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Ionisation energies - Boron and Oxygen

Why do Boron and Oxygen not follow the general trend of ionisation energies across period 2.
I know it’s something to do with the orbitals not being filled but I’m not sure how or why it happens.
Thanks
Reply 1
Avatar for Pigster
Pigster
20
Can you write out the electronic configuration of Be and B?

Do you know about Hund's rule?
Reply 2
Avatar for Sarah_g_24
Sarah_g_24
OP
13
Original post by Pigster
Can you write out the electronic configuration of Be and B?

Do you know about Hund's rule?


Yes so if the p orbital for example held 5 electrons, 2 orbitals would hold two and the third orbital would hold only one

Be would be 1s^2, 2s^2
B would be 1s^2, 2s^2, 2p^1
(edited 6 years ago)
Reply 3
Avatar for Pigster
Pigster
20
Original post by Sarah_g_24
Be would be 1s^2, 2s^2
B would be 1s^2, 2s^2, 2p^1


What do you know about the relative energy levels of the 2s and 2p sub-shells?

Original post by Sarah_g_24
Yes so if the p orbital for example held 5 electrons


Minor (but important) correction: an orbital can only ever hold two electrons. Also, in this case, as you have pointed out: the p sub-shell only has one electron in.
Reply 4
Avatar for Sarah_g_24
Sarah_g_24
OP
13
What do you know about the relative energy levels of the 2s and 2p sub-shells?

In the second sub-shell for example, 2s has a slightly lower energy than 2p and will therefore be filled first.
The same occurs with a 3d orbital for example being of a slightly higher energy level than 3p.
However the 4s orbital is a lower energy than 3d so this would be filled before you begin filling up 3d.
For example the electron configuration of Iron (III) would be: 1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^2, 3d^6

But why do Boron and Oxygen have lower ionisation energies compared to the rest of group 2?
Reply 5
Avatar for Pigster
Pigster
20
Original post by Sarah_g_24
In the second sub-shell for example, 2s has a slightly lower energy than 2p and will therefore be filled first.
But why do Boron [and Oxygen] have lower ionisation energies compared to the rest of group 2?


You have answered the boron question yourself.

Be has its highest energy e- in the 2s sub-shell, whereas B has its in the higher energy 2p sub-shell. It will take less energy to remove.

You might think that C should also be lower in energy as it too has a 2p e- being removed, but C has an extra p+ which lowers the energy of all of its e-, making them need more energy to remove than Be or B.

For O: find out about Hund's rule.

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