What does the pauli exclusion principle state ?

No two electrons in an atom can have the same set of four quantum numbers.

Step-by-step explanation:

Another way of saying it is, An orbital can contain only two electrons, and they must have opposite spins.

For example, the first electron added to an atom has the quantum numbers

n = 1, l = 0, mₗ = 0, mₛ = +½

For the second electron added, at least one of the quantum numbers must be different. If n = 1, the only quantum number that can change is mₛ.

The second electron must have quantum numbers

n = 1, l = 0, mₗ = 0, mₛ = -½

beta decay is the answer