Which statement describes the Aufbau Principle?

Which statement describes the Aufbau Principle?

• A. An electron occupies the lowest-energy orbital that can receive it
• B. An electron occupies the highest-energy orbital that can receive it
• C. Electrons fill subshells in random order
• D. Electrons pair in the same orbital before occupying the next orbital

Answer: (A)

The Aufbau principle is about building up electron configurations by adding electrons to the lowest-energy available orbitals first. This minimizes the atom’s energy, so an electron goes into the lowest-energy orbital that can receive it rather than jumping to a higher one. In practice, electrons fill orbitals in increasing energy order (1s, then 2s, then 2p, 3s, 3p, etc.), with subtleties like 4s filling before 3d due to small energy differences. Within a subshell, electrons prefer to occupy separate degenerate orbitals before pairing up, maximizing stability through exchange energy (Hund’s rule). This means electrons do not pair in the same orbital before there are empty orbitals in the same subshell. So the statement that best describes the Aufbau principle is: an electron occupies the lowest-energy orbital that can receive it. The other ideas—filling the highest-energy orbital, filling randomly, or pairing before other orbitals are occupied—conflict with energy minimization and observed electron configurations.

The Aufbau principle is about building up electron configurations by adding electrons to the lowest-energy available orbitals first. This minimizes the atom’s energy, so an electron goes into the lowest-energy orbital that can receive it rather than jumping to a higher one. In practice, electrons fill orbitals in increasing energy order (1s, then 2s, then 2p, 3s, 3p, etc.), with subtleties like 4s filling before 3d due to small energy differences.

Within a subshell, electrons prefer to occupy separate degenerate orbitals before pairing up, maximizing stability through exchange energy (Hund’s rule). This means electrons do not pair in the same orbital before there are empty orbitals in the same subshell.

So the statement that best describes the Aufbau principle is: an electron occupies the lowest-energy orbital that can receive it. The other ideas—filling the highest-energy orbital, filling randomly, or pairing before other orbitals are occupied—conflict with energy minimization and observed electron configurations.