1. Electron configuration exceptions

2. There are several exceptions to the order of electron filling for some of the transition metals.  Chromium (24) is [Ar]4s 1 3d 5 and not [Ar]4s 2 3d 4 as expected.  Copper (29) is [Ar]4s 1 3d 10 and not [Ar]4s 2 3d 9 as expected. The reason for these anomalies is the slightly greater stability of d subshells that are either half-filled (d 5 ) or completely filled (d 10 ). Electrons move from s energy to nearest d energy level 4s4s3d3d3d3d3d3d3d3d3d3d [Ar]Cr Greater stability with half-filled 3d subshell Electron Configurations and the Periodic Table

3. activity Say yes or no to exception. Explanation should tell what the electron configuration actually is.