Question

What element in the fourth period is an exception to the Aufbau principle?

a.
Li
c.
Ca
b.
Cr
d.
Br

Answer 1

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