The ability of an orbital to approach the nucleus; s penetrates more than p, d and f in the same shell.
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Penetration describes how close an orbital's electron density can come to the nucleus. S orbitals have appreciable density near the nucleus, whereas p, d and f orbitals have a node at the nucleus, so s electrons penetrate closer. An electron that penetrates feels more of the nuclear attraction and less shielding, so it has lower energy. Within a shell the ordering is s, then p, then d, then f in terms of penetration and energy. This splitting is why 4s lies below 3d, so potassium and calcium fill 4s before 3d.
Key ideas: Electrons in orbitals with a high probability near the nucleus feel more Z_eff. Penetration order within a shell: s > p > d > f. That is why in many-electron atoms energy order is ns < np < nd < nf and why 4s lies below 3d. The 2s electron has a small inner maximum, so it is more penetrating than 2p.
Common mistakes and connections: Students think energies depend on n only, as in hydrogen, so they predict 2s and 2p equal energy in Li. In fact 2s lies lower. Penetration explains Aufbau order, the ionization energy dip from Be to B, and why the s-block is more reactive than expected from size alone.
The 2s radial plot has a small inner peak near the nucleus, so 2s penetrates the core better and is lower in energy than 2p. The interactive lesson shows this as a live animation or chart.
The 2s electron of lithium is lower in energy than a 2p electron because the 2s orbital penetrates closer to the nucleus.
The ability of an orbital to approach the nucleus; s penetrates more than p, d and f in the same shell.
Penetration of Orbitals is a advanced topic in Atomic Structure & Periodicity. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Atomic Structure & Periodicity first: Effective Nuclear Charge, Shielding Effect, Slater's Rules.
The next topics in Atomic Structure & Periodicity are Periodic Law and Mendeleev's Periodic Table.
Yes. The interactive lesson includes a self-check question on Penetration of Orbitals, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Penetration of Orbitals is topic 73 of 115 in Atomic Structure & Periodicity.