Answer:
4d orbital.
Explanation:
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In this case, since zirconium's atomic number is 40, we fill in the electron configuration up to 40 as shown below:
[tex]1s^2,2s^2,2p^6, 3s^2,3p^6,4s^2,3d^{10},4p^6,5s^2,4d^2[/tex]
Thus, the orbital 4d is partially filled.
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