Posted by J.Nandhini on Saturday, September 22, 2012 at 11:02pm.
Through element #20 (Ca), it is
1s2 2s2 2p6 3s2 3p6 4s2.
For elements 21 through 30 (Sc through Zn) you fill the d orbitals. The way to remember how many d electrons (there are two exceptions) is it's the second number of the atomic number. That is Sc has 21 (from the 1 in 21), Ti has 22, V has 23 etc. The two exceptions are Cr and Cu.Cr,#24,--instead of [Ne]3d4 4s2 it is [Ne] 3d5 4s1--that is the d and s orbitals are BOTH half filled). The same kind of thing happens at Cu, #29. Instead of [Ne]3d9 4s2 it is 3d10 4s1. I call this the DrBob rule.
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