Understanding Halogens

Halogen is the name given to an element belonging to Group 17 of the periodic table. Older textbooks in the USA may refer to this Group as VIIA, while European texts may use VIIB. Group 17 is the terminology used by the International Union for Pure and Applied Chemistry (IUPAC). The name Halogen comes from the Greek and means salt-former.

There are five halogen elements Fluorine (symbol F, atomic number 7), Chlorine (symbol Cl, atomic number 17), Bromine (symbol Br, atomic number 35), Iodine (symbol I, atomic number 53) and Astatine (symbol As, atomic number 85). All halogens have seven electrons it their outer orbital shell. This gives them an oxidation number of -1.

The short-lived radioactive element, astatine, is very rare in nature. The total of the earth’s crust contains approximately 30 grams of astatine at any time. Very little of the element has been made, so few of its properties have been studied.

If you look at the periodic table you will see a sixth element in Group 17 – Ununseptium (symbol Uus, atomic number 117). This is an IUPAC holding name as the element has yet to be made.

Elements within the groups of the periodic table show trends in their physical and chemical properties.

The melting point, boiling point, atomic radius, and density of the elements in the halogen group increase as their atomic number increases. At room temperature, fluorine and chlorine exist as gases; bromine is a liquid, while iodine and astatine are both solids.

The electronegativity of an element is a measure of its ability to attract a bonding pair of electrons. The higher the electronegative value of an element the more likely it is to form a compound with another element. An element’s electronegativity is measured on the Pauling scale. On this scale, fluorine, with a value of 4.0, is the most electronegative of all the elements in the periodic table. The electronegativity of the halogens decreases as their atomic numbers increase. Fluorine is one of the few elements that can form compounds with elements in Group 18 of the periodic table. Group 18 consists of the nearly inert noble gases.

An illustration on the reactive nature of fluorine comes from its first production as a free element. In 1869, George Gore used electrolysis to produce a small amount of fluorine gas. The fluorine immediately combined with the hydrogen produced at the other electrode resulting in an explosion that destroyed the electrolysis equipment.

Atoms of a halogen can oxidize the ions of a halogen lower down in the table. If sodium bromide is exposed to chlorine gas the chlorine replaces the bromine to form sodium chloride. Likewise fluorine will replace chloride ions to form sodium fluoride.

In their elemental form all the halogens are toxic in some degree. The degree of toxicity decreases down the group. Fluorine is highly toxic and chlorine gas was used as a chemical weapon during World War One. Bromine and iodine will cause burns to skin, eyes and mucous membranes. The toxicological aspects of astatine are unknown but it does present a radiological hazard.

Although the elements are toxic many halides are not. Common table salt is made of sodium chloride. Fluoride is added toothpaste and some drinking water supplies to prevent tooth decay.