WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

WJEC Chemistry for AS Level

Successive ionisation energies Further evidence for shells and subshells comes from the successive ionisation energies needed to remove all the electrons from an atom. An element has as many ionisation energies as it has electrons. Sodium has eleven electrons and so has eleven successive ionisation energies. For example, the third ionisation energy is a measure of how easily a 2+ ion loses an electron to form a 3+ ion. An equation to represent the third ionisation energy of sodium is: Na 2+ (g) →  Na 3+ (g) + e − Successive ionisation energies always increase because: ▪ There is a greater effective nuclear charge as the same number of protons are holding fewer and fewer electrons. ▪ As each electron is removed there is less electron–electron repulsion and each shell will be drawn in slightly closer to the nucleus. ▪ As the distance of each electron from the nucleus decreases, the nuclear attraction increases. As the ionisation energies are so large we must use logarithms to base 10 (log 10 ) to make the numbers fit on a reasonable scale. Remember, electrons are removed in order, starting with the furthest from the nucleus. The graph below shows the successive ionisation energies of sodium.

Key term Successive ionisation energies are a measure of the energy needed to remove each electron in turn until all the electrons are removed from an atom.

Stretch & challenge For any positive number n , log 10 of n is the power to which the base (in this case 10) must be raised to make n . For example, for the number 100, log 10 100 = 2 i.e. 100 = 10 2 10 Knowledge check The first four ionisation energies (in kJmol –1 ) for an element are: 738, 1451, 7733 and 10541. The element belongs to Group in the periodic table because there is a between the and ionisation energies.

These 8 electrons are in the second shell. They experience less nuclear attraction than the rst shell.

5

These 2 electrons are in the rst shell, closest to the nucleus.

4

3

2 log 10 rst ionisation energy

This electron is in the third shell, furthest from the nucleus.

6 7 8 9 10 11

1 2 3 4 5

Exam tip A large increase in successive

Number of electrons removed

▲ Graph of sodium’s IE DRAFT For sodium there is one electron on its own which is easiest to remove. Then there are eight more electrons which become successively more difficult to remove. Finally there are two electrons which are the most difficult to remove. Notice the large increases in ionisation energy as the 2nd and 10th electrons are removed. If the electrons were all in the same shell, there would be no large rise or jump. 01.02.15 AS Chemistry Eduqas ionisation energies shows that an electron has been removed from a new shell closer to the nucleus and gives the group to which the element belongs. Li has a large energy jump between 1st and 2nd IE therefore it’s in Group 1. Al has a large energy jump between 3rd and 4th IE therefore it’s in Group 3. 26