Overview
This lecture explains how to predict and write electron configurations for atoms and ions, discussing related periodic table trends, exceptions, and the significance of valence electrons.
Orbital Energies and Atomic Structure
- Atomic orbital energy increases with principal quantum number (n); within a shell, energy increases in order: s < p < d < f.
- In larger atoms, some subshells (e.g., 4s and 3d) overlap in energy.
- Shielding reduces the nucleus' attraction on outer electrons, further affecting orbital energies.
- Electron configuration notation: principal quantum shell (n), subshell type (l), and number of electrons as a superscript (e.g., 2p⁴).
The Aufbau Principle and Filling Order
- The Aufbau principle states electrons fill the lowest available energy orbitals first.
- Electrons fill orbitals in the observed energy order, not strictly by n value alone.
- The Pauli exclusion principle: no two electrons in an atom have the same four quantum numbers.
- Hund's rule: electrons occupy degenerate orbitals singly before pairing.
Writing Electron Configurations
- Each atom’s electron configuration follows a specific filling sequence (e.g., H: 1s¹, He: 1s², Li: 1s²2s¹).
- Orbital diagrams use arrows to depict paired and unpaired electrons in boxes for each orbital.
- Condensed notation uses noble gas symbols to represent core electrons (e.g., Na: [Ne]3s¹).
Electron Configuration Patterns and Exceptions
- Periodic table groups share similar valence shell configurations, explaining chemical similarities.
- Exceptions occur where subshell energy differences are minimal, often stabilizing half-filled or filled subshells (e.g., Cr, Cu).
- Transition and inner transition metals show additional irregularities in electron removal and addition.
Electron Configurations for Ions
- Cations form by electron removal; anions form by electron addition, following the typical order of orbital energies.
- In transition metals, s electrons are removed before d electrons when forming cations.
Classifying Elements by Electron Configuration
- Main group elements: last electron enters s or p orbital; highest n electrons are valence.
- Transition elements: last electron enters d orbital; valence includes ns and (n–1)d electrons.
- Inner transition elements: last electron enters f orbital; valence includes (n–2)f, (n–1)d, and ns electrons.
Key Terms & Definitions
- Electron Configuration — arrangement of electrons in orbitals for an atom.
- Aufbau Principle — electrons occupy lowest energy orbitals available.
- Pauli Exclusion Principle — no two electrons in the same atom have the same four quantum numbers.
- Hund's Rule — electrons fill degenerate orbitals singly before pairing.
- Valence Electrons — electrons in the atom's outermost shell involved in chemical reactions.
- Core Electrons — inner electrons not involved in bonding.
- Transition Elements — elements with last electron added to a d orbital.
- Inner Transition Elements — elements with last electron added to an f orbital.
- Shielding — reduction in effective nuclear charge on outer electrons by inner electrons.
Action Items / Next Steps
- Practice writing full and condensed electron configurations for elements and ions.
- Use the periodic table to predict electron configurations and classify elements.
- Review and memorize the filling order of atomic orbitals for quick recall.