Development of the Atomic Theory

Democritus
He named the atom for the Greek word atomos meaning “indivisible” in 450 B.C. He originated the idea that all matter is made of some elemental particle.

John Dalton
In 1803, he revived the atomic theory of matter. He stated 4 postulates:
• Each element is composed of extremely small particles called atoms.
• All atoms of a given element are identical, but differ from those of any other element.
• Atoms are neither created nor destroyed in any chemical reaction.
• A given compound always has the same relative numbers & kinds of atoms.

Sir J. J. Thompson
In 1898, he began studying cathode rays emitted from the cathode ray tube (CRT). In doing so, he discovered that the particles coming from the CRT was actually a stream of negative particles. He had discovered the electron! He proposed the “Plum Pudding” Model of the Atom that was the idea that atoms are like a positive charged pudding with negative electrons attached on the surface like raisins would be in plum pudding. He won the Nobel Prize in Physics in 1906, and he was “Knighted”.

Ernest Rutherford
From 1896 – 1908, he did a series of experiments on radiation. He did an experiment in which he named the 2+ charges “alpha radiation” and the high-speed electrons “beta radiation”. He is perhaps best known for the famous Gold Foil Experiment in which he discovered that atoms are mostly empty space with a small, dense core of positive charge that he named the “nucleus”. He proposed the “Nuclear or Planetary” model of the atom that replaced J.J. Thomson’s model. This was the idea that the atom is like a miniature solar system with the positive particles in the middle and the negative particles orbiting as the planets do around the center. He won the 1903 Nobel Prize in Chemistry, and he was “Knighted.”

Dimitri Mendeleev
In 1896, he organized the elements into what is now known as the Modern Day Periodic Table. He is known as the “Father of the Periodic Table”

Henri Becquerel
In 1896, he accidentally discovered a phenomenon that later became known as “spontaneous radioactivity”. He noticed that some photographic plates that had a piece of Uranium rock lying on top of it in a dark drawer had exposed the film. This was the first time that this phenomenon was known about. This initial investigation & discovery gave rise to the field of nuclear chemistry & nuclear physics, as well as, giving insight into the substructure of the atom. He shared the 1903 Nobel Prize in Physics with the Curies.

Max Plank
In 1900, he proposed the idea that there is a fundamental restriction on the amount of energy that an object emits or absorbs. He called each of these pieces of energy a “quantum” which means, “fixed amount”. He was able to establish the mathematical relationship between the frequency & wavelength of a particular radiation and the energy with which it is associated. He derived a mathematical universal constant. The formula is E = hv – the “h” represents this constant. He was able to predict how the spectrum of radiation emitted by object changes with its temperature. Einstein later used this formula in his relativity studies. He won the Nobel Prize in Physics in 1918.

Marie Curie
She was the first person to ever win two Nobel Prizes in two different fields. Marie Curie, Pierre Curie, and Becquerel won the Nobel Prize in Physics in 1903 for their discovery of spontaneous radioactivity. Marie won a 2nd Nobel Prize in another field – Chemistry in 1911 for discovering the radioactive elements radium & polonium and for her continued work in the field of radiochemistry. She coined the word “radioactive”. She founded the Radium Institute, and became a world-renown scientist.

Albert Einstein
He won the Nobel Prize in Physics in 1921. He proved the “particle nature of light”. His first and only Nobel Prize was for work proving that electrons are ejected from the surface of a metal when light shines on the metal, as well as, for his contributions to theoretical physics. This is called the photoelectric effect. He developed the ideas that mass & energy are equivalent which is the theory of special relativity. (E = mc2) He also developed the theory of general relativity by proposing the idea that the interactions of bodies (gravity) are explained as the influence of “space & time as the 4th dimension.”

Henry Moseley
He was a student of Ernest Rutherford’s. He proved (using x-ray technology that existed at the time) that each element contains a unique positive charge in the nucleus or that an “atom’s identity comes from the number of protons in the nucleus”, and this number is still called the “atomic number.” The Periodic Table was reordered accordingly by atomic number in 1913 replacing the older order by atomic mass.

Niels Bohr
From 1911 to 1922, he developed the concept that electrons are allowed to have only certain orbits corresponding to different amount of energy. He labeled each energy level or each orbit by a “quantum number n. He proposed the idea that electrons must absorb energy to jump to a higher energy level and then emit energy in the form of radiation when they return to a lower energy level or ground state. He won a Nobel Prize in 1922. He worked at the Los Alamos on the development of the Atomic bomb. His model of the atom is also known by his last name. His model is the “iconic depiction” of the atom to this day.

Louis Victor DeBroglie
In 1924, he discovered the “wavelike behavior” of electrons. This settled the dispute that had been going on for years -” do electrons travel as particles or as waves?” Electrons have a dual nature – they can travel as both a particle and a wave. This idea is one of the foundations of the modern Quantum Model of the Atom.He derived a mathematical relationship between mass & velocity of a moving particle and the wavelength that it would have. He won the Nobel Prize in Physics in 1929 for his work.

James Chadwick
He discovered the existence of a heavy, uncharged particle in the atomic nucleus that became known as the “neutron” in 1932. This led to nuclear fission and the development of work on the atomic bomb. He won the 1935 Nobel Prize in Physics. He worked at the Los Alamos Lab from 1943 -45.

Wolfgang Pauli
He developed the piece of the atomic puzzle, which is known as the “Exclusion Principle”. He discovered that “two electrons can never exist in the same state and have 4 identical quantum numbers”. This led to the discovery of the 4th quantum number, which assigns a value to electrons according to their spin as “clockwise” or “counterclockwise.” This became useful in determining the properties of atomic nuclei and electrical conductivity properties in metal. He won the 1945 Nobel Prize in Physics.

Werner Heisenberg
He developed the idea known as the “Uncertainty Principle”. This tells us that we cannot know the exact location & momentum of any electron simultaneously. This information is one of the foundations of understanding the quantum mechanical model of the atom today.

Murray Gell-Mann
He was an American scientist who discovered the particles that make up protons and neutrons called “quarks”.

Ernest Schrodinger
He developed many of the mathematical descriptions of the wave mechanics of orbiting electrons. His theories also became one of the foundations of quantum mechanics. He shared the 1933 Nobel Prize in Physics with Paul Dirac.

Irene-Joiloit Curie
She was the 1st person to synthesize a radioactive substance. She won the 1935 Nobel Prize in Chemistry. She was the daughter of Marie & Pierre Curie.

Enrico Fermi
He achieved the 1st controlled nuclear fission chain reaction at the University of Chicago in 1942. He investigated the products of artificial radioactivity by bombarding over 60 elements with neutrons. He also worked at Los Alamos. He won the Nobel Prize in Physics in 1938.

Lisa Meitner
In 1938, she provided the mathematical explanation for nuclear fission of U-235. Two other German scientists had accomplished splitting a uranium atom earlier, but they were unable to explain why new elements had been formed. This explanation provided the necessary foundation that made the idea of an “atomic” bomb and nuclear energy possible.

Glen Seaborg
He discovered the elements Plutonium and nine other “transuranium” elements. He also discovered 100+ isotopes of other existing elements. He did his work at the University of California at Berkeley Laboratory. He won the Nobel Prize in Chemistry in 1951. He is the only living person to have an element named in his honor. He died in 1998.