Atoms and Subatomic Particles — Quick Notes

History and Concept of the Atom

  • Early Greek idea: matter was explored by cutting into smaller pieces; eye-limits prevented seeing the atom; matter once thought infinitely divisible.
  • Dalton (early 1800s): atoms combine in simple, whole-number ratios; law of multiple proportions established; led to the concept of definite atomic compositions.
  • Brownian motion (Einstein, Perrin): provided evidence for atoms existing in motion.
  • Electron discovery via cathode-ray tube: electrons exist; concept of charge-to-mass ratio (e/m).
  • Millikan oil-drop experiment: measured electron charge magnitude and established e ≈ −1.60imes10−19-1.60 imes 10^{-19} C; electron mass ≈ 9.11imes10−289.11 imes 10^{-28} g.
  • Rutherford gold foil experiment: most of the atom is empty space; nucleus at center; protons and neutrons in a tiny dense core.
  • Plum pudding model (early atomic model): electrons embedded in a positively charged substance; replaced by the nuclear model after Rutherford’s results.
  • Nuclear atom: nucleus contains protons and neutrons; electrons orbit in the surrounding space; atom is mostly empty space.
  • Size scale: atom ≈ 10−10extm10^{-10} ext{ m} (Å); nucleus is tiny and very dense; most of the atom is empty space.
  • Mass relationship: proton mass ≈ 2000 × electron mass (mp ≈ 2000 me); neutrons similar to protons.
  • Bridging micro and macro: Avogadro’s constant, NA=6.022imes1023extmol−1N_A = 6.022 imes 10^{23} ext{ mol}^{-1}; connects atomic scale to moles.
  • Angstrom unit: 1extA˚=10−10extm1 ext{ Å} = 10^{-10} ext{ m}.

Key Experiments and Models

  • Thomson cathode-ray tube: deflection by electric and magnetic fields showed electrons carry charge; established electron as a component of atoms.
  • Electric/magnetic deflection and electron beam experiments (electron gun): allowed measurement of charge-to-mass interactions and the concept of e/m.
  • Bar magnet deflection: magnetic fields affect moving charges (electrons) and helped confirm charge-based deflection.
  • Millikan oil-drop: determined absolute electron charge e=−1.60imes10−19extCe = -1.60 imes 10^{-19} ext{ C}; combined with e/m to obtain electron mass me=9.11imes10−28extgm_e = 9.11 imes 10^{-28} ext{ g}.
  • Rutherford scattering: alpha particles vs. gold foil revealed a dense nuclear center and mostly empty space, leading to the nuclear model.
  • Nuclear model vs. plum pudding: shift from distributed positive charge to a central positively charged nucleus with surrounding electrons.

Atomic Structure Today

  • Atom: nucleus (protons + neutrons) + electrons in surrounding space; most of the atom’s volume is empty space.
  • Nucleus is extremely dense; protons are positively charged; neutrons neutral; electrons are negatively charged.
  • Mass hierarchy: m<em>poughly2000imesm</em>em<em>p oughly 2000 imes m</em>e; neutron mass ≈ proton mass.
  • Ionization changes mass negligibly; electrons add/remove only tiny mass relative to nucleons.
  • Atomic behavior and chemistry are governed by the arrangement and interactions of electrons around a dense nucleus.

Subatomic Particles and Fundamental Constants

  • Electron: charge e=−1.60imes10−19extCe = -1.60 imes 10^{-19} ext{ C}; mass me=9.11imes10−28extgm_e = 9.11 imes 10^{-28} ext{ g}.
  • Proton: charge +e; mass m<em>pext(≈2000imesm</em>e)m<em>p ext{ (≈ }2000 imes m</em>e).
  • Neutron: charge 0; mass ≈ proton mass.
  • Avogadro's constant: NA=6.022imes1023extmol−1N_A = 6.022 imes 10^{23} ext{ mol}^{-1}; bridges microscopic atoms to macroscopic moles.
  • Angstrom: 1extA˚=10−10extm1 ext{ Å} = 10^{-10} ext{ m};
  • Law of multiple proportions (Dalton): compounds form in simple integer ratios; ratios by mass reflect atomic composition.
  • Example of mass ratios (same element C and O in different compounds):
    • CO₂: rac{mO}{mC} ig|{CO2} o 2.67
    • CO: rac{mO}{mC} ig|_{CO} o 1.33 (approx.)

Quick Concepts and Takeaways

  • Atom = nucleus (protons + neutrons) + electrons; nucleus is dense; electrons occupy surrounding space.
  • Protons and neutrons reside in the nucleus; electrons orbit; most of the atom is empty space.
  • Electron mass is ~1999/2000 of the proton’s mass difference; mp ≈ 2000 me.
  • Central experiments shaped the modern view: Dalton’s ratios, Brownian motion, Thomson’s electron, Millikan’s charge, Rutherford’s nucleus.
  • Units and constants to memorize for quick recall: e=−1.60imes10−19extC,extm<em>e=9.11imes10−28extg,extN</em>A=6.022imes1023extmol−1,ext1extA˚=10−10extme = -1.60 imes 10^{-19} ext{ C}, ext{ } m<em>e = 9.11 imes 10^{-28} ext{ g}, ext{ } N</em>A = 6.022 imes 10^{23} ext{ mol}^{-1}, ext{ } 1 ext{ Å} = 10^{-10} ext{ m}
  • Avogadro’s constant enables conversion between microscopic atoms and macroscopic quantities.