Lecture Notes: Introduction to Electron Configuration

Key Concepts:

• Electron Configuration: Arrangement of electrons in the orbitals of an atom.
• Atomic and Mass Numbers: Atomic number indicates the number of protons (and electrons in a neutral atom). Mass number is the total number of protons and neutrons.
• Energy Levels and Sublevels:
  • 1st energy level: 1s
  • 2nd energy level: 2s, 2p
  • 3rd energy level: 3s, 3p, 3d
  • 4th energy level: 4s, 4p, 4d, 4f
• Sublevel Capacities:
  • s: 2 electrons
  • p: 6 electrons
  • d: 10 electrons
  • f: 14 electrons

Electron Configuration Examples:

Nitrogen Atom (N):

• Atomic Number: 7 (7 electrons)
• Electron Configuration:
  • Start with 1s: 1s2
  • Move to 2s: 2s2
  • Then 2p: 2p3
• Total: 2 + 2 + 3 = 7 electrons

Aluminum Atom (Al):

• Atomic Number: 13 (13 electrons)
• Electron Configuration:
  • Start with 1s: 1s2
  • Then 2s: 2s2
  • Continue with 2p: 2p6
  • Move to 3s: 3s2
  • Finally, 3p: 3p1
• Total: 2 + 2 + 6 + 2 + 1 = 13 electrons

Iron Ion (Fe 2+):

• Atomic Number of Fe: 26
• Fe 2+ has 24 electrons (loss of 2 electrons)
• Process:
  • Write configuration of neutral Fe: 1s2 2s2 2p6 3s2 3p6 4s2 3d6
  • Remove two electrons from the highest energy level (4s): 1s2 2s2 2p6 3s2 3p6 3d6

Chloride Ion (Cl-):

• Chlorine Atomic Number: 17
• Cl- has 18 electrons (addition of 1 electron)
• Electron Configuration:
  • Start with 1s: 1s2
  • Then 2s: 2s2
  • Continue with 2p: 2p6
  • Move to 3s: 3s2
  • Finally, 3p: 3p6
• Total: 2 + 2 + 6 + 2 + 6 = 18 electrons

Important Points:

• For neutral atoms, electron configuration matches atomic number.
• For ions:
  • Positive ions: Subtract electrons based on charge.
  • Negative ions: Add electrons based on charge.
  • For transition metals, write the configuration for the neutral atom first, then adjust for the ion.
• Remember to remove/add electrons from/to the highest energy level first when dealing with ions.