GCSE Chemistry · AQA 8462 · Topic 4.1.2
The periodic table
Key points
- Elements are arranged in order of atomic number. Rows are periods; columns are groups. The group number tells you the number of outer-shell electrons.
- Mendeleev left gaps for undiscovered elements and sometimes swapped the order of atomic weights; his predictions were later confirmed. Isotopes explain why order by atomic weight did not always work.
- Metals form positive ions and are on the left; non-metals do not usually form positive ions and are on the right.
- Group 0 (noble gases) have full outer shells, so they are unreactive. Their boiling points increase down the group.
- Group 1 alkali metals react with water to give hydrogen and an alkaline hydroxide. Reactivity increases down the group because the outer electron is further from the nucleus and is lost more easily.
- Group 7 halogens exist as diatomic molecules (e.g. ). Reactivity decreases down the group, melting and boiling points increase, and a more reactive halogen displaces a less reactive one from a solution of its salt.
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The periodic table is not just a list: its layout reflects the electron arrangements of the atoms. Once you understand that, you can predict how an element behaves from where it sits.
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