Atoms were ancient Greek philosophers’ answer to the question, What happens if you could take a piece of matter and cut it in half, then again, halving it until it was reduced to the smallest possible size? The revival of atomic theory in the 19th century pictured atoms as nothing more than little balls, indivisible chunks of matter that were unchanged while being exchanged and reorganized during chemical reactions.

With the discovery of the electron in 1898, it became apparent that atoms are divisible. The race was on when Rutherford discovered in 1912 that the atomic nucleus was much smaller than the atom itself.

Since then physicists and chemists have discovered that matter is made from a panoply of particles.

The atomic nucleus was found to contain protons and neutrons, the number of which determined the type of element and isotope.

When atom smashers began colliding high-energy protons, a mess of hundreds of particles came flying out, and it seemed that any simple classification was not to be had.

Eventually physicists realized that all of the hundreds of new particles could be classified into only a few types of fundamental particles. Physicists had to abandon the simplicity of the earlier proton and neutron nucleons and replace it with the Standard Model, which contains the most fundamental particles that we know.

In the Standard Model there are three kinds of particles: three pairs of leptons, three pairs of quarks and five bosons.

Of the bosons, there are gluons that mediate the strong force, photons that mediate the electromagnetic force, and W and Z bosons that mediate the weak force. The Higgs boson is responsible for the mass of quarks and leptons.

Quarks and leptons each come in three generations, with each generation representing a higher-energy state. Almost the entire universe is made from first-generation particles. Second- and third-generation particles are unstable and quickly decay into their first generation counterparts.

Quarks have a fractional electric charge of either plus 2⁄3 (up quarks) or minus 1⁄3 (down quarks). They come in three types, referred to as "color." They are not really colored; it is just a term coined by physicists to help understand how quarks combine and exchange gluons.

Quarks only exist in groups with other quarks to form composite particles called hadrons. Baryons such as protons and neutrons are hadrons that comprise three quarks. A proton contains two up quarks, each with a charge of plus 2⁄3, and one down quark with a charge of minus 1⁄3, giving the proton a total charge of plus 1.

A neutron comprises two down quarks, each with a charge of minus 1⁄3 and one up quark with a charge of plus 2⁄3, giving the neutron zero charge.

As one goes deeper into the Standard Model, it becomes steadily more complex with a total of 61 so-called fundamental particles, a total that many physicists believe is way too high to be truly fundamental. But the Standard Model is the best there is to date, and it says that the universe is made of six quarks, six leptons and five bosons.

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Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.