The new periodic table

The Periodic Table has changed! Below are copies of the new and old versions to compare.

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The new version has two main changes (only one of which you need to remember at IGCSE and GCSE level), they are:

1. The atomic masses of the elements are now NOT nice whole numbers. The atomic masses are now a spread of values from the smallest possible to the largest possible. This is because most elements have isotopes (same number of protons but a different number of neutrons in the nucleus), and a SPREAD of values is now used because of of the different amounts of isotopes of a particular element present on the Earth.

For example, the atomic mass of sodium (Na) is now not 23 but 22.98977 Why? Because there are two isotopes of sodium (Na22 and Na24) that exist on Earth in very small quantities. However, at GCSE and IGCSE levels it is much more practical to use the atomic mass of sodium as 23. So you need not remember this first change.

2. The second change concerns the numbering of the Groups. You will notice above that the Group numbers now run from 1 to 18. In the old Periodic Table (see below) the Groups ran from 1 to 8 (I – VIII) with the transition metals in the middle left blank.

This was alright because the number of the Group an element was in, told you how many electrons that element had in its OUTER electron ‘shell’.

For example, nitrogen was in Group 5 (V), so it had 5 electrons in its OUTER shell. But now nitrogen is in Group 15. This may seem confusing but you just need to take the ‘5’ in ‘15’ to get the correct number of electrons in nitrogen’s outer shell..

But why the change? Because the transition metals are now included in the Groups instead of being left blank. For example, iron, Fe, is now in Group 8, and there are 8 electrons in its OUTER shell. But why, you might ask, are chemists so concerned about electrons in the OUTER shell of atoms? Because, knowing how many electrons there are in an element’s OUTER shell can give you a good idea of how reactive that element is likely to be.

The ‘Row’ numbers (or ‘Periods’) have not changed. What do they tell us ? A Row number tells you how many ‘SHELLS’ are needed for an element’s electrons.

(‘Shells’ are really energy levels). For example, potassium, K, is in Row 3, this tells you immediately that potassium’s 19 electrons are distributed over 3 shells – and the same is true, of course, for all the other elements in Row 3 from potassium right through to krypton, Kr, in Group 18.

When the exam boards begin to use the new version of the Periodic Table in their exam papers, is anyone’s guess. Most other countries now use the new version.

THE OLD PERIODIC TABLE

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