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Question

Which of the following represents the set of quantum numbers for the last electron filled in beryllium?

A
n = 1, l = 0, ml = 0, ms = 0
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B
n = 1, l = 1, ml = + 1, ms = + 1/2
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C
n = 2, l = 0, ml = 0, ms = - 1/2
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D
n = 2, l = 1, ml = 0, ms = 0
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Solution

The correct option is C n = 2, l = 0, ml = 0, ms = - 1/2
Each electron in an atom is described by a set of four quantum numbers n, l, ml and ms.
Here n = principal quantum number
l = azimuthal quantum number
ml = magnetic quantum number
ms = spin quantum number.
Beryllium has 4 electrons.
So, electronic configuration of beryllium is 1s22s2.
Thus, last electron is filled in 2s orbital. Hence, n = 2, l = 0, and ml = 0 and electron can have any spin positive or negative.
So, set of quantum number could be n = 2, l = 0, ml = 0, ms = - 1/2

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