Answer :
Final answer:
When navigating down the periodic table, the 4s subshell is filled first before the 3d subshell. This order is due to considerations around electron orbital shapes and energy levels, with exceptions noted for certain atoms like potassium.
Explanation:
On your journey down the periodic table, the subshell that is filled first between the 3d subshell and the 4s subshell is the 4s subshell. Normally, electron subshells fill according to their energy levels starting with the lowest, however, there are exceptions due to the shape and proximity of these electron probability clouds to the nucleus. One such exception is observed for the atom of potassium, where the 4s subshell starts to fill before the 3d.
Here is a sequence that can help illustrate this: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s... Notice that the 4s comes before the 3d. Transition metals which contain the d subshell, such as chromium (Cr) or copper (Cu), also show interesting configurations due to stability considerations.
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