Answer
Period 3 only contains eight elements because the 3d-transitional elements (Z=21-30) have lower energy than the 4s elements (K, Z=19, and Ca, Z=20) and, thus, do not take precedence over them in occupying their strata with electrons.
Work Step by Step
1. In Period 3, the maximum number of electrons is 18 (2\timesn^{2} = 2\times3^{2} = 2\times9 = 18).
2. The energy of 4s strata is higher than the energy of the 3d strata.
3. There must be two elements that occupy their 4s strata before the 3d begins to be occupied. This is the law of occupying electrons strata according to their energy.
