Effective Nuclear Charge trend, zeff down the group
Introduction
We have already covered the Representation of the s−Orbitals in the previous unit (see below).
Example: 12Mg atom
Here, we will focus on the effective nuclear charge acting on the outermost electron(s).
Effective Nuclear Charge: outermost electron(s)
Zeff = Z − S
Where:
Zeff: Effective Nuclear Charge
Z: Number of protons in the nucleus
S: Average number of electrons between the nucleus and the outermost electron (s), called "core electrons"
Explanation
Electrons are attracted to the nucleus, but repelled by the electrons that screen it from the nuclear charge.
→ As the number of these screening electrons increases, Zeff decreases.
The nuclear charge experienced by an electron depends on its distance from the nucleus and the number of core electrons.
→ As the distance from the nucleus increases, S increases and Zeff decreases.
So, the question is: How does the trend of Zeff look like in a periodic table ?
Trend
Zeffincreases as atomic number increases across a period.
Number of core electrons stays the same, but the actual nuclear charge increases, causing more attraction to the outermost electrons.
The outer-shell electrons added to counterbalance the increasing nuclear charge shield each other ineffectively
Zeff slightly increases as atomic number increases down a group:
Although larger cores are added as we move down a group, these are less able to shield the outer electrons from the nuclear charge.
