Preparation for Lecture

• Print two worksheets labeled 'one' and 'two' before proceeding.
• Worksheets will be used to write down information during the lecture.
• Take a moment to pause if you're ready to proceed.

Introduction to Electron Configurations

• This lecture focuses on:
  • Building up the elements in periods one and two of the periodic table.
  • Understanding ground state electron configurations using new notation.
  • Describing the spin orientation of an electron through a new quantum number, m sub s.
  • Learning the Pauli exclusion principle which governs electron filling of orbitals.
  • Applying Hund's rule for distributing electrons in orbitals of the same energy.

Historical Context and Purpose

• Before quantum mechanics, chemistry students faced difficulties in memorizing properties and reactions of individual elements.
• Patterns in element behavior led to recognition of their arrangement in the periodic table (columns and rows).
• Goal of the lecture: Understand the electron configurations to explain the formation of the periodic table.

Quantum Mechanics Overview

• Previous focus was on one-electron atoms, especially hydrogen (wave functions, orbitals):
  • Types of orbitals discussed: 1s, 2s, 2p, 3s, 3p, 3d, etc.
• Many-electron atoms: Understanding begins with Helium as the simplest example.
• Examples of many-electron atoms:
  • Lithium
  • Sodium
  • Potassium

Impact of Multiple Electrons

• With more than one electron, electron-electron repulsions occur.
• Resulting energy effects:
  • Orbitals with the same principal quantum number (n) no longer have equivalent energy when differing in l.
  • Example: 2s and 2p orbitals are no longer energetically identical.
  • Similarly, three s, p, and d orbitals differ in energy levels.

Ground State Electron Configurations

• The atomic number (Z) indicates the number of protons, which equals electrons in a neutral atom.
• Each successive element in the periodic table has an atomic number that increases by 1.
• Task: Identify the arrangement of electrons to minimize their total energy.
• Energy level diagram provided for electron distribution.

Notation for Electron Configurations

• Two methods to represent electron configurations:
  • Orbital Diagram: Depicts each orbital and its electron distribution.
  • SPDF Notation: Further abbreviates configurations for convenience.
  • 1s^1 for one electron in 1s orbital.

Example: Hydrogen Atom (Atomic Number 1)

• Configuration: 1 electron goes into the 1s orbital.
  • Representation: Up arrow for spin up.
  • Notation: 1s^1

Example: Helium Atom (Atomic Number 2)

• Configuration: First electron in 1s (up arrow), second electron in 1s (down arrow).
  • Representation: Spin states defined as an up arrow and a down arrow.
  • Quantum numbers for first electron:
  • n = 1
  • l = 0
  • msubl = 0
  • msubs = +1/2 (spin up)
  • Quantum numbers for second electron:
  • n = 1
  • l = 0
  • msubl = 0
  • msubs = -1/2 (spin down)
  • Resulting notation: 1s^2

Quantum Spin and the Pauli Exclusion Principle

• Spin represented by a fourth quantum number, m sub s:
  • Values: +1/2 (spin up) or -1/2 (spin down).
  • Electron viewed as a tiny bar magnet with this spin orientation.
• Pauli Exclusion Principle:
  • No two electrons in an atom can share all four quantum numbers.
  • Each orbital can hold a maximum of two electrons with opposite spins.

Example: Lithium Atom (Atomic Number 3)

• Configuration: Three electrons are placed as follows:
  • Fill 1s with two electrons (one up, one down).
  • The third electron goes to the next available orbital (2s).
  • Configuration result: 1s^2 2s^1

Filling Higher Orbitals

• After filling 1s and 2s, electrons will proceed to fill higher subshells:
  • Filling pattern proceeds similarly to lower energy levels.
  • For example, next elements will progress to 2p orbitals.

Conclusion

• Exhaustive examination of electron configurations from hydrogen to neon completed.
• Next elements will begin to fill the 3s and 3p subshells.
• Attendees should be prepared for assessments related to this material and encouraged to revisit challenging concepts for clarification.