What determines the maximum number of electrons in an orbital?

The maximum number of electrons in an orbital is determined by the Pauli Exclusion Principle.

The Pauli Exclusion Principle is a fundamental concept in quantum mechanics, named after the physicist Wolfgang Pauli who proposed it. This principle states that no two electrons in an atom can have the same four quantum numbers. Quantum numbers are essentially 'addresses' for electrons, describing their energy level, shape of orbital, orientation in space, and spin.

The first three quantum numbers (principal, azimuthal, and magnetic) define the orbital in which an electron resides, while the fourth (spin) describes the direction in which the electron spins. The spin quantum number can only have two values: +1/2 or -1/2, often referred to as 'up' or 'down'. This means that only two electrons can occupy a single orbital, one with an 'up' spin and one with a 'down' spin.

Therefore, the maximum number of electrons in an orbital is two. This is a fundamental rule of atomic structure and is crucial in understanding the arrangement of electrons in atoms, known as electron configuration. It also explains the structure of the periodic table, where each row represents a new energy level and each block corresponds to a different type of orbital.

In summary, the Pauli Exclusion Principle, a key principle in quantum mechanics, dictates that an orbital can hold a maximum of two electrons. These two electrons must have opposite spins, as defined by their spin quantum numbers. This principle is fundamental to our understanding of atomic structure and the arrangement of the periodic table.