ELECTRON CONFIGURATION

Modern Electron Theory

  • Describes electrons using quantum mechanics; electrons occupy energy levels and orbitals, not fixed paths.

Atomic Spectroscopy

  • Energy absorption excites electrons to higher levels, releasing light of specific colors upon return.

  • Each element has a unique line spectrum; practical applications include fireworks and astronomy.

Bohr Model

  • Proposed by Niels Bohr (1913); electrons in specific orbits at certain distances from the nucleus.

Electron Cloud

  • Are space in which electrons are likely to be found.

  • Location of electrons depends upon how much energy the electron has.

  • Electron with the lowest energy are found in the energy level closest to the nucleus.

  • Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.

Wave Model

  • Based on wave mechanics; electrons do not follow definite paths.

  • Electron cloud indicates probable locations rather than precise paths.

  • Louis de Broglie (1924) suggested that not only light but also

    matter (like electrons) behaves as a wave.

Electron microscopes

  • Can see objects much smaller than light microscopes because electrons have tiny wavelengths.

Heisenberg Uncertainty Principle

  • States that both position and momentum of an electron cannot be precisely determined simultaneously.

  • Describes electron locations as probability clouds.

  • Instead of sharp orbits, we talk about probability clouds—places where an electron is most likely to be.

Quantum Mechanics

  1. The Schrödinger Model

    • Erwin Schrödinger developed equations to describe electrons as

    waves in 3D space.

    These areas of high probability are called orbitals.

    • Types of orbitals:

    • s: spherical

    • p: dumbbell-shaped

    • d and f: more complex

  2. • Each electron is described by four quantum numbers:

    • Principal number (n) – the energy level or shell (1, 2, 3…).

    • Angular number (l) – shape of the orbital (s, p, d, f).

    • Magnetic number (mₛ) – orientation of the orbital in space.

    • Spin number (mₛ) – spin direction (+½ or –½).

Quantum Numbers

  • Four quantum numbers describe electrons:

    • Principal (n): energy level

    • Angular (l): shape of orbital

    • Magnetic (m): orientation of orbital

    • Spin (s): direction of electron spin.

Pauli’s Exclusion Principle

  • No two electrons can have the same four quantum numbers; each orbital holds 2 electrons with opposite spins.

Aufbau’s Principle

  • Electrons fill lower-energy orbitals first; order of filling: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p…

Hund’s Rule

  • Electrons fill orbitals of the same energy singly before pairing to reduce repulsion; stability increases.

Electronic Configuration

  • Represents the arrangement of electrons in an atom.

  • Notation rules follow Pauli's, Aufbau's, and Hund's principles.

  • Examples include:

    • Hydrogen (Z=1): 1s¹

    • Oxygen (Z=8): 1s² 2s² 2p⁴.

Summary

  • Atomic spectroscopy reveals unique electron movements and spectra.

  • Wave-particle duality described by De Broglie; uncertainty in electron location affirmed by Heisenberg.

  • Quantum mechanics defines electron behavior; theoretical principles guide orbital filling.

  • Electronic configuration aids in predicting chemical behavior based on arrangement.