Hund's Rule and Electron Configuration

Atomic Structure

• An atom is electrically neutral because the number of protons are equal to the number of electrons.
• Atoms do not lose or gain protons in any chemical reaction, but they do lose or gain electrons to become ions.

Nuclear Atomic Model

• Thomson's model: proposed a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces.
• Rutherford's model: atoms have a positive charge concentrated in the nucleus (dense central core within an atom).
• Dual nature of electrons: electron energy is quantized, and electron is both particle and wave.
• De Broglie's postulate: electron has wave-like properties.
• Heisenberg's uncertainty principle: not possible to know the unlimited accuracy both the position and momentum of an electron.

Bohr's Atomic Model

• Bohr's model: energy of the atom is quantized, and the amount of energy in the atom is related to the electron's position in the atom.
• Postulates:
  • The single electron in H atom moves around the nucleus in orbits or energy levels.
  • The H atom has only certain energy levels, which are called stationary states.
  • Each state is associated with a fixed circular orbit of the electron around the nucleus.
  • The higher the energy level, the farther the orbit is from the nucleus.
  • When the H electron is in the first orbit, the atom is in its lowest energy state, called the ground state.

Quantum Numbers

• Principal quantum number (n): describes the energy level of an electron.
  • n can be any integer ≥ 1.
  • The larger the value of n, the more energy the orbital has.
  • The larger the value of n, the larger the orbital.
• Angular momentum quantum number (l): describes the shape of the orbital.
  • l = 0, 1, 2, 3, … n-1.
  • Shape of the orbital changes with l.
• Magnetic quantum number (ml): describes the orientation of the orbital.
  • ml = -l, …., 0, ….
  • The total number of orbitals is 1 + 3 + 5 = 9.
• Spin quantum number (ms): describes the spin of an electron.
  • ms = +½ or −½.
  • Spin up or spin down.

Electron Configurations and Orbital Diagrams

• Electron spin: a fundamental property of all electrons.
• Orbital diagrams: use a square to represent each orbital and a half-arrow to represent each electron in the orbital.
• By convention, a half-arrow pointing up is used to represent an electron in an orbital with spin up.
• Spins must cancel in an orbital with 2 electrons (paired e-).

Electronic Configurations

• Partial orbital diagrams: show only the highest energy sublevels being filled.
• Condensed electron configurations: have the element symbol of the previous noble gas in square brackets.
• Hund's rule: the most stable arrangement of electrons in subshells is the one with the greatest number of parallel spins.

Checkpoints

• Checkpoint 3: write the four quantum numbers for an electron in a 3p orbital.
• Checkpoint 4: the maximum number of electrons that can reside in the orbitals is 2 × 9 = 18.
• Checkpoint 5: write the four quantum numbers for each of the eight electrons in the ground state of an oxygen atom.