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− halide ion
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