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The empty 3d block is higher in energy than the 4s block. Because electrons fill up lower energy shells first, the 4s fills before the 3d orbitals. But once one electron is present in the 3d orbitals, it drops to a lower energy than the 4s. This is why an atom with filled 4s and 3d orbitals has 4s electrons as its outermost valence electrons.
You only fill the 3d orbital before finishing the 4s orbital for Chromium and Copper (for this class because we don't go past the first row of the d-block). This happens because Cr almost has a half-filled d orbital and copper almost has a fully filled d orbital. A half or fully filled d orbital is more stable than a full 4s orbital, so they take the electron that is supposed to be in the 4s orbital and put it in the 3d orbital making it Cr = [Ar] 3d5 4s1 or Cu = [Ar] 3d10 4s1.
Venus is correct. But in any case, the 4s orbitals fill first before the 3d orbitals. In copper and chromium, the pre-existing electron from 4s2 fills up a spot in the lower energy 3d5 and 3d10 orbitals, resulting in 4s1 configuration.
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