Gen Chem: energy levels

[EnginrTheFuture]

So aamc #5 PS item 34 is a question about which electrons in the shells form a bond. There is only one viable option in the multiple choices (3d elctrons) so it's fairly obvious but it had me wondering...

For a transition metal like Ti ( [Ar] 3d2 4s2 ) we know that the 4s electrons are at a higher energy level and further from the nucleus... why aren't these the electrons forming the bond? It confuses me that when we ionize the atom, we pull 4s first yet when we share electrons in a bond all of a sudden we're digging into the 3rd discrete energy level not the highest energy level?!

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[aSagacious]

Electron shells are very stable when they are full (and, to a lesser extent, half full). As such the atom would much rather bond with an open (partially unoccupied), lower energy shell than disrupt a closed (full), higher energy shell.

 

[EnginrTheFuture]

Beautiful.... makes plenty of sense. Thanks!