The history of the periodic table

The elements are ordered by mass number

The periods are ordered by atomic number

There are blank spaces for elements which are predicted to exist, but have not yet been found.

There are about 50 elements listed

The period to which an element is allocated depends on the number of electron shells around it

There are 10 groups of elements with similar properties

There is a group called the Noble Gases

Elements are allocated to a group depending on the number of electrons on the outside shell.