Effective Nuclear Charge

[blazinfury]

Effective Nuclear Charge (Zeff) is defined as how tightly the valence e- are held. I understand that Zeff decreases down a group because there is an increase in the number of principle energy levels (PEL) and as such, the valence e- are further away from the nucleus. However, Zeff increases left to right across a period. I am trying to understand this, but am having a difficult time. I understand that the number of protons and e- increase across a period, but how is that conducive to a tighter attraction with the nucleus?

 

Thanks.