Question

a. In your own words, state Hund’s rule. b. What is the basis for this rule?

Short answer

Hund’s rule states that electrons fill degenerate orbitals singly first, with parallel spins, to minimize repulsion and lower energy.

Step-by-step solution

- Stating Hund’s Rule

Hund's rule states that electrons will fill degenerate orbitals (orbitals with the same energy level) singly before they pair up. Each electron in singly occupied orbitals will have parallel spins.

- Basis for Hund’s Rule

The basis for Hund’s rule is to minimize electron-electron repulsions and thus lower the energy of the atom. Electrons are negatively charged and repel each other, so by filling degenerate orbitals singly, the repulsions between electrons are minimized.

Key concepts

electron configuration

Electron configuration refers to the arrangement of electrons within an atom. These electrons are distributed among various energy levels or shells, which are further divided into subshells (s, p, d, f). Each shell or energy level can hold a specific maximum number of electrons, determined by the formula 2n², where n is the principal quantum number of the shell.

Electrons fill these shells and subshells in a specific order, starting from the one with the lowest energy. This order is guided by the Aufbau principle, the Pauli exclusion principle, and Hund's rule.

• The Aufbau principle states that electrons fill the lowest energy subshells first.
• The Pauli exclusion principle tells us that no two electrons in the same atom can have the same set of quantum numbers.
• Hund's rule, the focus of this discussion, states that degenerate orbitals must be filled singly with electrons of the same spin before any pairing starts.

This arrangement ensures that the atom is in its most stable (lowest energy) configuration.

degenerate orbitals

Degenerate orbitals are orbitals within the same subshell that have the same energy. For instance, the three p orbitals in any p subshell (px, py, pz) are considered degenerate because they are of equal energy.

Hund's rule comes into play specifically for these degenerate orbitals. When filling orbitals, electrons will first occupy each degenerate orbital singly with parallel spins.

• This means that for the p subshell with three orbitals, one electron will go into each of the three p orbitals before any of them start pairing.
• This leads to a configuration where we may have one electron each in px, py, and pz before adding a second electron to px, for example.

This distribution minimizes the repulsion between electrons, thereby lowering the energy of the atom and making it more stable.

electron-electron repulsion

Electron-electron repulsion occurs when negatively charged electrons in an atom interact and repel each other. Since electrons have the same charge, they naturally repel one another, and this force needs to be considered when determining the most stable arrangement of electrons in an atom.

Hund's rule helps in minimizing this repulsion by ensuring that electrons fill degenerate orbitals singly before pairing up. By placing electrons in separate orbitals, the repulsion between them is decreased, leading to a more stable, lower energy arrangement.

• If electrons were to pair up immediately instead of occupying degenerate orbitals singly, the repulsion between them within the same orbital would increase.
• The increased repulsion would raise the energy of the atom, making it less stable.

Thus, Hund's rule, combined with the overall understanding of electron configurations and degenerate orbitals, ensures that atoms remain in their lowest possible energy state. This is crucial because atoms inherently strive for stability through the lowest energy configuration.