Summary
The periodic table is the way of ordering all the chemical elements that exist —oxygen, iron, gold…— by their atomic number, that is, the number of protons in their atom. Dmitri Mendeleev devised it in 1869 when he realised that, placed in order, the elements repeated their properties at regular intervals: hence "periodic".
It is read in rows and columns. The horizontal rows are called periods and there are 7. The vertical columns are called groups, there are 18, and elements in the same group behave alike: for example, those in group 1 are very reactive metals and those in group 18 are noble gases that barely react. Broadly, metals are on the left and non-metals on the right.
Key points
- It orders the elements by atomic number (the number of protons).
- Mendeleev devised it in 1869; today the table has 118 elements.
- Periods = horizontal rows (7); groups = vertical columns (18).
- Elements in the same group have similar chemical properties.
- Metals (left), non-metals (right) and metalloids (on the border).
- Each box shows the symbol, the name, the atomic number and the atomic mass.
- Group 1: alkali metals, very reactive; group 18: noble gases, almost inert.