# Why Zn has highest ionisation enthalpy in 3d series?

Zn which has the highest ionisation enthalpy in 3d series.The reason given in my textbook is:

The value of zinc is higher because it represent ionisation from 4s level.

This is not correct because for every 3d element ionisation is from 4s orbital. The explanation I found for this is from this site is:

https://www.chemguide.co.uk/atoms/properties/ies.html

Increase in nuclear charge outweigh the electron electron repulsion of 4s electrons rendering Zn an higher ionisation enthalpy than Cu.

This seems like a probable explanation but if Zn has higher ionisation enthalpy than Cu due to its increased nuclear charge.Why can't Zn have smaller atomic radius than Cu due to its increased nuclear charge which can decrease the size of the atom. But experimentally it has been found that Zn is bigger in size than Cu.

So why does Zn has the highest ionisation enthalpy in 3d series?

TEXT SOURCE:NCERT textbook (chapter:d and f block)

• Where is the atomic radius of Zn rendered larger than that of Cu? In Wikipedia they are the same and so are a few preceding elements to boot. – Oscar Lanzi Feb 10 at 0:55
• In my textbook it's given that Zn has a higher atomic radii than Cu – Grace Feb 10 at 1:48
• The reference for the textbook being ... – Oscar Lanzi Feb 10 at 2:02
• The real reason for this happening is because the orbitals self overlap in the d-block; refer to the band theory. pci.tu-bs.de/aggericke/PC4e/Kap_II/Metalle.htm and check out this question that I've asked chemistry.stackexchange.com/questions/123802/… – DDD4C4U Feb 12 at 11:18
• I think a more suitable question would be "is atomic radii and ionization independent properties" – DDD4C4U Feb 12 at 11:20

In the Zinc atom, the levels $$4s$$ and $$3d$$ are completely occupied. This increases the stability of the atom. And electrons need more energy to be removed.