What is this EXCHANGE ENERGY?

[mooncrater]

Homework Statement

While reading d and f block elements from my book I got this line:
"The irregular trend in the first ionization enthalpy of the 3d metals can be accounted for considering that the removal of one electron alters the relative energies of 4s and 3d orbitals. So the unipositive ions have dⁿ configurations with no 4s electrons. There is thus a REORGANISATION ENERGY accompanying ionisation with some gains in exchange energy as the number of electrons increases and from the transference of s electrons into d orbitals. "

Homework Equations

None

The Attempt at a Solution

First of all, if any unipositive element has dⁿ configuration then why would it have no electrons in its s orbital? (As the second line says)
And the third line, I didn't get anything of it... [/B]

 

[Simon Bridge]

The context suggests that, prior to ionization, there was only one 4s electron.

 

[mooncrater]

The context suggests that, prior to ionization, there was only one 4s electron.

I think you're saying it on behalf of:

considering that the removal of one electron alters the relative energies of 4s and 3d orbitals.

Am I correct?
Though now I understand the second line, but third one is still a mile away from my understanding.

 

[Simon Bridge]

Well I got it off the first sentence.

Take a look at the order of states across the 3 and 4 shells.