# Is half-full rule and full rule allowed on 6th and 7th period?

Is half-full rule and full rule allowed on 6th and 7th period?
Note: Half-full rules is Hund Rules.

Example: What is the right electron configuration?
$W [Xe] 6s^2 4f^{14} 5d^4$ (doesn't use half full rule) or
$W [Xe] 6s^1 4f^{14} 5d^5$ (use half full rule)

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Short answer: no! The first known exception to occur in the periodic table is in period 5: niobium’s ground state electronic configuration is:

Nb: [Kr] 5s1 4d4

which is not warranted by the “usual” rules for determining electronic configuration. So, the “half-full rule” is not sufficient in periods 5 and higher.

Let’s look at Wikipedia’s list of exceptions to Madelung’s rule. In period 4, the only exceptions are Cr and Cu, which are accounted for by the “half-full rule”. (Don’t bother too much about the dispute for Ni.)

In period 5, the exceptions to Madelung’s rule are classified in three groups:

• Mo and Ag: they are the analogues of Cu and Cr
• Pd is 5s0 4d10: a different type of application of the explanation that “fully filled shells are particularly stable”, because in this case two electrons from the s shell were pulled into the d shell. Not what you usually learn.
• Nb is 5s1 4d4, and Ru is 5s1 4d7: these cannot be explained by simple rules, and one has to perform complex quantum chemistry calculations to understand these electronic configurations.

Period 6 contains further inconsistencies, because of the introduction of f orbitals into the mix.

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