Potassium-krypton period

The next four rows include the elements known as transition elements (potassium krypton, 18 elements rubidium xenon, 18 elements cesium-radon, 32 elements francium and above, 26 elements discovered so far) and are therefore long periods. 

But I want to return to my claim that quantum mechanics does not really explain the fact that the third row contains 18 elements to take one example. The development of the first of the period from potassium to krypton is not due to the successive filling of 3s, 3p and 3d electrons but due to the filling of 4s, 3d and 4p. It just so happens that both of these sets of orbitals are filled by a total of 18 electrons. This coincidence is what gives the common explanation its apparent credence in this and later periods of the periodic table. As a consequence the explanation for the form of the periodic system in terms of how the quantum numbers are related is semi-empirical, since the order of orbital filling is obtained form experimental data. This is really the essence of Lowdin s quoted remark about the (n + , n) rule. 

For each of the following select an element from Period 4, potassium to krypton, that matches the description. 

In this period, the valence electrons of the elements may be in 4s, 3d or 4p orbitals. This period starts with potassium (K) and ends with krypton (Kr). 

Potassium (K) begins the period. Krypton (Kr) ends it, and there are 18 elements in the period. 

The second period contains eight elements lithium, Li, through neon, Ne. The fourth period consists of eighteen elements potassium, K, through krypton, Kr. 

After calcium the 3 d orbital is more stable than the 4s, and in the elements from scandium (Z = 21) to zinc (Z = 30) we find electrons entering the 3 d orbital of the M shell in preference to the N shell (see table 2.03). Only after zinc is the development of the N shell resumed by the occupation of the 4p orbitals to give the sequence of elements gallium (Z = 31) to krypton (Z — 36). The elements from scandium to zinc are termed the elements of the first transition series if they are imagined to be removed from the Periodic Table the remaining elements from potassium to krypton would form a fourth period exactly analogous to the third period from sodium to argon. 

Period 4 includes eighteen elements, eight as before from alkali metal potassium, K, to inert gas krypton, Kr, with ten elements inserted between calcium, Ca, and gallium, Ga. 

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