I'm thinking it's because the nuclei are further from each other, so if the inverse square law applies they'll have a relatively tiny effect compared to the electrons, as they have equivalent charge. (See the powers of ten clip for a visual aid for the distance)
The Pauli Exclusion Principle is the actual scientific reason if I understand it correctly (my physics is slightly rusty). Philosophically Liebniz's Law implies two objects can't co-exist in the same space at the same time (otherwise they'd be one object) which has always been sufficient explanation for me until I really examined my understanding of it scientifically.
I did find some interesting things about electron shells and subshells-
The quantum number n first appeared in the Bohr model. It determines, among other things, the distance of the electron from the nucleus; all electrons with the same value of n lay at the same distance. Modern quantum mechanics confirms that these orbitals are closely related. For this reason, orbitals with the same value of n are said to comprise an "shell". Orbitals with the same value of n and also the same value of l are even more closely related, and are said to comprise a "subshell".
Electron subshells are identified by the letters s, p, d, f, g, h, i, etc., corresponding to the azimuthal quantum numbers (l-values) 0, 1, 2, 3, 4, 5, 6, etc. Each shell can hold up to 2, 6, 10, 14, 18, 22 and 26 electrons respectively. The notation 's', 'p', 'd', and 'f' originate from a now-discredited system of categorizing spectral lines as "sharp", "principal", "diffuse", or "fundamental", based on their observed fine structure. When the first four types of orbitals were described, they were associated with these spectral line types, but there were no other names. The designations 'g', 'h', and so on, were derived by following alphabetical order.
http://en.wikipedia.org/wiki/Electron_shell |
