OCR AS Chemistry Unit 1 - The Halogens

The group 7 elements are fluorine, chlorine, bromine, iodine, and astatine. These elements are known as the halogens.

The halogens have low melting and boiling points.

As you move down the group the number of electrons increases, so the Van der Waals forces between the molecules increase. The boiling points of the halogens increase down the group.

The physical states of the halogens at RTP show the classic trend of gas to liquid to solid as you move down the group. They exist as diatomic molecules i.e. F₂, Cl_2, I_2,.  

The elements in group 7 have 7 electrons in their outer shell.

The highest energy electrons are in the p-sub shell, meaning they form part of the p-block in the periodic table.

They have one electron less than the electronic configuration of a noble gas.

They have an outer p sub-shell containing 5 electrons.

The halogens are the most reactive non-metals in the periodic table and are strong oxidising agents removing electrons in reactions. The oxidising power of a halogen is a measure of the strength with which a halogen atom is able to attract and capture an electron to form a halide ion.  The halogens become less reactive down the group as their oxidising power increases. As halogens gain an electron in their reactions the reactivity decreases down the group because:

·         The atomic radius increases.

·         The electron shielding increases.

·         The ability to gain an electron into the p sub shell decreases to form a halide ion.

Redox reactions can show that the halogens become less able to form halide ions down the group. We can show the decrease in reactivity using redox reactions of:

Aqueous solutions of halide ions: Cl⁻ (aq), Br⁻  (aq) and I⁻  (aq).

With aqueous solutions of halogens: Cl₂  (aq), Br₂ (aq) and I₂ (aq).

Each halogens is mixed with aqueous solutions of the different halides. A more reactive halogen will oxidise and displace a halide of a less reactive halogen. This is often called a displacement reaction. Halogens form solutions with different colours, so any change in colour will show whether a redox reaction has taken place.