Ionisation Energy & Sub-shells
- Successive ionisation data can be used to:
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- Predict or confirm the simple electronic configuration of elements
- Confirm the number of electrons in the outer shell of an element
- Deduce the group an element belongs to in the Periodic Table
- We can look at calcium as an example
- The first electron removed in calcium has a low IE1 as it is easily removed from the atom due to the spin-pair repulsion of the electrons in the 4s orbital
- The second electron is more difficult to remove than the first electron as there is no spin-pair repulsion
- The third electron is much more difficult to remove than the second one corresponding to the fact that the third electron is in a principal quantum shell that is closer to the nucleus (3p)
- Removal of the fourth electron is more difficult as the orbital is no longer full, and there is less spin-pair repulsion
- The graph shows there is a large increase in successive ionisation energy as the electrons are being removed from an increasingly positive ion
- The big jumps on the graph show the change of shell and the small jumps are the change of subshell
Ionisation Energies of Calcium Table
Successive ionisation energies for the element calcium
- By analysing where the large jumps appear and the number of electrons removed when these large jumps occur, the electron configuration of an atom can be determined
- Na, Mg and Al will be used as examples to deduce the electronic configuration and positions of elements in the Periodic Table using their successive ionisation energies
Successive Ionisation Energies Table
Sodium
- For sodium, there is a huge jump from the first to the second ionisation energy, indicating that it is much easier to remove the first electron than the second
- Therefore, the first electron to be removed must be the last electron in the valence shell thus Na belongs to group I
- The large jump corresponds to moving from the 3s to the full 2p subshell Na 1s2 2s2 2p6 3s1
Magnesium
- There is a huge increase from the second to the third ionisation energy, indicating that it is far easier to remove the first two electrons than the third
- Therefore the valence shell must contain only two electrons indicating that magnesium belongs to group II
- The large jump corresponds to moving from the 3s to the full 2p subshell
Mg 1s2 2s2 2p6 3s2
Aluminium
- There is a huge increase from the third to the fourth ionisation energy, indicating that it is far easier to remove the first three electrons than the fourth
- The 3p electron and 3s electrons are relatively easy to remove compared with the 2p electrons which are located closer to the nucleus and experience greater nuclear charge
- The large jump corresponds to moving from the third shell to the second shell Al 1s2 2s2 2p6 3s2 3p1
Worked example
Values for the successive IEs of an unknown element are:
IE1 = 899 kJ mol-1, IE2 = 1757 kJ mol-1, IE3 = 14850 kJ mol-1, IE4 = 21005 kJ mol-1
Deduce which group of the periodic table of elements you would expect to find the unknown element.
Answer:
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- The largest jump is between IE2 and IE3 which will correspond to a change in energy level.
- Therefore the unknown element must be in group 2
Worked example
The table shows successive ionisation energies for element X in period 2.
Identify element X.
Answer:
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- The largest jump in ionisation energy is between IE6 and IE7 meaning that the 7th electron is being removed from an energy level closer to the nucleus
- Therefore element X must be group 6
- If element X is in group 6 and in period 2 it must be oxygen
