The periodic table

Before protons, neutrons and electrons were discovered, scientists ordered elements by atomic weight. Their tables were incomplete, and some elements ended up in the wrong groups because only their weight was considered.

In 1869, Dmitri Mendeleev improved on this. He:

• left gaps for elements he thought had not been discovered yet, and predicted their properties
• swapped the order of some elements, such as tellurium and iodine, so they fitted with elements of similar properties

When elements such as germanium were later found and matched his predictions, his table was accepted. The discovery of isotopes finally explained why ordering by atomic weight is not always correct.

In the modern periodic table, elements are arranged in order of atomic (proton) number.

• The columns are groups. Elements in the same group have the same number of electrons in their outer shell, so they have similar chemical properties. For groups 1 to 7, the group number tells you how many outer electrons there are.
• The rows are periods. The period number tells you how many shells of electrons an atom has.

This means you can predict how an element will react from its position in the table.

Metals are on the left and towards the bottom of the periodic table. Most elements are metals. Non-metals are on the right and towards the top.

Metals react by losing electrons to form positive ions. Non-metals do not form positive ions.