JEE Advance - Chemistry (1982 - No. 6)

When there are two electrons in the same orbital, they have opposite spins. This phenomenon is a consequence of the Pauli Exclusion Principle, which states that no two electrons in an atom can have the same set of four quantum numbers. The quantum numbers describe the properties of electron orbitals around an atom, including the principal quantum number (n), the azimuthal quantum number (l), the magnetic quantum number (m_l), and the spin quantum number (m_s). When two electrons are in the same orbital, they share the same n, l, and m_l values but must have different m_s values. Since the spin quantum number can only take on two values, +1/2 or −1/2, the two electrons in the same orbital must have one with +1/2 spin and the other with −1/2 spin, thus ensuring their spins are opposite.