Aufbau Diagrams and Electron Configuration

The Aufbau Principle and Energy Levels

• Aufbau Principle: Derived from the German word Aufbauen ("to build"), this states that electrons fill lower-energy atomic regions before filling higher-energy ones.
• Energy Levels (Shells): The period number of an element indicates the number of energy levels in its atom. Energy increases as the distance from the nucleus increases ($n=1$ has the lowest energy).
• Electron Capacities:
  • Energy level 1: Max 2 electrons.
  • Energy level 2: Max 8 electrons.
  • Energy level 3: Max 8 electrons (based on provided diagrammatic scope).

Atomic Orbitals

• Definition: Atomic orbitals are three-dimensional regions of space around the nucleus where electrons are found with high probability.
• Shapes:
  • s orbital: Spherical shape.
  • p orbital: Hourglass shape.
• Distribution:
  • Energy level 1: Contains one s orbital ($1s$).
  • Energy level 2: Contains one s orbital ($2s$) and three p orbitals ($2p$).
  • Energy level 3: Contains one s orbital ($3s$) and three p orbitals ($3p$).

Rules for Drawing Aufbau Diagrams

• Representation: Orbitals are represented as blocks or squares, and electrons are shown as arrows.
• Energy Order: Within an energy level, s orbitals are at a lower energy than p orbitals.
• Orbital Limit: Each individual orbital can hold a maximum of 2 electrons. Consequently, a p subshell (3 orbitals) can hold 6 electrons.
• Pauli's Exclusion Principle: Two electrons sharing an orbital must have opposite "spin," represented by arrows pointing in opposite directions.
• Hund's Rule: Electrons will occupy empty orbitals within a subshell singly before pairing up to minimize repulsion. Electrons in singly occupied orbitals must have the same spin.
• Building Sequence: Electrons fill the lowest available energy orbital completely (e.g., $1s$) before occupying higher energy levels (e.g., $2s$).