Atom (from Greek átomo "indivisible"): basic, indivisible building block of matter.

The Greek philosopher Democritus (460 - 371 B.C.) challenged the view that matter was infinitely divisible. Instead, he postulated an infinite number of atoms to be the smallest particles of matter. His atoms were solid bodies that came in various sizes and shapes: angular, round, smooth, rough, even and uneven. For Democritus, all material phenomena -- earth, air,  water, fire, plants, animals, and human beings -- are composed of different kinds of atoms. Thus, the perceivable properties of objects are due solely to the combinations of atoms making them up. "An object is only apparently colored, only apparently sweet or bitter. In reality, there are only atoms and void space. " Even the soul of a being is composed of soul-atoms that disperse after death and may later become part of yet another soul.

The Indivisible ...

Democritus's theory found few followers in its time, but was revived in the seventeenth century by various philosophers, including ►Leibniz with his ►theory of monads. In the early nineteenth century, John Dalton formulated the theory of atoms as a basis for chemistry. At first, atoms were regarded as solid, indivisible balls. In 1896, however, Henri Becquerel discovered nuclear disintegration and the fact that atoms of one type can transform themselves into atoms of another. It was also discovered that atoms combine positive and negative electric charges. And in 1905, ►Albert Einstein used atomic theory to explain Brownian motion. This constituted the first direct proof of the existence of atoms. One year later, Ernest Rutherford discovered by way of scattering experiments that atoms are not solid, but consist of a positively charged nucleus with a negatively charged shell. This model basically rendered the very word "atom" inappropriate as a name for what it is supposed to describe, for it proposed that atoms could contain smaller particles and thus are not in principle indivisible.

... is Subject to Possible Fission

According to the Rutherford model and Niels Bohr's subsequently refined version of it, the shell of the atom consists of negatively-charged electrons and has a radius of approximately 0.0000001 mm. The nucleus, although it accounts for nearly all of the atom's mass, is approximately ten thousand times smaller than that. Thus, if an atom were the size of a soccer stadium, its nucleus would be about as large as the umpire's whistle. The nucleus consists of neutrons and positively-charged protons, each of which in turn is composed of three quarks. At quarks, however, the divisibility of matter ends, according to this model; there is as yet no evidence that quarks are composed of yet smaller particles. The positive electrical charge of the protons within the nucleus is balanced out by the negatively-charged electron shell, so that atoms are usually electrically neutral with regard to other atoms.

The current standard model of physics for the atomic structure of matter recognizes 48 types of elementary particles. These are regarded as indivisible elementary constituents of matter. The 48 types of particles are divided into three families, which differ from one another only with regard to particle mass. Now, all atoms and all known types of matter in the ►universe are constituted by particles in the first family. But we don't know why there are two other particle families, why there are exactly 48 types of particles, or how it is that they have the properties they do. The standard model of physics does not explain this particle zoo. For lack of an alternative, physicists are by and large resigned to regarding the number of particle and mass types as ►physical constants for the time being. Nonetheless, physicists hope that future physical theories, especially string theory, will be able to provide a more fundamental and satisfying explanation of this assortment of material particles.

The World Consists of Forty-eight of Particles ...

Elementary particles differ with regard to their mass and electrical charge, as well as in the forces by which they are attracted or repelled. Physics recognizes four elementary forces: gravity, electromagnetic force, and two types of nuclear power, strong and weak. These last two differ not only in strength, but also in their causal effects. While gravity affects all particles, electromagnetic force acts only on electrically-charged particles, and strong nuclear power only on quarks. The following table lists the properties of the various types of elementary particles:

Electrical charges are here displayed as multiples of the electron charge and mass, as is common in elementary particle physics, in the unit MeV (mega-electron volt; 1 MeV =  4.655 ∙ 10^-31 kg). There are three sub-categories for each type of quark; by convention, these are labeled red, green, and blue, but they have nothing to do with colors. Any proton or neutron must have a red, a green, and a blue quark in order to be part of an atomic nucleus. In addition, for each particle in the above table there is a corresponding anti-particle that carries the opposite charge. This combination of particles altogether yields 48 types of elementary particles.

... But Eight Would Suffice

The first family of particles, consisting of the electrons, neutrinos, and the up and down quarks in their respective three sub-categories, would entirely suffice to explain the structure of the universe as we know it. Everything we know of consists of these eight particle types (sixteen, if we count the corresponding anti-particle types as well). Perhaps some future theory will explain why there are two additional particle families.

Now, the world consists not only of elementary particles, but also of the fields of the four basic forces mentioned above (gravity, electromagnetic force, and strong and weak nuclear forces). The standard model also provides for seven additional types of particle, the so-called bosons, to make sense of the ways these forces are mediated between the particles of matter. For example, electrical or magnetic attraction is created by an exchange of virtual photons (light bosons). This is why the effect of forces is limited by the maximal velocity of photons – the ►speed of light. Each electromagnetic wave -- which includes radio waves and light rays -- is a cluster of photons. In addition, there is the Higgs field, which does not correspond to any of the basic forces but provides all particles of matter with inertial mass.

The greater a particle's mass, the more difficult it is to prove its existence, since creating even a particle with comparatively little mass in a particle accelerator takes an enormous amount of energy. This is why we have so far not managed to provide clear evidence of the heaviest of all particles, the ►Higgs boson; its existence has merely been theoretically postulated. However, our as-yet unsuccessful attempts do at least enable us to ascribe to this particle's mass a bottom limit of approximately 115000 MeV. Physicists hope to be able to produce, and thus prove the existence of, Higgs bosons with the new super accelerator launched in 2008, the Large Hadron Collider located at the CERN Institute in Geneva.

Links Related to the Topic

■ The Elegant Universe
■ Particle Physics Germany

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