A Complete Timeline for Atomic Structures

Experiments into atomic structures have been going on for a very long time. Theories are constantly being proposed and disproved. Over the centuries, much advancement has been made in the field of atomic structures. Here’s a complete timeline about the biggest events in the research about atomic structures.

• Greek era: Democritus, a Greek philosopher, proposed the idea that there was a “void” in the world, or a “great nothing.” In his own words, he wrote, "by convention bitter, by convention sweet, but in reality atoms and void." He was the first to propose such an idea.
• 1704: Isaac Newton was one of the first people to propose a “mechanical universe.” He believed that the universe is made up of small solid masses that are constantly in motion. This idea is known as “The Mechanical Universe” idea.
• 1803: John Dalton formulated the Atomic Theory which states that: atoms of an element are different than atoms of other elements; atoms of one element are the same; that atoms of different elements can be combined; that atoms cannot be divided or separated; and that elements are made of tiny particles called atoms.
• 1832: Michael Faraday, the man who discovered electro-magnetic induction, studied how electricity affected things. He split molecules using electricity and developed the laws of electrolysis. Faraday even coined the term “electrolysis” because of his many experiments.
• 1859: Julius Plucker studied cathode rays, better known as electron beams, and invented the cathode ray tube that allowed for people to actually see those electron beams.
• 1869: Dmitri Mendeleev is the one who proposed what is now known as the Periodic Law. He organized all elements into seven categories sharing similar properties. They were arranged by the “functions of their atomic weights.”
• 1873: Scottish physicist James Clerk Maxwell is most known for his experiments with classical electromagnetic theory, where he proposed that not only did “the void” exist, it was filled with both electric and magnetic fields. His work into electromagnetism has been called the “second great unification in physics.”
• 1874: G.J. Stoney was the first physicist to propose that electricity is actually made up of subatomic particles that carry negative electric charges. Electrons are what combine with protons and neutrons and make up atoms, the building blocks of the universe.
• 1879: Sir William Crookes was the one who identified the properties of cathode rays. He discovered that they travel in straight lines from the cathode, impart a negative charge to any object they strike, cause glass to fluoresce and pinwheels in their path to spin, indicating they have mass, and that they are naturally deflective by electric fields and magnets, which suggests that they have a negative charge.
• 1886: Eugene Goldstein used cathode ray tubes to discover what he called canal rays, rays that passed through the tube, and emitted a glow at the end. This flow allowed him to discover that there are separate rays that travel from the anode or a tube to the cathode.
• 1895: Wilhelm Roentgen used cathode ray tubes in his experiments and discovered that chemicals he stored nearby actually glowed. He experimented further and discovered that unlike different types of rays, rays coming from the CRTs could not be deflected by a magnetic field. He decided to name these rays “X-rays.”
• 1896: Henri Becquerel was the man who discovered radioactivity. He was conducting experiments on photographic film and discovered that some chemicals actually lose energy, or decompose, and then give off ionization particles and radiation. This decomposition produces a certain type of atom that radiates very penetrative rays.
• 1897: Sir Joseph John “J.J”. Thomson was perhaps the most important man to influence the scientific understanding of the electron. His experiments determined the charge mass ratio of an electron (1.759 x 108 coulombs/gram).
• 1897: Thomson also studied canal rays and discovered that they could be associated with the proton H+. He also made many other electron and proton-related discoveries.
• 1898: Ernest Rutherford studied the radiation that came from thorium and uranium. He called them alpha radiation and beta radiation. Rutherford’s experiments earned him the unofficial title of “father of nuclear physics.”
• 1898: Marie Sklodowska Curie, aka Madame Curie, was one of the pioneers into the study of uranium and thorium. The experiments formed the groundwork for scientific understanding of radioactivity, which is the decaying process for uranium and thorium. Her husband, Pierre, also helped her discover the elements known as radium and polonium, which are also radioactive elements. They invented the term “radioactivity.”
• 1900: Frederick Soddy, Ernest Rutherford’s partner, observed what he called the “spontaneous disintegration of radioactive elements” into what he terms isotopes, elements that were totally new. He also discovered what’s known as “half-life,” the idea that a fixed fraction of an element will decay within a unit of time. Soddy also made some of the first calculations into energy released during decay.
• 1900: Max Planck set out to prove that there were new things to be discovered in the field of physics and he was able to use the idea of quanta, which is a packet of radiation that has energy, to explain hot glowing matter (i.e., how it became that way).
• 1903: Masato Nagaoka proposed that the atom was, in effect, “Saturian” because it had flat rings of electrons revolving around its center, a positively charged particle.
• 1904: Richard Abegg performed experiments and discovered that inert gases are chemically inactive because their electrons have stable configuration. In other words, the electrons within the gases do not shift.
• 1906: Hans Geiger invented the Geiger Counter, an electronic instrument that could detect and count alpha particles by “clicking” when the instrument was struck with said particles.
• 1909 Robert Andrew Millikan conducted a seven-year study on the electron where he discovered both the charge (e=1.602 x 10-19 coulomb) and the mass (m = 9.11 x 10-28 gram) of an electron. These experiments became known as the “oil drop experiments.”
• 1911: Ernest Rutherford, in one of many experiments, used alpha particles as a kind of atomic bullet. He was able to prop the atoms present in a piece of gold tin foil. Doing this, he established that the nucleus was small, dense, and positively charged. He also found out that the electrons were located outside of the nucleus.
• 1914: H.G.J. Moseley, using cathode rays and x-ray tubes, was able to bombard targets made up of different kinds of metal. These metals then emitted x-rays that were so unique, they could be considered fingerprints whereby "The atomic number of an element is equal to the number of protons in the nucleus." The heavier the mass, the shorter the wavelength. This work led to the reorganization of the periodic table.
• 1919: F.W. Aston used the mass spectrograph to discover that isotopes existed by separating electrically charged particles. This instrument is sometimes used to measure the masses of those isotopes.
• 1922: Niels Bohr is the one who was able to explain atomic structures that underlie regularities of the periodic table of elements. His special model put atoms on the table in a successive order of the orbital shells of electrons.
• 1923: Louise de Broglie discovered that electrons possess a dual nature just like particles and waves. de Broglie was also an Einstein supporter. It was his work, along with Einstein’s and several others, that led to the forming of the Particle/wave duality theory which states that all energy exhibits both wave-like and particle-like properties.
• 1927: Werner Heisenberg was the first to describe atoms by using a formula that connected them to their place on spectral lines. This led to the formation of the Proposed Principle of Indeterminacy, the idea that you can’t measure both the position and velocity of a particle at the same time.
• 1929: John Douglas Cockroft and Ernest Thomas Sinton Walton invented a linear accelerator so that they could bombard lithium with protons in an attempt to produce alpha particles. 
• 1930: Erwin Schrodinger introduced the term “wave mechanics” as a mathematical model of an atom. He also viewed electrons as “continuous clouds.”
• 1930: Paul Dirac was the one to propose the idea of “anti-particles.” Two years later, in 1932, William H Anderson discovered the positron, known at the time as the anti-electron. Then, in 1955, Serge and Chamberlain discovered the anti-proton, as a result of Dirac’s work.
• 1932: James Chadwick used what’s known as alpha particles to discover a neutral atomic particle that had a mass close to a proton. In other words, he discovered the neutron.

• 1938: Lise Meitner and partners Hahn and Strassman conducted experiments that helped lay the groundwork for the discovery of nuclear fission. They conducted experiments that verified that heavy elements are able to capture neutrons and then form unstable products which then undergo fission, or the splitting of an atom’s nucleus into smaller parts. This ejects more neutrons and continues the fission change reaction.
• 1941 - 51: Glenn Seaborg was able to actually artificially synthesize 6 transuranium elements. With the creation of these elements, the periodic table had to be changed to include them.
• 1942: Enrico Fermi was able to conduct the first controlled experiments that allowed the releasing of energy from an atom’s nucleus. These experiments led to the assembly of an “atomic pile” or what is known today as a “nuclear reactor.”
• 1950's: In the years after these groundbreaking experiments, other discoveries are being made as well. New findings are made all the time, in fact, and theories are always changing.