4s 3d subshells are partially filled in
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It is because, According to the Aafbau principle, the shells with less energy comes first (here, 4s shell has less energy than 3d). But if we talk about the removal of electrons( Ionisation), then, as I mentioned before, 4s shell has less energy than 3d shell, and also having only 2 electrons.
Fill in subshells till all electrons are used •4. Last subshell may be partially filled. Standard Notation ... Chromium steals a 4s electron to half fill its 3d ...