How to explain Group 1 and Group 17 trends

Group 1 and Group 17 are the two groups SPM asks you to compare most often, and the reason is neat: they behave in opposite directions. Reactivity increases as you go down Group 1 but decreases as you go down Group 17, and the same underlying idea, atomic size, explains both. Master this one contrast in the periodic table and you have a reliable source of marks. This guide walks through each group and then the crucial comparison.

Group 1: the alkali metals

Group 1 contains lithium (Li), sodium (Na), potassium (K) and the heavier metals below them. Their electron arrangements, Li is 2.1, Na is 2.8.1, K is 2.8.8.1, all share one valence electron. That single outer electron is the whole story of the group.

To reach a stable full outer shell, a Group 1 atom needs to lose its one outer electron, forming a singly charged positive ion such as Na⁺. So the reactivity of an alkali metal is really a question of how easily it loses that electron.

Reaction with water. Alkali metals react with water to form an alkaline metal hydroxide and hydrogen gas. For sodium:

2Na + 2H₂O → 2NaOH + H₂

Lithium reacts steadily, fizzing on the surface. Sodium reacts more vigorously, it melts into a shiny ball and darts across the water. Potassium is more vigorous still, igniting the hydrogen with a lilac flame. The trend is clear: reactivity increases down the group.

Why reactivity increases down Group 1. As you go down, each atom has one more electron shell, so the atom is larger and the single outer electron is further from the nucleus and more shielded by inner shells. The pull of the nucleus on that outer electron is therefore weaker, so it is lost more easily, and losing the electron is exactly what makes the metal react. Bigger atom, easier to lose the electron, more reactive.

Group 17: the halogens

Group 17 contains fluorine (F), chlorine (Cl), bromine (Br) and iodine (I). Their electron arrangements, F is 2.7, Cl is 2.8.7, all share seven valence electrons. They exist as diatomic molecules: F₂, Cl₂, Br₂, I₂.

Physical states and colour. A favourite Paper 1 fact is the change in physical state at room temperature going down the group:

• Chlorine is a greenish-yellow gas
• Bromine is a reddish-brown liquid
• Iodine is a purplish-black solid

The colour gets darker and the melting and boiling points rise down the group, because the molecules get larger and the forces between them get stronger.

Why reactivity decreases down Group 17. To reach a stable octet, a halogen atom needs to gain one electron, forming a singly charged negative ion such as Cl⁻. Going down the group, the atom is larger, so the outer shell where the incoming electron must go is further from the nucleus and more shielded. The atom therefore attracts an extra electron less strongly, so it gains an electron less easily, meaning reactivity decreases down the group. Fluorine and chlorine are very reactive; iodine much less so.

Displacement reactions. Because a more reactive halogen holds on to electrons more strongly, it can displace a less reactive halogen from a solution of its salt. Chlorine displaces bromine:

Cl₂ + 2KBr → 2KCl + Br₂

You would see the colourless solution turn reddish-brown as bromine is set free. Chlorine can displace both bromine and iodine; bromine can displace iodine; but iodine cannot displace either, a clean way to prove the reactivity order. You can see reactions like this written out in the reactions reference.

The comparison that earns marks

Here is the contrast to state precisely in an exam:

• Group 1 reactivity increases down the group because atoms lose their single outer electron more easily as they get larger.
• Group 17 reactivity decreases down the group because atoms gain an electron less easily as they get larger.

The deep point is that the same cause, a larger atom with its outer shell further from the nucleus and more shielded, pushes the two groups in opposite directions, because one group needs to lose an electron and the other needs to gain one. If you can say that sentence, you understand both trends at once.

A quick self-check

Test yourself: which is more reactive, lithium or potassium? (Potassium, it is lower in Group 1.) Which is more reactive, chlorine or iodine? (Chlorine, it is higher in Group 17.) If both answers came with the reason, not just the position, you are ready for the exam.

If the two trends keep blurring together, a very common mix-up, it usually helps to see them side by side with a teacher once. You can explore each element on our interactive periodic table, and our online one-to-one lessons run in English from RM50 an hour with a paid one-hour trial if you would like the reasoning taught against your own weak points; see how it works.