CHEM Topic 3c lecture

Electron Configuration

• How electrons are arranged in an atom.
• Based on quantum numbers (energy level, sublevel, orbital, spin).
• Uses diagrams to show where electrons go.

Order of Filling Energy Levels and Sublevels

• Filling order isn't simple due to overlapping energy levels.
• Sublevels: s, p, d, and f.
  • Energy level 1: only s.
  • Energy level 2+: s and p.
  • Energy level 3+: s, p, and d.
  • Energy level 4+: s, p, d, and f.

Diagram Construction

1. List sublevels (s, p, d, f) in columns.
   • Start with 1s, 2s, 3s, up to 7s.
   • Add 2p next to 2s, 3p next to 3s, up to 7p.
   • Include 3d, 4d, 5d, 6d, and 7d.
   • Add 4f, 5f, 6f, and 7f (theoretical).
2. Draw diagonal lines to show filling order.
   • Filling sequence: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, etc.
   • Atoms fill up to the 7p sublevel.

Orbital Filling Diagrams

• Visual way to show electron arrangement.
• Example: Lithium (3 electrons).
  • 1s sublevel: 2 electrons.
  • 2s sublevel: 1 electron.

Diagram Representation

• Each line is an orbital.
  • 1s sublevel: 1 orbital.
• Each orbital holds 2 electrons (opposite spins, shown by arrows).
• Diagram shows all four quantum numbers.
• Beryllium (4 electrons): $1s^2 2s^2$
• Sodium (11 electrons):
  • Fills 1s, 2s, and 2p.
  • p sublevel: 3 orbitals.
  • Electrons fill each orbital one by one before pairing (Hund's rule).
  • Ends with 1 electron in the 3s sublevel.

Electron Configuration

• Short way to write electron arrangement.
• Shows number of electrons in each sublevel, but not individual orbitals or spin.
• Lithium: $1s^2 2s^1$
• Sodium: $1s^2 2s^2 2p^6 3s^1$

Relation to the Periodic Table

• Periodic table shows electron configuration.
  • Lithium (Li): s-block, fills 2s sublevel.
  • Sodium (Na): below lithium, fills 3s sublevel.
  • Carbon: p-block, fills 2p sublevel ($2p^2$).
• Rows (periods): energy level being filled.
• Columns (groups): valence electrons.

Example: Oxygen

• Atomic number 8: 8 electrons.
• Orbital filling:
  • 1s: 2 electrons
  • 2s: 2 electrons
  • 2p: 4 electrons (fill one by one before pairing).
• Electron configuration: $1s^2 2s^2 2p^4$

Large Element Example: Germanium (Ge)

• Atomic number 32.
• Fills many sublevels.
• Filling order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.
  • s sublevel: 1 orbital.
  • p sublevel: 3 orbitals.
  • d sublevel: 5 orbitals.
• Fills up to 4p with 2 electrons.
• Electron configuration: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^2$

Alternative Notation

• Can group sublevels by energy level:
  • $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^2$
• Both ways are correct.

Lewis Dot Diagrams

• Show valence electrons (important for reactions).
• Focus on s and p block elements.
• Oxygen: 6 valence electrons ($1s^2 2s^2 2p^4$).

Diagram Construction

1. Write element symbol.
2. Draw each side as an orbital.
3. Add valence electrons, pairing only after each side has one.
4. Oxygen: 6 electrons around O.

• Nitrogen: 5 valence electrons (2 in s, 3 in p).

Ions

• Charged atoms.
• Sodium loses an electron to chlorine.
• Positive ions (cations): no dots or brackets.
• Negative ions (anions): brackets and charge.
• Chlorine gains an electron: 8 dots and negative charge in brackets.

Shortcuts

• Group number = valence electrons (for s and p block elements).
  • Group 1: 1 valence electron.
  • Group 2: 2 valence electrons.
  • Skip transition metals.
  • Group 13: 3 valence electrons, etc.

Practice Examples

1. Aluminum (Al):
   • Atomic number 13.
   • Electron configuration: $1s^2 2s^2 2p^6 3s^2 3p^1$
   • Lewis dot diagram: 3 dots around Al.
2. Magnesium (Mg