Noble gases and halogens

Chlorine is a gas, bromine a liquid and iodine a solid at room temperature, yet all three sit in the same column.

The last two columns of the table sit side by side but behave as opposites. Group 17, the halogens, are among the most reactive elements known. Group 18, the noble gases, are among the least. The difference between them is a single electron, and looking at that one electron explains a large part of chemistry.

Noble gases: full shells

Helium, neon, argon, krypton, xenon and radon have completely filled outer shells: 2 electrons for helium, 8 for the rest. A full shell is a very stable arrangement. The atoms have no strong tendency to lose, gain or share electrons, so they exist as single atoms rather than molecules and react only under special conditions. Xenon does form compounds with fluorine and oxygen, but helium, neon and argon form no stable compounds at all under normal conditions.

Low reactivity makes them useful. Argon makes up almost 1% of the air and is used to fill light bulbs and to shield hot metal while it is welded, because it will not react with the glowing filament or the molten metal. Helium is lighter than air and does not burn, which is why it fills party balloons and airships instead of hydrogen. Neon glows red-orange when electricity passes through it, which gave neon signs their name.

Halogens: one electron short

Fluorine, chlorine, bromine, iodine and astatine have 7 outer electrons, one short of a full shell. Gaining a single electron gives them the same arrangement as the noble gas next door, so they pull electrons from other atoms eagerly. That is why they have such high electronegativity and why they are dangerous as pure elements. Chlorine gas attacks the lungs; fluorine reacts with almost everything, even glass. The name halogen comes from Greek words meaning "salt maker", because they combine with metals to make salts such as sodium chloride.

Going down the group the molecules (F₂, Cl₂, Br₂, I₂) get bigger and heavier, and the forces between them grow. At room temperature fluorine and chlorine are gases, bromine is a red-brown liquid and iodine is a dark solid that gives off violet vapour. The PubChem melting points in the model show the climb: fluorine 53.53 K, chlorine 171.65 K, bromine 265.95 K and iodine 386.85 K, which is about 114 °C. Reactivity runs the other way: fluorine is the most reactive and each halogen below it is a little less eager for an electron.

Tamed inside compounds, halogens are everywhere in daily life. Table salt is sodium chloride. Small amounts of chlorine are added to tap water and swimming pools to kill germs. Many toothpastes contain fluoride, and salt is often sold with added iodine, because the body needs a little iodine to make thyroid hormones. In each case the halogen is present as a negative ion or in a compound, not as the dangerous pure element.