Electron Arrangement in Atoms OBJECTIVES 1 Describe how

Electron Arrangement in Atoms OBJECTIVES 1. Describe how to write the electron configuration for an atom. 2. Explain why the actual electron configurations for some elements differ from those predicted by the aufbau principle

ELECTRON CONFIGURATIONS • Three principles are used to distribute electrons in orbitals • Aufbau or Building up Principle. Orbitals are filled in order of increasing energy: 1 s, 2 p, 3 s, 3 p, 4 s, 3 d, 4 p, 5 s, 4 d, 5 p, 6 s, 4 f, 5 d, 6 p. • Pauli Exclusion. No more than 2 electrons can occupy an orbital and then only if they have opposite spins. • Hund’s Rule. One electron is placed in each equal-energy orbital so that the electrons have parallel spins, before pairing occurs.

Aufbau Principle

Pauli Exclusion Principle

Hund’s Rule

Hund’s Rule Examples

Exceptions to the Rules! • The Aufbau orbital filling principle can be used for elements up to Vanadium (atomic # 23). • Some actual electron configurations differ because of their tendency to want to be stable. • These exceptions are due to subtle e- to e- interactions in orbitals with very similar energies. • Examples: Chromium and Copper

Exceptions to the Rules! • Cr 1 s 2 2 p 6 3 s 2 3 p 6 3 d 4 4 s 2 (aufbau) • Cr 1 s 2 2 p 6 3 s 2 3 p 6 3 d 5 4 s 1 (correct) • Cu 1 s 2 2 p 6 3 s 2 3 p 6 3 d 9 4 s 2 (aufbau) • Cu 1 s 2 2 p 6 3 s 2 3 p 6 3 d 10 4 s 1 (correct)

Practice!!!! In addition: include the orbital box diagrams for each and the planetary model for each.