Electronic Structure of Atoms

Introduction

• The energy of electrons is quantized
  • Quantization: only specific values of energy are possible
  • Shells only occur at quantized energies
  • 3 important effects balance each other
    • Electrostatic attraction that draws then electrons toward the nucleus
    • Electrostatic repulsion between the electrons
    • The wavelike nature of an electron that prefers to be delocalized, spreading the electron density over a large volume space
• Electron shells are identified by the quantum numbers 1,2,3,4,etc
• Each shell can contain up to 2n^2 electrons
  • n=the number of the shell
• Electrons in lower numbered shells are closest to the positively charged nucleus and are held most strongly by it
  • Lowest in energy
• Electrons in higher numbered shells are furthest from the post charged nucleus and are held less strongly
  • Higher in energy
• Shells are divided into subshells (spdf)
  • Within these sub shells, electrons are grouped into orbitals
  • Orbital: a region of space that can hold 2 electrons and has a specific quantized energy
  • Orthogonal: orbitals have no net overlap

Electron Configuration of Atoms

• Ground state electron configuration: electron configuration of lowest energy
• Aufbau principle: orbitals fill in order of increasing energy, from lowest to highest
  • 1s, 2s, 2p, 3s, 3p, etc
• Pauli exclusion principle: only 2 electrons can occupy an orbital and their spins must be paired
  • A given electron can exist in only two different spin states
  • 2 electrons with opposite spins are said to have paired spins
• Hund’s rule: when orbitals of equal energy (called degenerate) are available but there aren’t enough electrons to fill all of them completely, one electron is added to each orbital before a second one is added to any of them; spins of the single electrons in the degenerate orbitals should be aligned
  • Partially filling orbitals as much as possible minimizes electrostatic repulsion between electrons
• Energy level diagrams: pictorially designate where electrons are placed in an electron configuration
  • Moving up the diagram means higher energy
  • Electrons are drawn as arrows
  • Lines draw indicate relative energy

The Concept of Energy

• Energy: the ability to do work
• The higher in energy an entity is, the more work it can perform
• Excited state: a state of a system at higher energy than the ground state
  • Unstable in comparison to ground state
• All of nature seeks its lowest energy state
• When the electrons are rearranged back to the ground state, energy is released
• Electrons in lower energy orbitals are held tightly to the nucleus
  • Would take a large amount of energy to remove these electrons
  • Ionization potential: the energy it takes to remove an electron from an atom or molecule
    • The closer the electron is to the nucleus, the greater the ionization potential

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