Electron Configuration Principles and Orbital Diagrams
Analogy- You're organizing a library.
- Each book has a specific place on a shelf, and there are rules for how the books are arranged: the smallest books go on the lowest shelves first, no two books can occupy the same spot, and books of the same size prefer to spread out before stacking.
- Electrons in an atom follow a similar set of rules when they occupy atomic orbitals.
These rules, known as the Aufbau principle, Hund’s rule, and the Pauli exclusion principle, ensure that electrons are arranged in the most stable and efficient way possible.
Properties of Atomic Orbitals
Before diving into the principles of electron configuration, let’s clarify what an atomic orbital is.
Definition
Orbital
An orbital is a region of space around the nucleus where there is a high probability of finding an electron.
- Each orbital can hold a maximum of two electrons with opposite spins.
- This is due to the Pauli exclusion principle, which states that no two electrons in the same orbital can have identical quantum numbers.
Each main energy level can hold a maximum of $2n^2$ electrons, where $n$ is the principal quantum number.
Electron Configuration Principles
Electron Capacity of Sublevels
- Each sublevel has a specific electron capacity based on the number of orbitals it contains:
- s sublevel: 1 orbital → 2 electrons
- p sublevel: 3 orbitals → 6 electrons
- d sublevel: 5 orbitals → 10 electrons
- f sublevel: 7 orbitals → 14 electrons
- Each orbital can hold 2 electrons with opposite spins, as described by the Pauli exclusion principle later.
Aufbau Principle: Filling the Lowest Energy Orbitals First
- The Aufbau principle states that electrons fill orbitals in order of increasing energy.
- This means electrons will occupy the lowest-energy orbitals first before moving to higher-energy orbitals. The general order of orbital filling is:
$$1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p$$
TipTo remember the order of orbital filling, use the diagonal rule or refer to the periodic table, as it is structured according to the filling of sublevels.
Hund’s Rule: Maximizing Unpaired Electrons
- When electrons occupy orbitals of the same energy (degenerate orbitals), Hund’s rule states that electrons will fill each orbital singly before pairing.
- For example, in the 2p sublevel, the three p orbitals ($p_x$, $p_y$, $p_z$) will each get one electron before any of them gets a second.
- Consider nitrogen ($Z=7$).
- Its electron configuration is $1s^2 2s^2 2p^3$.
- The three electrons in the 2p sublevel will occupy the $p_x$, $p_y$, and $p_z$ orbitals singly, all with the same spin.
Pauli Exclusion Principle: Opposite Spins in the Same Orbital
- The Pauli exclusion principle ensures that no two electrons in the same orbital can have identical quantum numbers.
- This means that if two electrons share an orbital, one must have a spin of $+\frac{1}{2}$ (up arrow) and the other $-\frac{1}{2}$ (down arrow).
Do not place more than two electrons in a single orbital or assign the same spin to both electrons in an orbital.
Writing Electron Configurations
Full Electron Configurations
To write the full electron configuration of an atom, follow the Aufbau principle and distribute electrons among the orbitals based on the atomic number of the element.
