Atomic Theory:


An ingenious escape was proposed in the fifth century B.C. by Democritus. He hypothesized that all matter (plus space and time) is composed of tiny indestructible units, called atoms. This idea seems motivated by the question of how finely one can go on cutting up matter. While Democritus performed no experiments and had only the flimsiest evidence for postulating the existence of atoms, his theory was kept alive by the Roman poet Lucretius which survived the Dark Ages to be discovered in 1417. The atoms in Democritus theory themselves remain unchanged, but move about in space to combine in various ways to form all macroscopic objects. Early atomic theory stated that the characteristics of an object are determined by the shape of its atoms. So, for example, sweet things are made of smooth atoms, bitter things are made of sharp atoms.








Atomic theory is the field of physics that describes the characteristics and properties of atoms that make up matter. The key point to note about atomic theory is the relationship between the macroscopic world (us) and the microscopic world of atoms. For example, the macroscopic world deals with concepts such as temperature and pressure to describe matter. The microscopic world of atomic theory deals with the kinetic motion of atoms to explain macroscopic quantities.


Thomson Atom:

The next great step forward in the understanding of atoms was accomplished by John Thomson. Using a cathode ray scope, Thomson determined that all matter, whatever its source, contains particles of the same kind that are much less massive than the atoms of which they form a part. They are now called electrons, although he originally called them corpuscles.


By applying an improved vacuum technique, Thomson was able to put forward a convincing argument that these rays were composed of particles. Furthermore, these rays seemed to be composed of the same particles, or corpuscles, regardless of what kind of gas carried the electric discharge or what kinds of metals were used as conductors. From these ideas he developed the idea that atoms are made of negative electrons embedded in a gel of positive charge (a "plum pudding" model).

Thomson's conclusion that the corpuscles were present in all kinds of matter was strengthened during the next three years, when he found that corpuscles with the same properties could be produced in other ways; e.g., from hot metals. Thomson may be described as "the man who split the atom" for the first time, although "chipped" might be a better word, in view of the size and number of electrons.

Rutherford Atom:


By the turn of the 20th century, physicists knew that certain elements emitted fast moving particles of two flavors, alpha particles and beta particles. These elements were typically very heavy (i.e. their atom nuclei were massive) such as uranium and radium. Today we know that heavy nuclei are unstable and `decay', meaning that they spontaneously split into smaller nuclei and emit stray particles. This is called radioactivity.

The alpha particle was heavy and positively charged, we now know that it is the helium nuclei (2 protons and 2 neutrons). The beta particle was light and negatively charged, the electron. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. His experiment looked like the following:

The Rutherford beamed alpha particles through gold foil and detected them as flashes of light or scintillations on a screen. The gold foil was only 0.00004 centimeter thick, meaning on a few hundreds of atoms thick.

If the Thomson model of atoms was correct, then the alpha particles should pass through with relatively little deflection.

The expectation is that they will strike the fluorescent screen directly behind the foil.

In fact, what was observed was the following:

Most alpha particles were observed to pass straight through the gold foil. A few, however, were scattered at large angles, and some even bounced back toward the source. Only a positively charged and relatively heavy target particle, such as the proposed nucleus, could account for such strong repulsion.



Elementary Particles :

One of the primary goals in modern physics is to answer the question "What is the Universe made of?" Often that question reduces to "What is matter and what holds it together?" This continues the line of investigation started by Democritus, Dalton and Rutherford.

Modern physics speaks of fundamental building blocks of Nature, where fundamental takes on a reductionist meaning of simple and structureless. Many of the particles we have discussed so far appear simple in their properties. All electrons have the exact same characteristics (mass, charge, etc.), so we call an electron fundamental because they are all non-unique.


The study of particles is also a story of advanced technology begins with the search for the primary constituent. More than 200 subatomic particles have been discovered so far, all detected in sophisicated particle accerlators. However, most are not fundamental, most are composed of other, simplier particles. For example, Rutherford showed that the atom was composed of a nucleus and orbiting electrons. Later physicists showed that the nucleus was composed of neutrons and protons. More recent work has shown that protons and neutrons are composed of quarks.

Short History of Elementary Particles

Quarks and Leptons:

The two most fundamental types of particles are quarks and leptons. The quarks and leptons are divided into 6 flavors corresponding to three generations of matter. Quarks (and antiquarks) have electric charges in units of 1/3 or 2/3's. Leptons have charges in units of 1 or 0.

Normal, everyday matter is of the first generation, so we can concentrate our investigation to up and down quarks, the electron neutrino (often just called the neutrino) and electrons.

Note that for every quark or lepton there is a corresponding antiparticle. For example, there is an up antiquark, an anti-electron (called a positron) and an anti-neutrino. Bosons do not have antiparticles since they are force carriers (see fundamental forces).

Baryons and Mesons:




Quark Confinement:


