When there are two electrons in the same orbital, they have: A. Same spin B. Opposite spin C. Same or opposite spin D. No spin

The correct answer is: B. Opposite spin

Two electrons in the same orbital must have opposite spins. This is because of the Pauli exclusion principle, which states that no two electrons in an atom can have the same set of quantum numbers. The quantum number that determines the spin of an electron is called the spin quantum number, and it can have two values: $+1/2$ or $-1/2$. If two electrons are in the same orbital, they must have opposite spin quantum numbers in order

Option A is incorrect because two electrons in the same orbital cannot have the same spin. Option C is incorrect because two electrons in the same orbital must have opposite spin. Option D is incorrect because electrons do have spin.