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الكيمياء الصناعية | Effective Nuclear Charge |
 2953
 02:52 صباحاً
date: 4-11-2020 |
Author : University of Missouri System 
Book or Source : Organic Chemistry ii 
Page and Part : .................|
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Date: 29-5-2019
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Many of the periodic properties of atoms depend on electron configuration; in particular, the valence electrons and their level of attraction to the nucleus.
Valence electrons are simultaneously attracted to the positive charge of the nucleus and screened (repelled) by the negative charges of other electrons. This net nuclear charge felt by valence electrons is known as its Effective Nuclear Charge, Zeff (pronounced “zed-effective”). The effective nuclear charge is always less than the actual nuclear charge, and can be roughly estimated using the following equation:
Zeff = Z – S
Where Z is the nuclear charge (equal to the number of protons), and S is the screening constant which can be approximated to the number of non-valence, “core” electrons.
Example:Approximate the effective nuclear charge of magnesium.
Solution:First we must determine the electron configuration of magnesium to determine the number of core electrons.
Mg =1s2 2s2 2p6 3s2 = [Ne]3s2, therefore magnesium has 10 core electrons from its 1s2, 2s2, 2p6 orbitals.
Magnesium is element 12, so it has 12 protons and a nuclear charge of 12.
Zeff = 12 – 10
Zeff = 2+
Moving left to right across a period on the periodic table, each subsequent element has an additional proton and valence electron, but the core electrons which are responsible for the majority of screening remain the same. This results in a trend that in general the effective nuclear charge increases from left to right across any period of the periodic table.
Moving from top to bottom down a column of the periodic table, we might expect the elements to have a similar effective nuclear charge as they all have the same number of valence electrons. However, we actually see a slight increase in Zeff moving down a column of the periodic table. As the principal quantum number (n) increases, the orbital size increases making the core electron clouds more spread out. These core electron clouds that are more diffuse do not screen as well, giving a slight increase to Zeff (Figure 1.1)
Figure 1.1. The periodic trend for effective nuclear charge.
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