Final answer:
The principle of quantum mechanics that states that orbitals of equal energy are each occupied by a single electron before any orbital is occupied by a second electron is known as the Pauli exclusion principle.
Step-by-step explanation:
The principle of quantum mechanics that states that orbitals of equal energy are each occupied by a single electron before any orbital is occupied by a second electron is known as the Pauli exclusion principle. This principle, proposed by Wolfgang Pauli in 1925, states that no two electrons can have the same set of quantum numbers, which include the principal quantum number, the angular momentum quantum number, the magnetic quantum number, and the spin quantum number. It applies to any identical particles with half-integral intrinsic spin, such as electrons.