What element in the fourth period is an exception to the Aufbau principle? a. Li c. Ca b. Cr d. Br

The fourth period contains 18 elements, beginning with potassium and ending with krypton. As a rule, period 4 elements fill their 4s shells first, then their 3d and 4p shells, in that order; however, there are exceptions, such as chromium.

Cr. I think