Atomic Theory : A Detailed Timeline Presentation

Democritus (460-370 BC)

Democritus

• He developed the concept of the 'atom' around 400 BC.
• Democritus believed that everything in the universe was made up of atoms, and that all matter was formed of different types of tiny discrete particles and that the properties of these particles determined the properties of matter.
• Democritus did an experiment where he took a seashell and broke it in half over and over again to show that if you keep dividing a material, there should eventually be a point where you've reached the smallest representative element of your material.

John Dalton (1766-1844)

John Dalton

• Dalton proposed a theory of chemical combination in 1803 :
• Elements consist of indivisible small particles (atoms).
• All atoms of the same element are identical; different elements have different types of atom.
• Atoms can neither be created nor destroyed
• Compound elements’ are formed when atoms of different elements join in simple ratios and form ‘compound atoms’
• This theory is also known as Dalton's Law of Partial Pressures..
• It led him to find out that the total pressure of a mixture of gases amounted to the sum of the partial pressures that each individual gas exerted while occupying the same space.
• Dalton did a lot of experiments with gases that led him to propose his theory.

JJ Thomson (1856-1940)

JJ Thomson

• In 1897, Thomson performed multiple experiments that were designed to study the nature of electric discharge in a high-vacuum cathode-ray tube. This experiment showed that all atoms contain tiny negatively charged subatomic particles or electrons.
• Also in 1897, he discovered the electron
• In 1904, Thomson cam up with a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces (Plum Pudding Model)

Max Planck (1858-1947)

Max Planck

• Planck was a German physicist who originated quantum theory in 1900
• He proposed that energy is not emitted continuously and that energy is given off in chunks/bundles (Quanta)
• It won him the Nobel Prize for Physics in 1918.
• His experiments accurately predicted the wavelengths of light radiated by a black body.

Ernest Rutherford (1871-1937)

Ernest

Rutherford

• Rutherford is known for his studies of radioactivity and the atom.
• He discovered that there are two types of radiation, alpha and beta particles, coming from uranium.
• One of Rutherford's experiments was the Gold Foil Experiment in 1908
• This experiment showed that the atom is mostly empty space with a tiny, dense, positively-charged nucleus.
• From this experiment, Rutherford proposed the nuclear model of the atom.

Niels Bohr (1885-1962)

Niels Bohr

• In 1913, Niels Bohr proposed a theory for the hydrogen atom
• It was based on quantum theory that some physical quantities only take discrete values.
• The Bohr Model :
• Bohr discovered that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element..
• Bohr did many experiments to prove that atoms only emit electromagnetic energy at certain discrete frequencies.

Erwin Schrodinger (1887-1961)

Erwin Schrodinger

• Schrodinger proposed the quantum mechanical model of the atom.
• This treats electrons as matter waves.
• Schrodinger's wave equation (1926)
• This equation covers the form of the probability waves and the evolution over time of a wave function
• They govern the motion of small particles, and specifies how the waves are altered by external influences.
• Schrodinger's Cat Experiment
• " A cat is in a box with a vial of poison. The vial breaks if an atom inside the box decays. The atom is superposed in decay and non-decay states until it is observed, and thus the cat is superposed in alive and dead states."

James Chadwick (1891-1974)

James Chadwick

• Chadwick made the discovery of the neutron in 1932
• "In 1932, the physicist James Chadwick conducted an experiment in which he bombarded Beryllium with alpha particles from the natural radioactive decay of Polonium. The resulting radiation showed high penetration through a lead shield, which could not be explained via the particles known at that time."

Werner Heisenberg (1901-1976)

Werner Heisenberg

• Heisenbug discovered the uncertainty principle (1927)
• It states that a particle's position and momentum cannot both be known
• In his experiments, Heisenberg said that when a gamma-ray microscope measures the position of an electron, the measurement disturbs the electron's momentum.

Murray Gell-Mann (1929-2019)

Murray Gell-Mann

• In 1964, Mann proposed that observed particles are composite, that is, comprised of smaller building blocks called quarks.
• "In 1964 Gell-Mann and George Zweig independently proposed the existence of a new type of particle that made up particles such as neutrons and protons. Gell-Mann's decision to call them quarks came from his interest in language, which was evident at an early age"