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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.
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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.
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). 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.”
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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.
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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.
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.
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.
1930: Erwin Schrodinger introduced the term “wave mechanics” as a mathematical model of an atom. He also
viewed electrons as “continuous clouds.”
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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.