Course 9.3Lesson 2 of 3

Group 17 and group 18: halogens and noble gases

Group 17 has the halogens, which are non-metals with seven outer electrons and valency 1. They get darker and less reactive down the group, and a more reactive halogen displaces a less reactive one from its salt. Group 18 has the noble gases, which have full outer shells and do not react.

about 25 min1 simulation2 practice questions

Lesson 2 of 3Show all lessons

Try the code and the simulations beside the lesson.

Group 17: the halogens

Fluorine, chlorine, bromine, iodine and astatine are the halogens. The name means "salt makers", since they make salts with metals.

They have seven outer electrons, so they need one more. The valency is $1$ and they make $\text{X}^-$ ions.
They are non-metals, and they make molecules of two atoms: $\text{F}_2$ , $\text{Cl}_2$ , $\text{Br}_2$ and $\text{I}_2$ .
They react with metals and make salts: $2\,\text{Na} + \text{Cl}_2 \rightarrow 2\,\text{NaCl}$ .

Going down the group

Four bottles of the halogens. Fluorine is a pale yellow gas that boils at 85 kelvin. Chlorine is a green-yellow gas that boils at 239 kelvin. Bromine is a red-brown liquid that boils at 332 kelvin. Iodine is a dark grey solid that boils at 457 kelvin. Down group 17 the colour gets darker, the boiling point rises and the reactivity falls.

Halogen	State at room temperature	Colour	Boiling point ( $\text{K}$ )
Fluorine	gas	pale yellow	$85$
Chlorine	gas	green-yellow	$239$
Bromine	liquid	red-brown	$332$
Iodine	solid	dark grey	$457$

The colour is darker, and the boiling point rises, since the bigger molecules attract each other more.

Reactivity falls down the group. A halogen gains an electron more easily when its atom is small, as the nucleus is close. So fluorine is the most reactive.

Displacement

A more reactive halogen takes the place of a less reactive one in a salt.

\displaystyle \text{Cl}_2 + 2\,\text{KBr} \rightarrow 2\,\text{KCl} + \text{Br}_2

\displaystyle \text{Cl}_2 + 2\,\text{KI} \rightarrow 2\,\text{KCl} + \text{I}_2

\displaystyle \text{Br}_2 + 2\,\text{KI} \rightarrow 2\,\text{KBr} + \text{I}_2

If you add chlorine water to potassium bromide solution, the colourless solution turns orange-brown, since bromine is made. Iodine in water looks brown, and in a solvent such as hexane it is purple.

The other way round does not happen. Bromine cannot take the place of chlorine in potassium chloride.

You should find that the free halogen wins only when it sits above the halide in the list.

Group 18: the noble gases

Helium, neon, argon, krypton, xenon and radon are the noble gases.

Their outer shell is full: $2$ for helium and $8$ for the rest.
So the valency is $0$ . They do not lose, gain or share electrons. They are very unreactive, and they are called inert gases.
They are single atoms (monatomic), and they are gases.

Electron shell diagrams of helium with the arrangement 2, neon with 2, 8 and argon with 2, 8, 8. The outer shell of each is full, so a noble gas does not need to lose, gain or share electrons.

Uses. Helium fills balloons and airships, since it does not burn. Neon glows red in advertising signs. Argon fills electric bulbs, so that the hot filament does not burn.

Practise with new numbers

Try it in Python

This program uses the order of reactivity to decide if a halogen displaces another.

Remember these

Halogens: seven outer electrons, valency $1$ , molecules of two atoms. Reactivity falls down the group, so fluorine is the most reactive.
A more reactive halogen displaces a less reactive one from its salt.
Noble gases: full outer shell, valency $0$ , very unreactive.