Hal (Chemistry)

Hal is a general symbol used in chemistry to represent any one of the halogen elements. The halogens are the group of elements in period 17 of the periodic table: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). When a writer wishes to show a chemical formula without specifying which particular halogen, the symbol 'Hal' is used in its place.

The use of Hal appears most often in organic chemistry, particularly in writing general reaction schemes. For example, the formula R–Hal denotes a carbon centre bonded to a halogen, where R represents a hydrocarbon part. In this way a single reaction can be used to describe all the halogens, without needing to write out four or five separate formulas.

Usage of the symbol

Hal is a placeholder symbol, not a real element. When writing a specific formula, it is replaced with the symbol of the actual halogen: for instance CH3–Hal can become CH3–Cl (chloromethane) or CH3–Br (bromomethane). Other common placeholder symbols include R and R′ for alkyl groups, X which also typically represents a halogen, and M for a metal.

Why this symbol was chosen

The halogens share a similar chemical behaviour: they all have seven electrons in their outer shell and tend to gain one electron. Because the reactions they undergo are largely of the same type—only the bond energies and strengths differ—it is possible to write them with a single symbol. The most significant difference is that the C–F bond is very strong while the C–I bond is very weak, which affects the speed of substitution reactions.

Other meanings of 'Hal'

The term Hal also derives from the Greek root 'hals' meaning 'salt'. This is the origin of the name 'halogen', meaning 'salt-producer', because these elements form salts with metals such as sodium chloride. Similarly, the chemistry terms halide and halogenation are based on the same root.