Chemistry 6.6-6.10 - Halogen properties and reactions

Visible patterns and the chlorine test

The halogens are reactive non-metals in Group 7 of the periodic table. Elemental halogens exist as diatomic molecules: each molecule contains two atoms, so their formulae are $\ce{Cl2}$, $\ce{Br2}$ and $\ce{I2}$.

At room temperature their appearances are:

Do not confuse the dark grey iodine solid with iodine vapour, which is purple. The table asks about room temperature, so purple gas is not the appearance of iodine under those conditions.

Going down Group 7 from chlorine to bromine to iodine:

• the colour becomes darker;
• melting point and boiling point increase;
• the room-temperature state changes from gas to liquid to solid.

These trends support predictions. Fluorine, above chlorine, is predicted to be a paler gas with lower melting and boiling points. Astatine, below iodine, is predicted to be a darker solid with higher melting and boiling points. These are pattern-based predictions; astatine is not being observed in a school laboratory.

Testing for chlorine

Hold damp blue litmus paper in the gas. A positive result is:

blue → red → white

The chlorine dissolves in the water on the damp paper to form an acidic solution, so the paper first turns red. It is then bleached white. The red stage may be brief because bleaching can be rapid. The dampness is essential, and the gas must never be identified by smelling it.

Forming metal and hydrogen halides

A halide is a compound of a halogen with another element. A halide ion is the negative ion formed when a halogen atom gains one electron. In a metal halide, the halogen is present as a negative halide ion: chloride is $\ce{Cl-}$, bromide is $\ce{Br-}$ and iodide is $\ce{I-}$. This is different from the elemental molecules $\ce{Cl2}$, $\ce{Br2}$ and $\ce{I2}$.

Halogens with metals

Halogens react with metals to form metal halides:

metal + halogen → metal halide