The properties and reactions of the group 1 alkali metals, the group 7 halogens and their displacement reactions, and the group 0 noble gases, with the trends down each group.

What this dot point is asking

Edexcel wants you to describe the properties and reactions of the group 1 alkali metals, the group 7 halogens and their displacement reactions, and the group 0 noble gases, and to explain the trends in reactivity down each group in terms of electronic structure.

Group 1: the alkali metals

For example: sodium + water $\rightarrow$ sodium hydroxide + hydrogen. When added to water, an alkali metal floats (it is less dense than water), moves around the surface, fizzes as hydrogen is released, and (for the more reactive ones) melts into a ball or ignites. The solution left behind is alkaline, so it turns universal indicator purple. Because they are so reactive, the alkali metals are stored under oil to keep them away from air and water.

Group 7: the halogens

The group 7 elements (fluorine, chlorine, bromine, iodine) are the halogens, non-metals that exist as diatomic molecules ($Cl_2$). They are coloured, getting darker down the group: chlorine is a green gas, bromine an orange liquid, iodine a grey-black solid.

Reactivity decreases down group 7, because the atoms get larger and the outer shell is further from the nucleus, so an electron is gained less easily.

Group 0: the noble gases

The group 0 elements (helium, neon, argon and below) are the noble gases: colourless, unreactive (inert) gases that exist as single atoms. They are unreactive because they have full outer electron shells, so they have no tendency to gain or lose electrons. Their boiling points increase down the group as the atoms get larger and the forces between them increase. Their lack of reactivity makes them useful and safe: argon fills light bulbs to stop the filament burning, helium fills balloons because it is lighter than air and does not burn, and neon is used in glowing signs.

The whole point of these three groups is that the electronic structure explains the trends. Group 1 metals each have one outer electron to lose, group 7 non-metals each need to gain one electron, and group 0 already have a full shell and need to do neither. As you go down any group, the outer shell is further from the nucleus and more shielded, weakening the attraction, which is why electrons are lost more easily (so group 1 gets more reactive) but gained less easily (so group 7 gets less reactive). Always linking a trend back to the outer electrons is what earns the explanation marks.

Try this

Q1. Name the products when potassium reacts with water. [2 marks]

• Cue. Potassium hydroxide and hydrogen.

Q2. Explain why group 0 elements are unreactive. [1 mark]

• Cue. They have full outer electron shells, so they do not gain or lose electrons.