Fluorine is a very reactive gas. When fluorine atoms react with
another element, they gain electrons to become fluoride ions. These are supposed to make
your teeth stronger, so fluoride may be added to toothpaste and to drinking water.
Chlorine is also a very reactive gas, but it is not quite so reactive
as Fluorine. You know what Chlorine smells like if you have been to an indoor swimming
pool or used household bleach. They put Chlorine in swimming pools to kill bacteria and
fungi so that it is safe to swim. The Chlorine in household bleach is also used to kill
"germs".
When Chlorine atoms react with other elements they gain electrons to become chloride
ions. Chloride ions are present in Sodium Chloride (table salt). They are required by
nerves and muscles to work properly. Chloride ions are present in almost everything we
eat.
Bromine is a liquid at room temperatures. Bromine atoms also gain
electrons when they react; they become Bromide ions. Bromine is less reactive than
Chlorine and Fluorine. It is also toxic to bacteria and fungi. Putting bromide salts in
your food is supposed to reduce your interest in having sex; I don't know if this is true,
but I have heard that the British Army does this to soldiers tea. This is probably a joke
to frighten new soldiers.
Iodine is a solid at room temperatures. It is curious that when you
heat it it does not melt and make a liquid, instead it turns into a gas. When Iodine gas
is cooled it turns straight back into a solid. This process is called
"sublimation". Iodine is less reactive than Bromine, but it is also toxic. You
can put Iodine onto wounds to kill "germs". Iodine atoms gain electrons when
they react to form iodide ions. We need iodide ions in our diet to make a hormone called
thyroxine. This hormone controls how fast our bodies work. A deficiency of iodide causes
goitre. You may have seen "iodised table salt" in the shops. This is sodium
chloride with a little sodium iodide added to it.
