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The 3d state is higher in energy than the 4s state for potassium and calcium because Z(atomic number) is equal to 20. In addition, the d-electron are higher in energy than the p-electron on the same shell which results in higher energy than s-electron of the same shell too.
For K and Ca atoms, the 3d orbitals are higher in energy, but for Z > 20, the order switches as 3d is lower than 4s. This is a results of complex nuclear electrons interactions as well as electron-electron interactions due to resulting changes in the number of protons and the electrons. That is why you see 3d first than 4s for the transition metals. K and Ca do not have 3d electrons in the ground state, so you don't have issue writing 3d for them.
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