Bitesize has changed! We're updating subjects as fast as we can. Visit our new site to find Bitesize guides and clips - and tell us what you think!
Print

Science

The periodic table

Page:

  1. 1
  2. 2
  3. 3
  4. 4
  5. 5
  1. Back
  2. Next

The modern periodic table

Mendeleev's table needed one important modification before it became the modern periodic table – the use of atomic number to order the elements.

All atoms of the same element contain the same number of particles called protons, and this is called the element's atomic number.

Mendeleev put the elements in order of their relative atomic mass, and this gave him some problems. For example, iodine has a lower relative atomic mass than tellurium, so it should come before tellurium in Mendeleev's table. In order to get iodine in the same group as other elements with similar properties such as fluorine, chlorine and bromine, he had to put it after tellurium, so breaking his own rules.

Henry Moseley (1887 - 1915)

Using atomic number instead of atomic mass as the organising principle was first proposed by the British chemist Henry Moseley in 1913, and it solved anomalies like this one. Iodine has a higher atomic number than tellurium. So, even though he didn't know why, Mendeleev was right to place it after tellurium after all!

You need to be able to locate the positions of the:

  • alkali metals (group 1)
  • halogens (group 7)
  • noble gases (group 0)
  • transition metals
Group 1 - alkali metals, group 7 - halogens, group 0 -  noble gases. Transition metals are between group 2 and 3.

The modern periodic table

Page:

  1. 1
  2. 2
  3. 3
  4. 4
  5. 5
  1. Back
  2. Next

Back to Patterns in properties index

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.