The properties and reactivity trends of the Group 1 alkali metals, the Group 7 halogens including displacement reactions, and the Group 0 noble gases, and how these trends link to electron arrangement.

What this dot point is asking

CCEA Double Award wants the properties and reactivity trends of three groups: the Group 1 alkali metals, the Group 7 halogens (including displacement), and the Group 0 noble gases. You must link the trends to electron arrangement, because that is how the marks are earned in explanation questions.

Group 1: the alkali metals

The Group 1 metals (lithium, sodium, potassium) are soft and have low densities. They each have 1 outer electron, which they lose to form a +1 ion. They react vigorously with water to form an alkali (a metal hydroxide) and hydrogen gas:

sodium + water gives sodium hydroxide + hydrogen.

So potassium reacts more violently with water than sodium, which reacts more than lithium.

Group 7: the halogens

The Group 7 elements (fluorine, chlorine, bromine, iodine) are reactive non-metals. They each have 7 outer electrons and react by gaining 1 electron to form a -1 ion. They exist as diatomic molecules (such as Cl2).

A more reactive halogen displaces a less reactive one from a solution of its salt:

Group 0: the noble gases

The Group 0 elements (helium, neon, argon) have full outer shells (helium has 2, the others have 8). Because they do not need to gain or lose electrons, they are very unreactive (inert). This makes them useful where an unreactive gas is needed - helium in balloons, argon in light bulbs.

The noble gases are the opposite of the alkali metals and halogens: those react because their outer shells are nearly empty or nearly full, while the noble gases do not react because their shells are already complete. This is the clearest demonstration that chemical reactivity depends on the outer electrons - a full outer shell is the stable arrangement that other atoms react to reach.

Examples in context

Example 1. Storing sodium under oil

Sodium is so reactive it reacts with water vapour and oxygen in the air, so it is stored under oil to keep air and water away. This shows the high reactivity of Group 1 metals in practice.

Example 2. Using displacement to rank reactivity

By testing which halogens displace which from their salts, you can place them in order of reactivity: chlorine displaces bromine and iodine, bromine displaces iodine but not chlorine. This is a practical way to confirm the Group 7 trend, and the colour changes (chlorine to bromine giving orange, to iodine giving brown) make the result easy to read.

Example 3. Why helium and argon are chosen for different jobs

Helium is lighter than air and unreactive, so it is used to fill balloons and airships safely. Argon is denser and also unreactive, so it fills light bulbs to stop the hot filament reacting with oxygen. Both are picked because the full outer shell makes them inert, showing the noble-gas property put to practical use.

Try this

Q1. What ion does a Group 1 metal form? [1 mark]

• Cue. A +1 ion.

Q2. Does reactivity increase or decrease down Group 7? [1 mark]

• Cue. It decreases.