The Structure of the Atom

Since the beginning of recorded history the underlying structure of matter has been studied by scholars and scientists.  Even in cultures widely separated by geography like ancient Egypt, China, India, and Greece, remarkably similar models had emerged by about 1500 B.C.  Most of these early scholars concluded that matter was composed of four basic elements ---- earth, air, fire, water.  These four elements were usually depicted as pairs of complimentary opposites --- fire and water, earth and air -- that like yin and yang, were both needed to form a whole.  The Greek philosopher Democritus (ca. 460-ca. 370 B.C.), went even further. He thought that matter could be subdivided into only two fundamental states: Atoms (atomos, meaning not cuttable) and void (derived from vacuus, meaning empty). Democratus' world view was rejected for almost 2 millennia. The idea of the void proved particularly difficult to reconcile with common, everyday experience. Anyone with eyes could see that, "Nature abhors a vacuum", as the 17th century mathematician Rene Descartes put it. As a result, the atomic theory of matter was largely abandoned until about 1827 when John Dalton began publishing experimental results that supported the existence of atoms. According to Dalton, an atom was the smallest unit of an element that still retains the properties of that element, and these atoms could be combined in a limited number of predetermined ways to form all known matter.  The work of Dalton and others from that time on confirmed the idea of atoms as the smallest form of matter.

However, by the twentieth century, scientists had discovered that even the atom can be sub-divided into more elementary particles. Although physicists have uncovered a veritable zoo of subatomic particles with intriguing names like quarks, leptons and hadrons, it is the proton, the neutron, and especially the electron that are of interest to the study of chemistry.

Near the center of the atom's volume, protons and neutrons combine together to form an unimaginably dense, but tiny nucleus. Even tinier electrons move around frenetically through most of the volume of the atom.  The positively charged proton and negatively charged electron are the modern scientist's version of yin and yang -- opposites that require each other to form a whole and neutral atom. For an atom to be neutral, the number of protons in the nucleus must be matched by an equal number of electrons.  Protons and neutrons have about the same mass, but either of these nucleon is almost 2000 times as massive as an electron.  Remarkably, most of the atom is indeed empty space!
 

Subatomic particles are held together by a variety of competing forces. For example, electrons carry a negative charge and so repel each other. However, the protons in the nucleus of the atom carry a positive charge that strongly attract the appositely charged electrons to the nucleus. Neutrons are comparable in mass to protons but they are not charged particles and so they neither attract nor repel other charged subatomic particles. Only the electron is considered to be a fundamental particle Electrons can exist independently outside of the atom for an infinite length of time, but a free neutron can only exist outside of the nucleus of an atom for very brief periods of time, (half-life of 17 minutes). When the neutron decays a proton, an electron, and an antineutrino (one of the members of the subatomic zoo), are all that remain.