Bohr Model and Octet Rule - Key Concepts

Bohr Model and Electron Shells

  • Nucleus contains protons and neutrons; electrons orbit the nucleus.
  • In electrically neutral atoms, electrons = protons.
  • Electron shells are energy levels designated by the principal quantum number nn; the lowest energy shell is closest to the nucleus.
  • Electrons fill the lowest available shell first; energy input (e.g., from a photon) can promote them to a higher shell, but they typically decay back to the ground state.
  • Shell capacities (pattern continues beyond the third shell): K=2K=2, L=8L=8, M=18M=18; the sequence continues for higher shells.
  • Bohr diagrams depict electrons orbiting in shells: K, then L, then M, outward from the nucleus.
  • Examples of neutral atom configurations (outer shells shown):
    • Lithium: K(2), L(1)\mathrm{K}(2),\ \mathrm{L}(1)
    • Fluorine: K(2), L(7)\mathrm{K}(2),\ \mathrm{L}(7)
    • Aluminum: K(2), L(8), M(3)\mathrm{K}(2),\ \mathrm{L}(8),\ \mathrm{M}(3)
  • Noble gases (Group 18) have full outer shells and are highly unreactive.

Octet Rule and Stability

  • The octet rule: atoms are most stable when, aside from the first shell, their outermost shell contains 88 electrons.
  • Hydrogen is an exception; its valence shell can hold a maximum of 22 electrons.
  • Helium has a complete outer shell of 22 electrons.
  • Neon has a complete outer shell of 88 electrons.
  • Atoms with incomplete outer shells tend to gain or lose electrons to reach a full octet.

Ions and Periodic Trends

  • Group 1 elements (e.g., hydrogen, lithium, sodium) have 1 electron in their outer shell and tend to lose it to form positive ions, e.g., Na+\mathrm{Na}^{+}.
  • Group 17 elements (e.g., fluorine, chloride) have 7 electrons in their outer shell and tend to gain 1 electron to form negative ions, e.g., F−\mathrm{F}^{-}.
  • The columns of the periodic table reflect the tendency to gain/lose electrons and the resulting common charge.
  • Group 18 elements (He, Ne, Ar) have full outer shells and are inert.

Lewis Symbols

  • Lewis Symbols are simplified Bohr diagrams showing only the electrons in the outermost energy level.

Summary

  • The nucleus contains most of the atom’s mass; electrons contribute minimally to mass but determine charge.
  • Electrons fill inner shells first; stability is associated with a full outer shell (octet, 88 electrons after the first shell).
  • Atoms without a full outer shell tend to gain or lose electrons to reach a stable configuration.
  • Ions form by gain or loss of electrons; periodic table groups reflect outer-shell configurations and chemical properties.
  • Lewis symbols condense Bohr diagrams to the outer-shell electron arrangement.