Electron Shells in Atoms

Periodic Properties of the Elements

Electron Shells in Atoms
When we move down the column of the periodic table, we change the principle quantum number 'n' of the valence electrons of the atom

We have referred to all orbitals with the same principle quantum number 'n' as a shell

What does the quantum mechanical description of probability distributions for all electrons in an atom look like?

He	1s²
Ne	1s² 2s² 2p⁶
Ar	1s² 2s² 2p⁶ 3s² 3p⁶

The overall electron distribution can be calculated using supercomputers and the result is a spherically symmetrical distribution termed radial electron density

Helium shows a single "shell"
Neon shows two "shells"
Argon shows three "shells"

Each of these maxima corresponds to electrons that have the same principle quantum number 'n'

In He the 1s electrons have a maximum probability distribution at around 0.3 Å from the nucleus
In Ne the 1s electrons have a maximum at around 0.08 Å, and the 2s and 2p electrons combine to form another maximum at around 0.35 Å (the n=2 "shell")
In Ar the 1s electrons have a maximum at around 0.02 Å, the 2s and 2p electrons combine to form a maximum at around 0.18 Å and the 3s and 3p electrons combine to form a maximum at around 0.7 Å

Why is the 1s shell in Argon so much closer to the nucleus than the 1s shell in Neon, and why is that closer than the 1s shell in helium?

The nuclear charge (Z) of He = 2+, Ne = 10+, Ar = 18+
The inner most electrons (1s shell) are not shielded by other electrons, therefore the attraction to the nucleus is greater with higher number of protons
Likewise, the n=2 shell of Ar is closer to the nucleus than the n=2 shell of Ne. The Z_eff for the 2s subshell of Ne would be (10-2) = 8+, and for Ar would be (18-2) = 16+. Thus, the 2s subshell electrons in Ar would be closer to the nucleus due to the greater effective nuclear charge.

Three properties that provide important insights into chemical behavior include:

atomic size
ionization energy
electron affinity

1996 Michael Blaber