The periodic table is arranged by atomic number

1. The elements are placed in order of increasing atomic number, which is the number of protons.
2. They are arranged so that elements with similar chemical properties line up in the same column.
3. Metals are on the left and centre; non-metals are on the right.
4. The pattern that repeats across the table is why it is called periodic.

Groups: columns with the same number of outer electrons

1. A group is a vertical column of the periodic table.
2. The group number tells you the number of electrons in the outer shell of each atom.
3. Elements in the same group react in similar ways, because chemical reactions depend on the outer electrons.
4. For example, every Group 1 element has 1 outer electron, and every Group 7 element has 7 outer electrons.

Periods: rows showing the number of shells

1. A period is a horizontal row of the periodic table.
2. The period number tells you the number of occupied electron shells in each atom.
3. Moving one step across a period, the atomic number increases by one, adding one proton and one electron.
4. The added electrons fill the same outer shell until it is full, which is why properties change gradually across a period.