Reactivity and Electron Configuration — Group 7: The Halogens | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 6% of your exam marks.

stable

Low

Stable6%

Displacement reactions and physical property trends tested consistently.

Each halogen has seven outer electrons

Fluorine (proton number 9): 2, 7
Chlorine (proton number 17): 2, 8, 7
Bromine (proton number 35): 2, 8, 18, 7
A halogen reacts by gaining one electron to complete its outer shell, which then has the noble gas configuration of the following group 0 element
That captured electron makes the atom a 1−

Why reactivity decreases down Group 7

An extra electron shell is added at each step down the group
The outer shell therefore sits further from the positive nucleus
The pull from the nucleus on an incoming electron is weaker the larger the atom (more distance, plus shielding by the inner shells)
The atom finds it harder to grab a passing electron and complete its outer shell, so it reacts less readily
Fluorine sits at the top with the smallest atom, the strongest pull, and is therefore the most reactive halogen; iodine sits much lower and is the least reactive of the four common halogens

Exam tip

Explaining why reactivity decreases down Group 7

What comes up: a 4-mark question asks you to explain the relative reactivities of two halogens (for example, fluorine versus chlorine) using their electronic configurations.

Write (four marks): (1) fluorine is more reactive than chlorine (state the conclusion first); (2) the outer shell of electrons is closer to the nucleus in fluorine, because fluorine has fewer electron shells; (3) the attraction from the nucleus to an incoming electron is therefore stronger in fluorine (less shielding from inner shells); (4) so fluorine gains an electron more readily and reacts more vigorously.

Watch out: this is a "gain-electron" argument — the opposite logic to Group 1 metals, which react by losing electrons. Never say halogens "lose" electrons to explain reactivity. The mark scheme credits "smaller atomic radius" and "less shielding" as alternatives for M2 and M3 respectively.