The Periodic TableOxford AQA IGCSE Chemistry: Revision notes

The periodic table lists all the known elements in order of their atomic (proton) number, starting from hydrogen (1) and increasing left to right, row by row.

Elements are arranged into columns called groups (numbered 1 to 7, plus Group 0) and rows called periods.

Elements with similar properties appear in the same group, and similar properties recur at regular intervals as you move across the table — this pattern is called periodicity.

Elements in the same group have the same number of electrons in their highest occupied energy level (outer shell). Because chemical reactions involve these outer electrons, elements in the same group react in similar ways.

The current periodic table is based on the work of Mendeleev, who arranged known elements by atomic mass and left gaps for undiscovered elements, correctly predicting their properties. The modern table instead orders elements by atomic (proton) number, but Mendeleev's grouping of elements with similar properties still underpins its structure.

The noble gases (Group 0) are very unreactive because their atoms already have a stable arrangement of electrons — eight electrons in their outer energy level (except helium, which has a full outer shell of just two electrons).

Because their outer shells are already full, noble gas atoms have no tendency to gain, lose or share electrons, so they rarely react with other elements.