Download Factors Affecting Ionisation Energies: Understanding Electron Removal Energy and more Lecture notes Applied Chemistry in PDF only on Docsity!
Ionisation Energies
The energy needed to remove an electron from the outer shell of an atom is called the ionisation energy. This is different for all atoms and can be measured. The definition you need to know is:
The first ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous ions.
You can write equations for this process:
Na (g) Na+(g) + e-^ +496 kjmol-
O(g) O+(g) + e-^ +1314 kjmol-
There are a few factors that will affect the energy needed to remove the electron.
Factors affecting ionisation energy
- Nuclear Charge: The more protons there are in the nucleus, the more positively charged it is. This means that there is a much stronger attraction for the electrons.
- Distance from the nucleus: Attraction falls off very rapidly with distance. An electron close to the nucleus will be much more strongly attracted than one further away.
- Shielding: As the number of electrons between the outer electrons and the nucleus increases, the outer electrons feel less attraction towards the nuclear charge. The lessening of the pull of the nucleus by the inner shells of electrons is called shielding.
Second Ionisation Energy
In many cases you will want to remove more than one electron. This is where the second ionisation energy comes in. This is the energy needed to remove a mole of electrons from a mole of ions in a gaseous state.
O+(g) O2+(g) + e-
As you can see the oxygen ion had a +1 charge and it has had one more electron removed from it, this now makes it 2+. The same factors affect the amount of energy needed but you will notice that second ionisation energies are much larger than first because you are trying to remove an electron from a positive ion.
Successive Ionisation Energies
We can measure the energies required to remove the electrons one by one from an atom, starting from the outer electrons and working inwards.
Na(g) Na+(g) + e-^ +496 kjmol- Na+(g) Na2+(g) + e-^ +4563 kjmol- Na2+(g) Na3+(g) + e-^ +6913 kjmol-
On the next slide, there is a complete set of ionisation enthalpies for sodium. Plot the data on a SUITABLE graph. What does the data give strong evidence for?
0
1
2
3
4
5
6
1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th
Log IE
Total number of electrons removed
A graph to show how ionisation energy changes with number of removed electrons.
Trends in Ionisation Energies across a period
Ionisation energies generally increase across periods. This is because there is an increase in nuclear charge making it more difficult to remove an electron.
Nuclear Charge Increasing
If you plot a graph of these values it shows that the increase is not regular.
Na Mg Al Si P S Cl Ar 496 738 578 789 1012 1000 1251 1521
0
1
1st Ionisation energy Kj mol
(^1) -
Element
Trends in 1st Ionisation Energies across period 3
Trends in Ionisation Energies down a group
The trend for down groups is a general decrease in the energy needed to remove the outer electron. Even though as you go down the group the nuclear charge increases, the electrons are getting further away from the nucleus. Couple this with the shielding effect this causes it to need less energy to remove.
900
738 (^590 ) 503
0
100
200
300
400
500
600
700
800
900
1000
Be Mg Ca Sr Ba
Ionisation Energies Kj mol
(^1) -
Element
A graph to show how ionisation energies change down group 2
