Unformatted text preview: s, we are going to
assume that ALL atoms (or ions) that end in d4
or d9 are exceptions to the rule. This may or
may not be true, it just depends on the atom. (HONORS only) 57 Exceptions to the Aufbau Principle
d4 is one electron short of being HALF full
In order to become more stable (require
less energy), one of the closest s
electrons will actually go into the d,
making it d5 instead of d4.
For example: Cr would be [Ar] 4s2 3d4, but
since this ends exactly with a d4 it is an
exception to the rule. Thus, Cr should be
[Ar] 4s1 3d5.
Procedure: Find the closest s orbital. Steal
one electron from it, and add it to the d. (HONORS only) 58 Exceptions to the Aufbau Principle
OK, so this helps the d, but what about the
poor s orbital that loses an electron?
Remember, half full is good… and when an
s loses 1, it too becomes half full!
So… having the s half full and the d half
full is usually lower in energy than having
the s full and the d to have one empty
orbital. (HONORS only) 59 Exceptions to the Aufbau Principle
d9 is one electron short of being full
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