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Effective nuclear charge
The effective nuclear charge, also known as the kernel charge, is the net positive charge experienced by an electron in a multielectron atom. The term "effective" is used because the shielding effect of negative electrons prevents higher orbital electrons from experiencing the full nuclear charge.
Additional recommended knowledge
In an atom with one electron, that electron experiences the full charge of the positive nucleus. In this case, the effective nuclear charge can be calculated from Coulomb's law.
However, in an atom with many electrons, and the outer electrons are simultaneously attracted to the positive nucleus and repelled by the negatively charged electrons. The effective nuclear charge on such an electron is given by the following equation:
Note: Zeff is also often known as "Z* ".
A simple way to calculate the effective nuclear charge is to take the total protons minus all electrons excluding the valence electrons.
Down the Periodic Table (Top to Bottom)
Effective Nuclear Charge for the outermost valence electron slightly increases for elements, going down the Periodic Table. This is because elements have:
The increase in repulsion is less than the increase of electrostatic attraction due to the Protons; Thus Zeff increases going down the Periodic Table. - Note: this is wrong since the ionization energy or electron affinity decreases going down
Across the Periodic Table (Left to Right)
Effective Nuclear Charge generally increases for elements, going across the Periodic Table. Elements have:
The increase in the electrostatic force provided by the protons is greater than the increase in repulsion due to shielding and distance. Therefore, across the Periodic Table, Effective Nuclear Charge increases.
These trends can be used to explain other trends between elements. Properties such as atomic radius, Melting and Boiling Points, electronegativity and ionization energy can be discussed using the Effective Nuclear Charge theory.
Generally, as Zeff increases:
It is important to bear in mind that when speaking about the location of electrons with respect to the nucleus, we are talking about a probability (see Heisenberg's uncertainty principle). The actual effective nuclear charge is therefore in fluctuation due to the location of an electron relative to the nucleus, and also due to electron-electron repulsions. Any theorized effective nuclear charge is a kind of mean between larger and smaller positive charges experienced during the path of an electron.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Effective_nuclear_charge". A list of authors is available in Wikipedia.