The halogens found in seawater include fluorine, bromine and iodine. Chlorine is also found in seawater as part of salt and other chlorides. Halogens are found in group 17 of the periodic table.
These elements are normally very reactive because they are considered by chemists to be "unhappy." This means they have incomplete outer shells and wish to grab electrons from other elements.
Fluorine is one of the most reactive elements. It reacts so violently with water, for example, that it tears the hydrogen out of the water molecule and forms hydrofluoric acid. Fluorine eagerly combines with most other elements. This produces compounds known as fluorides.
Bromine is the only non-metallic element that is a liquid at room temperature. It is reddish and foul-smelling and raises burns on the skin, while its fumes attack the mucous membranes. As it is so reactive, it is never found in a free state; instead, it is found in bromide compounds.
Though people think of iodine as a liquid dabbed on minor cuts, it is actually a blue-black solid at room temperature with a pungent odor. It is necessary for good health, and a lack of iodine in the diet leads to goiter. Sea vegetables such as kelp are rich in iodine.