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Electron Configuration

• Tells us where the electrons for an element are located.
• Follows three rules: Aufbau principle, Pauli’s exclusion principle, and Hund’s rule.

Aufbau Principle

• Electrons fill orbitals starting with the lowest energy level ($n=1$) and moving upwards.
• An electron occupies the lowest-energy orbital that can receive it (ground state).
• Subshell filling order is based on the energy order of subshells/sublevels.

Energy Levels and Sublevels

• Orbitals within a sublevel have the same energy (degenerate).
• Sublevels within an energy level are NOT degenerate.
• From the 4th energy level and higher, the $s$ sublevel is lower in energy than the $d$ sublevel of the previous energy level.
• The energy difference between levels becomes smaller for higher energy levels.

Subshell Filling Order

• To determine the filling order, draw a diagram with each energy shell on a row and list the subshells ($s, p, d, f$) for that shell in order of energy (left-to-right).
• Draw arrows through the diagonals, looping back to the next diagonal each time.

Pauli Exclusion Principle

• No orbital may have more than 2 electrons.
• If two electrons occupy the same orbital, they must have opposite spins.
  • $s$ sublevel: 1 orbital, 2 electrons max
  • $p$ sublevel: 3 orbitals, 6 electrons max
  • $d$ sublevel: 5 orbitals, 10 electrons max
  • $f$ sublevel: 7 orbitals, 14 electrons max

Hund’s Rule

• For orbitals in the same subshell, electrons fill each orbital singly before any orbital gets a second electron.
• The lowest energy is attained when the number of electrons with the same spin is maximized.
• Electrons pair up in an orbital only when all orbitals in the same sublevel have one electron.
• When single electrons occupy different orbitals of the same sublevel, they all have the same spin.

Full Electron Configurations

• A method of writing the location of electrons by sublevel.
• The sublevel is written followed by a superscript with the number of electrons in the sublevel (e.g., $2p^2$).
• The lowest energy sublevel is written first, then sublevels with increasing energies.

Writing Electron Configurations

1. Determine the number of electrons in the atom.
2. Arrange the energy sublevels according to increasing energy: $1s, 2s, 2p, …$
3. Fill each sublevel with electrons until all electrons are used.
4. The sum of the superscripts equals the atomic number of the element.