three groups, three patterns

Do not learn one rule such as “reactivity increases down a group” and apply it to every group. Work out whether the atoms lose or gain electrons first.