Discovery of the Atomic Structure

The discovery of the atomic structure is a story replete with a series of various discoveries that led to the final conceptualization of the final theory about its true mechanics.

The road that led to understanding the atomic structure began with John Dalton’s discovery of the Fundamental Law of Constant Proportions in 1808 which leads also to the chemically defined concepts of atom and molecule and forms in that respect the start of modern atomism; the fact that any two elements combine only in a definite limited quantity ratio points towards a limit of the smallest particles which can combine and leads from chemistry to the concepts of atom and molecule thus motivated him for his proposal of the Atomic Theory in the same year.

According to Dalton, matter is made up of tiny particles called atoms. The atom is the smallest particle of matter that takes part in a chemical reaction. Atoms are indivisible and cannot be created or destroyed. Further, atoms of the same element are identical in every respect.

In 1886, Eugen Goldstein a German physicist who was an early investigator of discharge tubes, the discoverer of anode rays, and is sometimes credited with the discovery of the proton discovered that discharge tubes with a perforated cathode also emit a glow at the cathode end. Goldstein concluded that in addition to the already-known cathode rays, later recognized as electrons moving from the negatively-charged cathode toward the positively-charged anode, there is another ray that travels in the opposite direction. Because these latter rays passed through the holes, or channels, in the cathode, Goldstein called them kanalstrahlen, or canal rays. They are composed of positive ions whose identity depends on the residual gas inside the tube. Goldstein’s work with anode rays of H+ was apparently the first observation of the proton.

The next great step forward in the understanding of atoms was accomplished by John Thomson through his discovery of the electrons in 1897.

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.

In 1911, Ernest Rutherford discovered the nucleus and provided the basis for the modern atomic structure through his alpha particle scattering experiment. According to Rutherford, the atoms is made of two parts: the nucleus and the extra-nuclear part. His experiments proved that the atom is largely empty and has a heavy positively-charged body at the center called the nucleus. The central nucleus is positively-charged and the negatively-charged electrons revolve around the nucleus.

Ernest Rutherford is considered the father of nuclear physics. Indeed, it could be said that Rutherford invented the very language to describe the theoretical concepts of the atom and the phenomenon of radioactivity. Particles named and characterized by him include the alpha particle, beta particle and proton. Rutherford overturned Thomson’s atom model in 1911 with his well-known gold foil experiment in which he demonstrated that the atom has a tiny, massive nucleus.

In 1932, James Chadwick made a fundamental discovery in the domain of nuclear science: he discovered the particle in the nucleus of an atom that became known as the neutron because it has no electric charge. In contrast with the helium nuclei (alpha particles) which are positively charged, and therefore repelled by the considerable electrical forces present in the nuclei of heavy atoms, this new tool in atomic disintegration need not overcome any electric barrier and is capable of penetrating and splitting the nuclei of even the heaviest elements. In this way, Chadwick prepared the way towards the fission of uranium 235 and towards the creation of the atomic bomb. For this important discovery he was awarded the Hughes Medal of the Royal Society in 1932, and subsequently the Nobel Prize for Physics in 1935.

Chadwick’s discovery made it possible to create elements heavier than uranium in the laboratory. His discovery particularly inspired Enrico Fermi, Italian physicist and Nobel laureate, to discover nuclear reactions brought by slowed neutrons, and led Otto Hahn and Fritz Strassmann, German radiochemists in Berlin, to the revolutionary discovery of “nuclear fission”, which triggered the development of the atomic bomb.

In 1913 Niels Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus. To remedy the stability problem, Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed size and energy. The energy of an electron depends on the size of the orbit and is lower for smaller orbits. Radiation can occur only when the electron jumps from one orbit to another. The atom will be completely stable in the state with the smallest orbit, since there is no orbit of lower energy into which the electron can jump.

Bohr’s starting point was to realize that classical mechanics by itself could never explain the atom’s stability. A stable atom has a certain size so that any equation describing it must contain some fundamental constant or combination of constants with a dimension of length. The classical fundamental constants–namely, the charges and the masses of the electron and the nucleus–cannot be combined to make a length. Bohr noticed, however, that the quantum constant formulated by the German physicist Max Planck has dimensions which, when combined with the mass and charge of the electron, produce a measure of length. Numerically, the measure is close to the known size of atoms. This encouraged Bohr to use Planck’s constant in searching for a theory of the atom.

The power in the Bohr model was its ability to predict the spectra of light emitted by atoms. In particular, its ability to explain the spectral lines of atoms as the absorption and emission of photons by the electrons in quantized orbits.

Niels Bohr provided the modern concept of the atomic model in which summarily according to him, the atom is made of a central nucleus containing protons (positively-charged) and neutrons (with no charge). The electrons (negatively-charged) revolve around the nucleus in different imaginary paths called orbits or shells.

Finally, our current understanding of atomic structure was formalized by Heisenberg and Schroedinger in the mid-1920’s where the discreteness of the allowed energy states emerges from more general aspects, rather than imposed as in Bohr’s model. The Heisenberg/Schroedinger quantum mechanics have consistent fundamental principles, such as the wave character of matter and the incorporation of the uncertainty principle