Chapter 30.2 Notes

Atomic Structure

 If you haven't yet explored the Particle Adventure online, I highly recommend that you do.

There are several groups of fundamental particles:


  • There are 6 "flavors" of quarks,(up, down, charm, strange, top and bottom)
  • Each can exist as one of three colors, making for a total of 18 different possible quarks.
  • Quarks can combine into quark-anitquark pairs called mesons, or 2 quark pairs, and 3-quark groups called baryons (protons and neutrons are examples).


  • There are 6 leptons.
  • electrons and neutrinos are leptons.

Force carrying particles

for each of the four fundamental forces.


All of these particles are very tiny, and their masses are measured in their energy equivalent in electronvolts (eV) or mega electronvolts (MeV)


  • One atomic mass unit is 1.66 x 10-27 kg. If this value is placed into Einsteins's formula E = mc2, we see that the energy equivalent of one AMU is 1.49 x 10-10 Joule.
  • An electronvolt is 1.602 x 10-19 Joule of energy, so one AMU can be expressed as 931 x 106 eV, or 931 MeV.
  • Check your text to see the masses of the various subatomic particles when expressed in this form.


It is unknown for certain why there are 6 quarks and 6 leptons that seem to organize into three families (or generations), as described in the text. The fact that there are patterns in the existence of these particles suggests that there may be even smaller units which make them up. I.e., puzzles have pieces!

The prevailing theory today is that there are in fact, tiny strings of matter, on the order of magnitude of 10-32 cm in length that make up the fundamental particles. These strings are continuous loops, and are not particles as such. They behave more like a looping rubber band, fluctuating predictably in shape. These specific fluctuations are what give the various fundamental particles their own particular natures and behaviours. This is called the "String Theory". String theory helps us to combine other theories into one that better explains the nature of all matter and energy.


Most everyday matter is composed of the up and down quarks, and the electrons and neutrinos.

The protons are made up of two up quarks (charge +2/3) and one down quark (charge -1/3). Neutrons are composed of one up quark and two down quarks.

Internet Project: The Particle Adventure. Get the .PDF version of the handout.