Updated on August 6, 2022
A.The two electrons in one orbital must have opposite spin.
B.The two electrons in one orbital must have the same spin.
C.The two electrons in one orbital can have either opposite or the same spin.
D.The two electrons in one orbital repel each other if they have opposite spin.
E.The two electrons in one orbital attract each other if they have the same spin.
DEC is correct : A
The reason being that electrons in the same orbital, with opposite spins, have an attraction for each other that is sufficient to overcome the tendency for the same-charge repulsive forces to kick each other out of the orbital. Think of the attraction as being due to an electric charge spinning and producing a magnetic field. With opposite spins, the poles of the magnets will be opposite each other and hence attract each other. This attraction is not strong enough for the electrons to “stick to” each other (don’t forget the opposing repulsion of the 2 negative charges) but it is enough to allow them to occupy the same orbital. This is an overly simplistic explanation, but hopefully gives you a handle on the phenomenon.