6.9 - Halogen reactions with metals

Naming metal halides

A metal halide is a compound of a metal and a halogen. The general word equation is:

metal + halogen → metal halide

The metal's name stays the same. For example, calcium reacting with iodine makes calcium iodide.

Writing and balancing equations

Word equations show the product pattern clearly:

sodium + chlorine -> sodium chloride
magnesium + bromine -> magnesium bromide
potassium + iodine -> potassium iodide

Symbol equations need the formulae. The halogens chlorine, bromine and iodine exist as diatomic molecules, so their formulae are $\ce{Cl2}$, $\ce{Br2}$ and $\ce{I2}$. Many simple metal halides have formulae you have already met in ionic formula work.

Worked examples:

2Na + Cl2 -> 2NaCl
Mg + Br2 -> MgBr2
2K + I2 -> 2KI

The coefficient in front of the metal sometimes changes because the halogen molecule contains two halogen atoms. Do not change $\ce{Cl2}$, $\ce{Br2}$ or $\ce{I2}$ to make balancing easier; change the coefficients in front of formulae.

For metals that form 2+ ions, such as magnesium or calcium, the simple formula is often MX2, where X is the halogen. For example, magnesium bromide is $\ce{MgBr2}$, so:

Mg + Br2 -> MgBr2

This equation is already balanced.