7.3.1 The noble gases

A full outer shell makes the noble gases inert

1. Helium, neon, argon, krypton and xenon all have a full outer shell of electrons.
2. A full shell is eight electrons, except for helium, which is full with two.
3. Reacting means gaining, losing or sharing electrons to reach a full shell.
4. A noble gas already has one, so it has nothing to gain from reacting.
5. They are therefore inert, and they exist as single atoms rather than as molecules.

Uses that depend on being inert

1. Argon fills filament light bulbs, where it stops the hot filament burning away.
2. Argon also shields the weld in arc welding, keeping air away from the hot metal.
3. Helium and argon protect delicate objects in storage from reacting with air.
4. An unreactive gas is chosen wherever a reaction with oxygen or nitrogen would cause damage.
5. Air would not do the job, because its oxygen reacts with hot or exposed material.

Uses that depend on low density and non-flammability

1. Helium has a very low density, so a balloon filled with it rises.
2. Helium is also non-flammable, which is why it replaced hydrogen in airships.
3. Hydrogen is lighter still, but it burns, which made it dangerous.
4. Neon glows red-orange when a current passes through it, which is used in display signs.
5. Each use is matched to a property, so the reason for the choice can always be named.

The trend in physical properties down the group

1. Boiling points rise steadily down the group, from helium at the top.
2. Helium boils at about $-269{}^{\circ }C$, the lowest of any element.
3. Density also rises down the group, so helium is the least dense and radon the most.
4. All the noble gases are gases at room temperature, despite the rise in boiling point.
5. The rise follows the same reason as in Group 7: larger atoms attract one another more strongly.

Predicting the properties of other noble gases

1. A gas lower in the group has a higher boiling point and a higher density than one above it.
2. Krypton therefore has a higher boiling point than argon and a lower one than xenon.
3. Radon, at the bottom, has the highest boiling point and the greatest density.
4. Every one of them stays inert, because they all have a full outer shell.
5. A prediction is made by placing the element in the group and reading the neighbouring values.