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4 December, 02:07

Calcium (atomic number 20) has electron configuration 1s2,2s2,2p6,3s2,3p6,4s2. The atomic number of scandium is 21. Which orbital does the 21st electron enter?

a. 3d b. 4p c. 4d d. 4f

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  1. 4 December, 04:53
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    a. 3d

    The electrons in an atom fill the shells in order from least energy to higher energy. The orbitals from lowest to highest are

    1s - 1 orbital, 2 electrons

    2s - 1 orbital, 2 electrons

    2p - 3 orbitals, 6 electrons

    3s - 1 orbital, 2 electrons

    3p - 3 orbitals, 6 electrons.

    4s - 1 orbital, 2 electrons. Note the energy for the 4s orbital is just barely

    lower than the 3d orbitals. So it fills up before the 3d orbitals.

    3d - 5 orbitals, 10 electrons.

    Since the 20th electron fills the 4s shell, the 21st electron goes into the next higher energy orbital which is one of the 3d orbitals.
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