The periodic table

Imagine someone hands you a map on which all 118 known elements are arranged so that you can read off their properties from their position. That is exactly what the periodic table is. It hangs in almost every chemistry room in the world, and once you can read it, it becomes your most useful tool.

Ordered by proton number

The elements are not scattered at random but strictly sorted by their proton number. Hydrogen with 1 proton comes first, followed by helium with 2, lithium with 3, and so on. This number is called the atomic number: it tells you right away how many protons sit in the nucleus of an atom.

Periods and main groups

The rows of the periodic table are called periods, the columns are called groups. The eight main groups are especially important. They even have their own names: the first main group is called the alkali metals, the seventh the halogens, and the eighth the noble gases. Hydrogen does sit at the very top of the first column, but it does not count as an alkali metal. So every element has a kind of address made of its period and group.

Neighbours with a family resemblance

Here is the genius part: elements in the same main group resemble each other in their properties. Sodium and potassium both react violently with water, helium and neon are both extremely unreactive. If you know one element of a group, you can often predict how its neighbours behave without ever running an experiment.