Reactivity and Electron Configuration — Group 1: The Alkali Metals | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

Reactions with water and trends in reactivity down the group are standard exam fare.

Each alkali metal has one outer electron

Lithium (proton number 3): 2, 1
Sodium (proton number 11): 2, 8, 1
Potassium (proton number 19): 2, 8, 8, 1
The lone outer electron is the only thing the atom needs to lose in order to react

Forming 1+ ions

When a Group 1 atom reacts, it loses its single outer electron to form a 1+ ion (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺)
The shell below becomes the new outermost shell — it is already full, so the ion has the noble gas configuration of the previous group 0 element

Why reactivity increases down the group

Going from lithium down to caesium, one extra electron shell is added at each step
The outer electron therefore sits further from the nucleus
The attraction between the outer electron and the positive nucleus is weaker (more distance, and inner shells partly shield the nuclear charge)
Less energy is needed to remove the outer electron, so the atom loses it more easily
An atom that loses its outer electron more readily reacts more readily — so the metals get more reactive as you descend the group

Exam tip

Explaining why reactivity increases down Group 1

What comes up: "Explain why potassium is more reactive than lithium. Refer to atomic structure." (3 marks — very common question).

Write (three marks): (1) Potassium has more electron shells than lithium, so its outer electron is further from the nucleus (or: potassium has a larger atomic radius than lithium). (2) The outer shell electron in potassium is therefore less strongly attracted to the nucleus. (3) So the outer electron in potassium is more easily lost.

Watch out: the mark scheme explicitly penalises answers that mention electrons in general without specifying the outer shell electron — you must say "outer shell electron" or "outer electron" to secure marks 2 and 3. Writing "more shielding" alone is accepted for mark 1, but does not substitute for the weaker attraction and easier loss points. Do not say "lithium has fewer outer shells" — that phrasing is rejected.