chemistry C1-C3

• How and why the atomic model has changed over time

  1. John Dalton proposed atomic theory

  2. Thomson proposed plum pudding model

  3. Rutherford proposed atoms with positive nucleus and negative electrons

  4. Bohr proposed electrons occupy shells around nucleus

  5. Geiger and Marsden carried out experiment that led to theory of nucleus existing within atom
• Atom


  • Positively charged nucleus surrounded by negatively charged electrons
  • Nuclear radius much smaller than atom
  • Most of mass is in nucleus
• Atoms and small molecules are incredibly small
• Small molecules are larger in size than atoms
• Typical atomic radii and bond length are in the order of 10^-10 m
• Relative charges and approximate relative masses of particles

  • Proton: +1, 1
  • Neutron: 0, 1
  • Electron: -1, 1/1836
• Calculating numbers of protons, neutrons and electrons in atoms and ions

  1. Atomic number = number of protons (= number of electrons if atom, not ion)

  2. Isotope = atoms of same element with different number of neutrons

  3. Mass number = number of protons + number of neutrons

  4. Ion = atom/group with positive or negative charge

  5. For atom: number of electrons = atomic number

  6. For ion: number of electrons = atomic number +/- charge