Overview
This lecture covers the interpretation of mass spectra, details of atomic structure (shells, subshells, orbitals), electron configuration—including exceptions for certain elements—and introduces ionization energy, its calculation, and factors affecting it.
Mass Spectrometry Review
- Mass spectrometer produces a mass spectrum (graph of mass/charge ratio vs relative or percentage abundance).
- A mass spectrum reveals the number of isotopes (count of peaks), mass of isotopes (x-axis), and their abundance (y-axis).
- Percentage abundance totals 100%; relative abundance may not.
- The mass spectrum helps identify isotopes and calculate relative atomic mass.
Atomic Shells, Subshells & Orbitals
- Main shells are labeled by quantum number n (e.g., n=1, 2, 3), with shells farther from the nucleus having higher energy.
- Each shell contains subshells (s, p, d), and subshells contain orbitals.
- An orbital is a region with >90% probability of finding an electron; orbitals have distinct shapes (s: spherical, p: dumbbell, d: double dumbbell).
- Maximum electrons: s = 2, p = 6, d = 10 per subshell.
Electron Configuration & Filling Rules
- Electrons fill from lowest to highest energy: 1s → 2s → 2p → 3s → 3p → 4s → 3d, etc.
- Aufbau Principle: fill lowest energy orbitals first.
- Pauli Exclusion Principle: each orbital holds two electrons with opposite spins.
- Hund's Rule: fill degenerate (equal energy) orbitals singly before pairing.
Special Cases in Electron Configuration
- Chromium (Cr) and copper (Cu) have stable "half-filled" or "fully-filled" d orbitals: Cr is [Ar] 3d⁵ 4s¹, Cu is [Ar] 3d¹⁰ 4s¹.
- For ions, remove electrons from the highest energy orbital (usually outermost s orbital) first.
Periodic Table: Groups, Periods, Blocks
- Groups are based on valence (outermost) electrons; periods are the number of electron shells.
- Blocks (s, p, d, f) are based on the highest energy orbital filled in the atom.
- Main group elements: Groups 1-2 and 13-18; transition metals: Groups 3-12 (d block).
Ionization Energy
- Ionization energy is energy required to remove one electron from a gaseous atom (measured in kJ/mol).
- First ionization energy: remove the first electron; subsequent ionization energies remove additional electrons.
- Successive ionization energies show "jumps" when electrons are removed from a new shell (closer to the nucleus).
Graphs & Analysis of Ionization Energy
- Small increases in ionization energy indicate removal from the same shell; large jumps indicate removal from an inner shell.
- Number of "jumps" on an ionization energy graph = number of shells; valence electrons = electrons before first big jump.
Factors Affecting Ionization Energy
- Atomic size: larger size → lower ionization energy.
- Nuclear charge (number of protons): higher nuclear charge → higher ionization energy.
- Shielding effect: more inner electrons (greater shielding) → lower ionization energy.
Key Terms & Definitions
- Mass Spectrum — graph showing mass-to-charge ratio vs. abundance of ions.
- Isotope — atoms with the same number of protons but different numbers of neutrons.
- Orbital — region where there’s >90% chance of finding an electron.
- Aufbau Principle — electrons fill lowest energy orbitals first.
- Pauli Exclusion Principle — no two electrons in the same orbital can have the same spin.
- Hund's Rule — electrons occupy degenerate orbitals singly before pairing.
- Ionization Energy — energy required to remove one mole of electrons from one mole of gaseous atoms.
- Shielding Effect — reduction in attraction between nucleus and outer electrons due to inner electrons.
Action Items / Next Steps
- Review electron configuration rules and exceptions (especially for Cr and Cu).
- Practice writing electron configurations for atoms and ions.
- Analyze ionization energy graphs for valence electron count and shell number.
- Prepare for a detailed discussion of factors affecting ionization energy in the next class.