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what could the fourth quantum number of a 2s^2 electron be?

D. ms=+1/2.

Explanation:

Hello there!

In this case, since the quantum numbers are known as:

* Principal quantum number (n) which cannot be zero. The allowed values of n are therefore 1, 2, 3, 4, and so on.

* Angular quantum number (l) is able to be any integer between 0 and n-1. If n = 2, for example, l can be either 0 or 1.

* Magnetic quantum number (m) is able to be any integer between -l and +l. If l = 3, m can be either -3, -2, -1, 0, +1, +2 or +3.

* Spin Quantum Number (ms): ms = +½ or -½. Accounts for the orientation of the spin axis of an electron as it is able to spin in only one of two directions.

Thus, since the fourth quantum number is ms, spin, we infer that the answer is D. ms=+1/2 because it represents the positive spin on 2s^2.

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