Penetration and shielding effect

[sanan...@gmail.com]

Hi,

I have a question about what is happening in this video at 00:19:34 -

http://www.youtube.com/watch?v=vjAyIHjxlUc#t=1174s.

In lecture 24, it was mentioned that 2S electron is penetrated more than 2P and hence shielding effect on 2S will be less than 2P. That is inverse relationship between penetration and shielding. In lecture 25 explaining about lanthanide contraction, it is mentioned that 4f electrons will be penetrated less and hence less shielding. i.e a direct relationship between both. Please give clarity on the exact relationship between shielding and penetration effects..

Thanks!

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Reference Key - Key('StudentQuestionEntity', 4557779229474816, namespace='ns_noc21_cy49')

[NOC21 CY49]

Dear Students:

The statement "In lecture 24, it was mentioned that 2S electron is penetrated more than 2P and hence shielding effect on 2S will be less than 2P" is actually a wrong statement. Herein I am explaining about penetration and shielding.

Orbital Penetration:

Penetration describes the proximity to which an electron can approach to the nucleus. In a multi-electron system, electron penetration is defined by an electron's relative electron density (probability density) near the nucleus of an atom. Electrons in different orbitals have different wavefunctions and therefore different radial distributions and probabilities (defined by quantum numbers n and ml around the nucleus). In other words, penetration depends on the shell (n) and subshell (ml). For example, we see that since a 2s electron has more electron density near the nucleus than a 2p electron, it is penetrating the nucleus of the atom more than the 2p electron. The penetration power of an electron, in a multi-electron atom, is dependent on the values of both the shell and subshell.

Within the same shell value (n), the penetrating power of an electron follows this trend in subshells (ml):

s>p>d>f

Shielding:

An atom (assuming its atomic number is greater than 2) has core electrons that are extremely attracted to the nucleus in the middle of the atom. However the number of protons in the nucleus are never equal to the number of core electrons (relatively) adjacent to the nucleus. The number of protons increase by one across the periodic table, but the number of core electrons change by periods. The first period has no core electrons, the second has 2, the third has 10, and etc. This number is not equal to the number of protons. So that means that the core electrons feel a stronger pull towards the nucleus than any other electron within the system. The valence electrons are farther out from the nucleus, so they experience a smaller force of attraction.

Shielding refers to the core electrons repelling the outer rings and thus lowering the 1:1 ratio. Hence, the nucleus has "less grip" on the outer electrons and are shielded from them. Electrons that have greater penetration can get closer to the nucleus and effectively block out the charge from electrons that have less proximity. For example, Zeff is calculated by subtracting the magnitude of shielding from the total nuclear charge. The value of Zeff will provide information on how much of a charge an electron actually experiences.

Because the order of electron penetration from greatest to least is s, p, d, f; the order of the amount of shielding done is also in the order s, p, d, f. Since the 2s electron has more density near the nucleus of an atom than a 2p electron, it is said to shield the 2p electron from the full effective charge of the nucleus. Therefore the 2p electron feels a lesser effect of the positively charged nucleus of the atom due to the shielding ability of the electrons closer to the nucleus than itself, (i.e. 2s electron).