I was wondering what was the reason that electron configurations seem to arbitrarily change between elements. To be more precise, while brushing up on the some basics with the periodic table, I noticed the fact that as you progress across a row [and the subshells are filled] electrons jump between different subshells for no apparent reason (not apparent to me, in any case). For example, between Nickel and Copper: Ni: [Ar] 4s2 3d7 Cu: [Ar] 4s1 3d10 The 4s subshell lost an electron to help fill the 3d subshell. Why did it choose to let it go now? Why not sooner, or later in the periodic table? An even better example is between Zirconium and Niobium Zr: [KR] 4d2 5s2 Nb: [KR] 4d4 5s1 The 5s subshell again lost an electron to help further fill the 4d subshell (which still has a long way to go before it is filled). Again, I'm confused as to why the electron jumped out of the 5s subshell for the 4d subshell. It's not as if the 4d subshell was even close to filling it's shell. Anyways I'm currently studying upper division quantum mechanics, so I wouldn't mind a slightly loaded answer. Thanks in advance.