Atomic Energy Levels: Key Concepts and Principles

Atomic Energy Levels (Part 2)

  • Aufbau principle: Atomic orbitals are filled with electrons, influencing an atom's properties and periodic table location.

Periodic Table Structure1

  • Elements are arranged by increasing atomic number and electron number.

  • Rows indicate the principal quantum number (n).

  • Columns represent groups with similar chemical properties.

  • Blocks (s, p, d, f) denote the highest energy filled orbital.

Aufbau Principle and Orbital Filling

  • Electrons occupy the most stable available orbital.

  • No two electrons have identical quantum numbers.

  • Orbital capacities: s (2), p (6), d (10), f (14) electrons.

  • Filling order follows the n + l rule; lower n fills first if n + l is equal.

Electron Configurations

  • Representation: quantum numbers, standard listing (e.g., 1s^2 2s^2 2p^2), or arrows.

  • Noble gas notation shortens configurations, emphasizing valence electrons (e.g., Carbon: [He] 2s^2 2p^2).

Valence Electrons

  • They are electrons furthest away or the outermost electrons from the nucleus that determine chemical behavior.

  • Core electrons (electrons closest to the nucleus) are up to the previous noble gas configuration and do not participate in reactions.

Hund's Rule

  • For degenerate orbitals, the lowest energy configuration maximizes unpaired electrons with the same spin orientation.

D-Block Electron Configuration

  • (n+1)s orbitals fill before nd orbitals due to lower energy.

  • Exceptions occur due to the stability of filled or half-filled orbitals (e.g., Cr: [Ar] 4s^1 3d^5).

Ions

  • Cations (positive charge) are smaller than neutral atoms; anions (negative charge) are larger.

  • Isoelectronic species have the same number of electrons (e.g., F^− and O^{2−}).

Periodic Trends

  • Atomic radii: Increase down a group (increasing n), decrease across a period (increasing effective nuclear charge, Z_{eff}).

  • Ionization energy: energy required to remove an electron from an atom. Decreases down a group, increases across a period.

  • Ionization energy mirrors orbital size