Overview
This lecture explains electron configuration and valence electrons according to quantum mechanical atomic theory, comparing it to Bohr's model and outlining rules for filling electron orbitals.
Electron Configuration in Quantum Theory
- Electron configuration describes how electrons are arranged in atomic orbitals.
- Three key rules: Aufbau principle (fill lowest energy first), Hund's rule (fill orbitals singly before pairing), and Pauli exclusion principle (no two electrons in an atom have all four quantum numbers the same).
- The maximum electrons per orbital type: s = 2, p = 6, d = 10.
- Electron filling order: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s, etc.
- Use an energy level diagram or arrows to determine the filling order.
Determining Maximum Electrons per Orbital
- 1s and 2s orbitals: max 2 electrons each.
- 2p orbital: max 6 electrons (three orbitals, each holds 2 electrons).
- 3s orbital: max 2 electrons.
- 3p orbital: max 6 electrons.
- 3d orbital: max 10 electrons (five orbitals, each holds 2 electrons).
Writing Electron Configuration
- Assign electrons to orbitals following the filling order until total matches atomic number.
- Only fill up to each orbital's maximum as per the rules.
Valence Electrons
- Valence electrons are those involved in forming chemical bonds.
- Main group elements (Group A): valence electrons found in ns and np orbitals.
- Transition group elements (Group B): valence electrons found in ns and (n-1)d orbitals.
- The pattern for valence electrons differs for main groups and transition metals.
Example Problems
- Sodium (Na, Z=11): 1s² 2s² 2p⁶ 3s¹; valence electron = 3s¹ (1 electron).
- Argon (Ar, Z=18): 1s² 2s² 2p⁶ 3s² 3p⁶; valence electrons = 3s² 3p⁶ (8 electrons).
- Scandium (Sc, Z=21): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹; valence electrons = 4s² 3d¹ (3 electrons).
Key Terms & Definitions
- Aufbau Principle — Electrons fill the lowest energy orbitals first.
- Hund's Rule — Electrons occupy orbitals singly before pairing.
- Pauli Exclusion Principle — No two electrons in an atom can have the same set of quantum numbers.
- Valence Electrons — Electrons in the outermost shell involved in chemical bonding.
Action Items / Next Steps
- Practice writing electron configurations using energy diagrams.
- Review valence electron patterns for main and transition groups.
- Prepare for further study on chemical bonding.