Introducing the Periodic Table

The periodic table is a chart that arranges all the known elements in a fixed order, so that elements with similar properties end up near one another.

Two words describe its shape:

• a period is a horizontal row;
• a group is a vertical column.

Elements in the same group behave in similar ways. That is the whole point of the table: once you know how one member of a column behaves, you can make a sensible prediction about its neighbours below it. Each box shows the element's symbol, its name, and the number that fixes its place in the order.

Reading the map. The table is split by a rough staircase line running down the right-hand side:

• metals occupy the left and the middle — the great majority of elements (iron, copper, gold, aluminium, platinum, manganese, chrome);
• non-metals sit in the top right corner (oxygen, nitrogen, carbon, sulfur, chlorine, and the gases in the last column).

Worked example — counting non-metals. From the list O, Fe, C, Cu, N, Au, which are non-metals? Oxygen (O), carbon (C) and nitrogen (N) sit in the top right of the table — 3 non-metals. Iron, copper and gold are metals.

Why the table exists. In 1869 Dmitri Mendeleev arranged the elements known at the time by their properties and deliberately left empty gaps, saying that elements would one day be found to fill them. He was right. That is the mark of a good scientific idea: it does not just organise what you already know, it predicts what you have not yet found.