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).
| 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 ?
| Zeff increases 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.
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