Atomic Structure - Group 7 Halogens and Displacement Reactions

The Halogens are the elements in Group 7 of the Periodic Table, including Fluorine ($F$), Chlorine ($Cl$), Bromine ($Br$), Iodine ($I$), and Astatine ($At$).

Atomic structure: All halogens have 7 electrons in their outermost shell, giving them a valency of $1-$ as they need to gain one electron to achieve a stable noble gas configuration.

Physical Trends: Down the group, the atomic radius increases and the relative molecular mass increases. This leads to stronger intermolecular forces, resulting in higher melting and boiling points ($F_2$ and $Cl_2$ are gases, $Br_2$ is a liquid, $I_2$ is a solid).

Chemical Trends: Reactivity decreases down the group. As the atom gets larger, the outer shell is further from the nucleus and more shielded by inner electrons, making it harder for the nucleus to attract and capture an incoming electron.

Displacement Reactions: A more reactive halogen will displace a less reactive halogen from its halide salt solution. For example, Chlorine can displace Bromine from Potassium Bromide.

Redox Nature: Displacement reactions are redox reactions. The more reactive halogen acts as an oxidising agent by gaining electrons (reduction: $X_2 + 2e^- \rightarrow 2X^-$), while the halide ion is oxidised (oxidation: $2Y^- \rightarrow Y_2 + 2e^-$).