Niobium: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d4 I have been learning a little about electron configuration rules and from what I have read online, I cannot understand why Niobium doesn't fill its 5s subshell before moving on to the 4d subshell. Wikipedia says "A d subshell that is half-filled or full (ie 5 or 10 electrons) is more stable than the s subshell of the next shell. This is the case because it takes less energy to maintain an electron in a half-filled d subshell than a filled s subshell." That's all well and good but withholding an electron from the 5s subshell in this case doesn't give the 4d subshell 5 electrons! So what's the actual reason for the 5s subshell not being filled?