Particulate Nature of Matter
Since the times of the ancient Greeks, scientists and philosophers have generally agreed that everything that takes up space, or matter, is made up of tiny bits of material that have come to be known as elementary particles. The only thing that has changed over the years is the scale of what we mean by the term particle.
The Greek philosopher Democritus, who conceived of the particulate nature of matter, believed that what he called atoms (from the Greek word atomos for indivisible) were the smallest constituents of matter. However, scientific advances have shown us that there are at least several smaller kinds of particles that make up each atom. Nevertheless, the atom remains the most stable and convenient way to understand and discuss the nature of matter--especially since it is the smallest particle that can enter into chemical combination.
The atomic theory of Democritus and his school fell by the wayside for many years as the science of Aristotle came into vogue. Aristotle preferred to think of the nature of matter in more concrete terms, disdaining such intangible abstractions as atoms. Therefore, he concluded that all matter consisted of four elements: air, fire, wind, and water. He added ether to the list to account for the heavens.(This last concept would persist into the 1800s.) Although there was much value in Aristotle's theories, some of which has been absorbed into modern-day science, he turned out to be wrong about the nature of matter. However, it would not be until about the start of the nineteenth century that Democritus's view would return to the forefront of science.
In the field of chemistry, there were several important events that solidified the concept of particulate matter in the early days of quantitative science. In the late 1700s, French chemist Antoine Lavoisier discovered that although the different components in a chemical reaction might change radically in terms of appearance and form, their total original mass remained the same. This prompted him to conclude that some basic part of the components did not change, although he could not see this part. In about 1800, scientists studying chemical reactions began wondering whether the behavior of matter could be ruled by something that even their microscopes could not perceive.
The next step came when another French chemist, Joseph Proust, extended Lavoisier's work by demonstrating conclusively that every pure chemical compound maintains the same constant proportions in weight of the elements that make it up. Soon afterward, in 1808, John Dalton proposed a scenario in which every element is composed of a huge number of identical atoms and that chemical compounds come about through the combination of a smaller number of atoms from different elements.
In 1811, Italian chemist and physicist Amadeo Avogadro cleared up some of the problems with Dalton's theories by showing that the basic components of a pure element were not necessarily individual atoms, but rather compound atoms existing as diatomic ("two-atom") molecules (e.g., HCl or H2). A scientific conference in 1860 officially adopted a combined form of Dalton's and Avogadro's theories, and soon afterward chemists began creating an accurate compilation of relative atomic masses. Yet what came to be known as the atomic-molecular theory of Dalton and Avogadro was developed without any real, direct proof of the particulate nature of matter.
The first hard evidence of the existence of fundamental particles came in 1897, when English physicist Joseph Thomson discovered electrons within an atomic nucleus. This was the end of the belief that the atom is the smallest particle that makes up matter. British physicist Ernest Rutherford confirmed and advanced Thomson's findings in the early 1900s, when he and his colleagues showed that every atom consists of a positively charged nucleus in the center that is surrounded by what they termed a "cloud" of negatively charged electrons. They also found that the atomic nucleus itself (except for hydrogen) consists of positive protons and electrically neutral neutrons. Collectively, these components are known as subatomic particles.
Rutherford's work gradually led to the conclusion that the different properties of chemicals arise from the different numbers of electrons in their respective clouds. However, it became clear that an atom's weight is determined by the contents of its nucleus (i.e., its protons and neutrons). This weight is the number assigned to each of the chemical elements in the periodic table.
Since the days of Rutherford, physicists and chemists have theorized that atoms of matter contain even smaller particles than electrons, protons, and neutrons. Since 1945, researchers have predicted and/or confirmed the existence of several hundred smaller particles, many of which only come into existence under extreme conditions and for very short times. These infinitesimal particles are usually grouped into four main categories: the mesons, the baryons, the leptons, and the photons (the most basic unit of electromagnetic radiation). Other categories include the hadrons and the bosons. In addition, as British physicist Paul Dirac predicted in 1930, every kind of elementary particle has an antiparticle (the basis of antimatter).
In 1963, physicists Murray Gell-Mann and George Zweig proposed the existence of an even smaller particle that they named the quark, which they postulated is the basis for the baryons and mesons. Subsequent research has experimentally confirmed the presence of six kinds of quarks, and in 1995, physicists actually detected one of them (the "top" quark).
As science becomes ever more sophisticated, it seems possible that even the unimaginably small quark will be found to contain yet tinier components. However, these discoveries only strengthen the fundamental truth of the particulate matter of nature.
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