@ionicbonded5: The aufbau principle says that electrons must fill atomic orbitals from lowest to highest energy. For example, the 1s orbital will fill before the 2s, because 1s is lower energy than 2s. In Cu and Cr, there is an excpetion. 4s is lower energy than 3d, so one woudl expect the e config for cu and cr to have a full 4s2 shell. however, they have a 4s1 shell becuase that missing 4s electron actually goes into the 3d orbital, to give 3d10 and 3d5 which are exceptionally more stable than the other configuration. . . . . . #periodictable #chemistrymemes #sciencememes #chemistry