GCSE Chemistry · Edexcel 1CH0 · Topic 1B
The periodic table
Key points
- Mendeleev ordered elements by atomic mass but swapped some (e.g. iodine and tellurium) to fit their properties, and left gaps for undiscovered elements, predicting their properties.
- The modern periodic table is ordered by atomic number. Isotopes explain why the order by mass sometimes seemed wrong.
- Groups are columns: elements in the same group have the same number of outer-shell electrons and similar reactions. Periods are rows: the period number is the number of occupied shells.
- Electronic configuration: shells fill 2, 8, 8, e.g. sodium (11) is 2.8.1, chlorine (17) is 2.8.7.
- Metals are on the left and middle; non-metals on the right. Metals tend to lose electrons to form positive ions.
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The periodic table is a map of the elements that lets you predict how an element behaves from its position. These notes explain how it was developed, how it is arranged today, and how electronic configuration links to groups and periods.
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