Group 7: The Halogens

Introduction to Halogens

The halogens are the elements found in Group 7 of the periodic table. They are all non-metals and include five elements:

• Fluorine (F)
• Chlorine (Cl)
• Bromine (Br)
• Iodine (I)
• Astatine (At)

These elements share similar chemical properties because they have seven electrons in their outer shell, making them very reactive non-metals. They tend to gain one electron to achieve a full outer shell, which explains their high reactivity.

Physical Properties

At room temperature (about 206C), the halogens exist in different physical states:

• Fluorine is a pale yellow gas.
• Chlorine is a greenish-yellow gas.
• Bromine is a red-brown liquid.
• Iodine is a dark grey solid that forms a purple vapour when heated.
• Astatine is a rare, radioactive solid (less commonly studied).

All halogens have a strong, unpleasant smell and are toxic. For example, chlorine gas is poisonous and was once used as a chemical weapon. Therefore, careful handling and safety measures are important when working with halogens.

Chemical Properties

Halogens are very reactive, but their reactivity decreases as you move down the group from fluorine to astatine. This is because the atoms get larger, so the outer electrons are further from the nucleus and less easily gained.

All halogens exist as diatomic molecules, meaning their atoms pair up to form molecules like $\mathrm{F_2}$, $\mathrm{Cl_2}$, $\mathrm{Br_2}$, and $\mathrm{I_2}$.

A key chemical property is their ability to undergo displacement reactions. A more reactive halogen can displace a less reactive halogen from a compound. For example, chlorine can displace bromine from potassium bromide solution, turning it orange-brown.

For example, chlorine can displace bromine from bromide ions:

$$\mathrm{Cl_2 (aq) + 2Br^- (aq) \rightarrow 2Cl^- (aq) + Br_2 (aq)}$$

This reaction shows chlorine (more reactive) displacing bromine (less reactive) from its salt solution.

Uses of Halogens

Chlorine is widely used as a disinfectant to kill bacteria in water supplies, swimming pools, and in bleach.

Halogen lamps contain halogens like iodine or bromine. These lamps produce bright light and last longer than ordinary bulbs because the halogen gas helps redeposit tungsten back onto the filament.

Some halogens have medicinal uses. For example, iodine is used as an antiseptic to clean wounds.

Because halogens are toxic, they must be handled with care using appropriate safety precautions.

Trends in Reactivity and Properties

As you go down Group 7:

• Reactivity decreases because atoms get larger and it is harder to gain an electron.
• Melting and boiling points increase due to stronger van der Waals forces between larger molecules.
• Electronegativity decreases because the nucleus has less attraction for bonding electrons as atomic size increases.

For example, fluorine is the most reactive and has the lowest melting and boiling points, while iodine is less reactive and has higher melting and boiling points.

The trend in melting points can be seen in their states at room temperature:

• Fluorine and chlorine are gases (low melting points)
• Bromine is a liquid (higher melting point)
• Iodine is a solid (even higher melting point)

Electronegativity is important in how halogens react with other elements, especially metals, to form ionic compounds.

Example: Predict which halogen is more reactive between bromine and iodine.

Since bromine is above iodine in Group 7, bromine is more reactive because it is smaller and can attract electrons more easily.