The modern periodic table

Atomic number and protons

The atomic number of an element was originally just its position on the periodic table. After the discovery of protons, scientists realised that the atomic number of an element is the same as the number of protons in its nucleus.

In the modern periodic table, the elements are arranged according to their atomic number - not their relative atomic mass.

the periodic table with the periods labelled vertically, and the groups labelled horrizontallyThe modern periodic table with some elements left out for simplicity

In the periodic table the elements are arranged into:

  • horizontal rows, called periods, in order of increasing atomic number
  • vertical columns, called groups, where the elements have similar properties

Metals and non-metals in the table

The metal elements are found on the left hand side of the periodic table, and the non-metal elements are found on the right. You can imagine a zigzag line, starting at B-Al-Si, separating metals from non-metals.

Period table with metals in red and non-metals in yellow.

Resolving pair reversals

When Mendeleev proposed his periodic table, he did not know about isotopes, but their existence is an explanation for pair reversals.

The positions of iodine and tellurium were reversed in Mendeleev's table because, although iodine has a lower relative atomic mass, its chemical properties show that it should be in the same group as chlorine and bromine.

It was only later, when scientists were able to work out the atomic number of elements, that it was discovered the atomic number of tellurium is 52 and the atomic number of iodine is 53. Therefore, Mendeleev was correct in the order that he placed these elements in the periodic table.