Group 7: The Halogens

As you move down Group 7, the melting and boiling points increase because the molecules become larger, leading to stronger intermolecular forces that require more energy to overcome.

The physical state of the elements changes from gas (Fluorine and Chlorine) to liquid (Bromine) to solid (Iodine) at room temperature.

The color intensity also increases down the group; for example, Chlorine is a pale green gas, Bromine is a dark red-brown liquid, and Iodine is a grey-black solid that produces purple vapor.

Reactivity decreases as you move down Group 7 because the atoms become larger and the outer shell is further from the nucleus.

The increased distance and electron shielding from inner shells weaken the electrostatic attraction between the positive nucleus and an incoming electron.

Consequently, it becomes harder for the atom to attract and capture the one electron needed to complete its outer shell, making larger halogens like Iodine less reactive than smaller ones like Fluorine.

A displacement reaction occurs when a more reactive halogen replaces a less reactive halogen from its salt solution (halide).

For example, adding Chlorine water to a solution of Potassium Bromide will result in the formation of Potassium Chloride and Bromine, visible as an orange color change.

These are redox reactions where the more reactive halogen is reduced (gains electrons) and the halide ion is oxidized (loses electrons), as shown in the ionic equation: $Cl_2 + 2Br^- \rightarrow 2Cl^- + Br_2$

It is vital to distinguish between the halogen element (the diatomic molecule, e.g., $Cl_2$) and the halide ion (the charged particle in a compound, e.g., $Cl^-$).

While Group 1 reactivity increases down the group, Group 7 reactivity decreases down the group due to the opposite requirement of gaining vs. losing an electron.

• Predicting Properties: Use the known trends to predict the properties of Astatine; it should be a dark solid with a very high melting point and the lowest reactivity in the group.

• Color Observations: Always specify the color of the halogen in the resulting solution during displacement (e.g., 'solution turns orange' for Bromine or 'brown' for Iodine).

• Ionic Equations: Practice writing ionic equations by removing spectator ions (like $K^+$ or $Na^+$) to focus on the electron transfer between the halogen and the halide.

• State Symbols: Ensure you use $(g)$ for Chlorine, $(l)$ for Bromine, and $(s)$ for Iodine at room temperature, but $(aq)$ when they are dissolved in water for reactions.