4/3/25 CCP Chemistry Lecture (Unit 3.4, Chap 9)

Chapter Nine: Electron Configurations and the Periodic Table

• Recap of Electron Orbitals

  • Electrons occupy orbitals in a specific order based on certain principles like the Pauli exclusion principle.
  • Filling order starts with:
  • $1s^2$, $2s^2$, $2p^6$, and continues for the first 118 elements.

• Memory Aids for Filling Order

  • While various memory aids exist (like diagonal lines on the periodic table), a more effective method involves using the periodic table.
  • The periodic table is divided into blocks that correspond to the types of orbitals being filled:
  • s block: Groups 1 and 2.
  • p block: Groups 13 to 18 (with relevant exceptions).
  • d block: Transition metals (Groups 3 to 12).
  • f block: Lanthanides and actinides, often placed below the main table.

• Understanding Periodicity and Electron Configuration

  • The correlation between each period (row) and the value of n, which indicates the principal quantum number.
  • For s and p blocks, $n$ equals the row number. For the d block, $n$ equals the row number minus one, and for the f block, $n$ equals the row minus two.

• Examples of Electron Configurations

  • For sulfur (S, atomic number 16):
  • Ends in $3p^4$; located in P block of period 3.
  • For chromium (Cr, atomic number 24):
  • Ends in $3d^4$; located in D block of period 4.
  • For gallium (Ga, atomic number 31):
  • Ends in $4p^1$; also in period 4.
  • For iron (Fe, atomic number 26):
  • Follows the pattern starting with $1s^2$ and ends in $3d^6$.

• Anomalous Configurations

  • Some elements have unexpected configurations (e.g., copper and lanthanum).
  • Copper (Cu) is expected to be $4s^2 3d^9$ but is actually $4s^1 3d^{10}$.
  • Lanthanum (La) should ideally have a configuration of $6s^2 4f^{14}$ but is $6s^2 5d^1$ instead.

• Standard Notation vs. Noble Gas Notation

  • Standard Notation: Complete electron configuration.
  • Noble Gas Notation: Uses brackets to represent the configuration up to the nearest noble gas, simplifying the configuration writing process. For example, calcium (Ca) can be written as [Ne]$3s^2$.

• Orbital Diagrams

  • Another method for illustrating electron configurations.
  • Represent subshells as boxes:
    • $s$ = 1 box, $p$ = 3 boxes, $d$ = 5 boxes, $f$ = 7 boxes.
  • Electrons are represented as arrows within these boxes, adhering to the Pauli exclusion principle and Hund’s Rule, which states “don’t pair until you have to”.

• Important Principles to Remember

  • Follow the order of subshell filling based on periodic table trends.
  • Understand the limitations of the periodic table due to configurations that involve f-block elements and their anomalous behaviors.
  • Focus on predicting configurations rather than memorizing them, leveraging the periodic table for guidance.