Touch this chemical to aluminum foil, and it bursts into flame. That makes for a pretty fire (although it's a pretty dangerous one), but the chemical is a fire retardant. Find out why bromine goes pyro the minute it touches aluminum foil.

If that seems like an incongruous job for an element we've just seen go fire-bug, well, bromine has always been a slightly sinister element. It corrodes many things, including human tissues. People who are exposed to bromine fumes suffer terrible chemical burns to their skin and their eyes; and people who inhale the fumes damage their lungs. Just as a bonus, bromine binds to the same receptors in the body that recognize iodine. Over-exposure to bromine makes the body believe it has too much iodine, and it starts shedding necessary iodine from body tissues. This does damage all over the body, but especially in the thyroid, which creates and secretes hormones in the body.

There's something of a controversy about bromine because it is added to many plastics to decrease the risk of them going up in a fire. Bromine in plastics insulates the material, reducing the heat that keeps the fire going. Bromine also doesn't catch fire easily, so when it is released into the atmosphere around the fire, it can choke the fire out.

Bromine doesn't help stop a fire in this particular reaction because it's a halogen, a group of elements that are usually found in salts. Aluminum reacts with all halogens, bromine included. The bromine in this reaction is reduced, meaning it grabs an electron from the aluminum. The aluminum is oxidized, losing its electron. The whole process can go down with a minimum of heat, but in this case, the transfer of the electron involves the release of a lot of energy... and a lot of heat. Flames shoot out, and at the end, the aluminum and bromine combine to form aluminum bromide. At least that won't evaporate into a gas that burns out your lungs.

[Via Chem Toddler, Paxymer, Reaction Between Aluminum and Bromine]