From Pauli's exclusion principle, two electrons in an atom can never have the same set of quantum numbers.

If n = 1, it implies that l = 0, m_{l} = 0

Therefore, the possible states are: (1, 0, 0, +1/2), (1, 0, 0, -1/2)

A second important result is that electrons will fill the lowest energy states available. This would seem to indicate that every electron in an atom should be in the *n*=1 state. This is not the case, because of *Pauli's exclusion principle*. The exclusion principle says that no two electrons can occupy the same state. A state is completely characterized by the four numbers *n*, *l*, *m**l*, and *m**s*, where *m**s* is the spin of the electron.

An important question is, How many states are possible for a given set of quantum numbers? For instance, *n*=1 means that *l*=0 with *m**l*=0 are the only possible values for those variables. Thus, there are two possible states: (1, 0, 0, 1/2) and (1, 0, 0, ?1/2). How many states are possible for *n*=2?

Express your answer as an integer.

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