Final answer:
The two arrows in an orbital diagram indicate opposite spins for electrons in accordance with the Pauli exclusion principle. Electrons in the same orbital must have opposite spins to have distinct quantum numbers, and an upward arrow represents one spin direction, while a downward arrow represents the opposite spin.
Step-by-step explanation:
The two arrows within a single block of an orbital diagram must be written in opposite directions because of a fundamental rule in quantum mechanics known as the Pauli exclusion principle. This principle asserts that no two electrons in an atom can have the same set of four quantum numbers. Essentially, if two electrons are in the same orbital, they must have opposite spins to comply with the Pauli exclusion principle, as each spin state represents a different quantum spin number.
This concept is also related to Hund's Rule, which further explains the distribution of electrons across orbitals of equal energy. Hund's Rule states that orbitals of equal energy within the same sublevel get filled with one electron each before any orbital gets a second electron, and all electrons in singly occupied orbitals must have parallel spins. Finally, the arrows are used in such diagrams to show the direction of the electronic spin, where an upward arrow indicates one spin direction and a downward arrow indicates the other.