why do chromium and copper have different electronic configuration
Answers
it is because their atomic number changes
You may have heard of Hund's rule - electrons tend to occupy orbitals singly before pairing up. It turns out that the 4s orbital and the 3d orbital are very close in energy, such that you can almost treat them as degenerate. In both cases, it is more stable to "promote" an electron from the 4s to 3d to satisfy a half-filled or fully-filled orbital (both of which are "preferred" due to their stability), with the added benefit of not having to pay the energetic cost of pairing up electrons in 4s, in the case of chromium.
I've put "promote" under quotations because the presence of electrons in the 4s orbitals is enough to make it higher in energy than 3d - this is the reason during ionization, 4s electrons are first to be ejected.
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