Fluorine Chlorine Bromine Iodine Astatine
F Cl Br I At

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.


Electrons have a very tiny mass: they don’t have “Zero Mass” as it says in the animation. However, you don’t need to take account of their mass when you are calculating molecular weights: for your purposes, the atomic mass is calculated by adding the number of neutrons to the number of protons.

Since electrons do actually have some mass, they cannot travel at the speed of light. They would need an infinite amount of energy to travel this fast. However, they do buzz around the atomic nucleus at very fast speeds.

When an atom is excited, ie. it has too much energy, electrons can escape from their normal orbits into higher energy orbits or escape the atom completely; this would leave the atom in an ionised state.

The animation shows electron pairs: since electrons have negative charges, they repel each other so they would normally be at opposite ends of their orbit.