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What makes an element electronegative. I understand, for example, $\ce{P(OR)3}$ is less basic than $\ce{PR3}$ because the oxygen in $\ce{P(OR)3}$ is more electronegative than P, so it pulls electron density away from the phosphorus, making the lone pair less available, and so, in turn making the phosphite less basic than the corresponding phosphine.

But, what about the oxygen makes it more electronegative than the phosphorus? Is it because of effects of shielding and penetration(?)* that makes oxygen better at pulling electron density. (because it has a higher Zeff)

*I don't think I fully understand what penetration is,so if does have something to do with penetration, could you explain from which orbital will the electrons penetrate to and from, directly referring to oxygen and phosphorus.

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    $\begingroup$ I'm afraid you don't understand * for a reason, and I've got no idea what would it supposed to be. $\endgroup$ – Mithoron Apr 2 '16 at 20:39
  • $\begingroup$ The presence of electron in shells of other electrons is called penetration. $\endgroup$ – Codefailure Apr 2 '16 at 20:45
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    $\begingroup$ I'm afraid it isn't. $\endgroup$ – Mithoron Apr 2 '16 at 20:48
  • $\begingroup$ For example, electrons occupying 2s orbitals can be found the 1s shell (and so also the core) because s orbitals have non-zero electron density at the core...but, 2p orbitals cant penetrate to the core, so 2s has a lower energy and experiences less shielding... To be honest, i'm not sure how penetration would be applied here...i was just making a suggestion... $\endgroup$ – Codefailure Apr 2 '16 at 20:52
  • $\begingroup$ chemistry.stackexchange.com/questions/13677/… and en.wikipedia.org/wiki/Electronegativity looks at least A-R scale is based on Zeff and covalent radius $\endgroup$ – Mithoron Apr 2 '16 at 20:58

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