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