ATOMIC NATURE OF MATTER
DOE-HDBK-1019/1-93
Atomic and Nuclear Physics
In 1661 the English chemist Robert Boyle published the modern criterion for an element. He
defined an element to be a basic substance that cannot be broken down into any simpler
substance after it is isolated from a compound, but can be combined with other elements to form
compounds. To date, 105 different elements have been confirmed to exist, and researchers claim
to have discovered three additional elements. Of the 105 confirmed elements, 90 exist in nature
and 15 are man-made.
Another basic concept of matter that the Greeks debated was whether matter was continuous or
discrete. That is, whether matter could be continuously divided and subdivided into ever smaller
particles or whether eventually an indivisible particle would be encountered. Democritus in about
450 B.C. argued that substances were ultimately composed of small, indivisible particles that he
labeled atoms. He further suggested that different substances were composed of different atoms
or combinations of atoms, and that one substance could be converted into another by rearranging
the atoms. It was impossible to conclusively prove or disprove this proposal for more than 2000
years.
The modern proof for the atomic nature of matter was first proposed by the English chemist John
Dalton in 1803. Dalton stated that each chemical element possesses a particular kind of atom,
and any quantity of the element is made up of identical atoms of this kind. What distinguishes
one element from another element is the kind of atom of which it consists, and the basic physical
difference between kinds of atoms is their weight.
Subatomic Particles
For almost 100 years after Dalton established the atomic nature of atoms, it was considered
impossible to divide the atom into even smaller parts. All of the results of chemical experiments
during this time indicated that the atom was indivisible. Eventually, experimentation into
electricity and radioactivity indicated that particles of matter smaller than the atom did indeed
exist. In 1906, J. J. Thompson won the Nobel Prize in physics for establishing the existence of
electrons. Electrons are negatively-charged particles that have 1/1835 the mass of the hydrogen
atom. Soon after the discovery of electrons, protons were discovered. Protons are relatively
large particles that have almost the same mass as a hydrogen atom and a positive charge equal
in magnitude (but opposite in sign) to that of the electron. The third subatomic particle to be
discovered, the neutron, was not found until 1932. The neutron has almost the same mass as the
proton, but it is electrically neutral.
NP-01
Page 2
Rev. 0
