History of the Atom

Democritus

415 BCE

Democritus theorizes that if a stone is cut in half over and over again you will eventually reach the basic particle that the stone is made of and cannot be cut anymore. This particle is called atomos and fills up the entire given space as it has no pores.

John Dalton

1805

John Dalton proposes that elements are made of unique atoms that cannot be altered or destroyed but can be combined together to form a more complex structure. Dalton also invents the 5 postulates that are still used to this day in chemistry.

J.J. Thomson

1898

J.J. Thomson determines that atoms were made of smaller particles and discovers the electron. Tomson does this by shooting cathode rays through a vacuum and he observes how the rays will bend in the direction of the positively charged plates and concluded there must be a negative charge in the atom. Thomson also creates the Plum Pudding Model, which states that an atom is a solid positively charged particle with negatively charged particles scattered throughout.

Max Planck

1903

Max Planck proposes that energy is radiated through very small increments, or quantums, rather than one continuous wave that was previously believed. Planck's discovers this through many mathematical equations and theories, this lays the foundation of the quantum theory.

Ernest Rutherford

1904

Ernest Rutherford debunks the Plum Pudding Model by discovering the positively charged part of the atom, the proton and that atoms are composed of mostly empty space. Rutherford does this by firing thin beams of alpha particles at a thin piece of gold foil. Rutherford's assumes that all of the alpha particles will pass through but instead some particles are deflected and bounce back at a 90 degree angle.

Robert Milikan

1911

Robert Millikan determines how to mathematically find the charge of an electron. Millikan and his college do this by dropping oil between two electrodes and measuring the force of the droplet as it passes through the electrical field.

Albert Einstein

1917

Albert Einstein mathematically proves the existence and size of atoms Einstein also proves the continuous erratic movement in particles, Brownian Motion and confirms that heat is the result of atoms in motion. He does this through many mathematical formulas that involved much trial and error.

Niels Bohr

1924

Niels Bohr comes up with the atomic model of a small positively charged nucleus surrounded by orbiting electrons. Bohr discovered that electrons travel in seperate orbitals and that whatever orbital the electron is in will determine the property of the element.

Ewin Shrödinger

1925

Ewin Shrödinger formulates the wave equation that accurately calculates the energy levels of electrons. Shrödinger also invents the Shrödinger equation which describes how wave systems evolve over time.

James Chadwick

1933

James Chadwick discovers the neutral part of the atom, neutrons. Many chemists and physicians observed that the atoms total mass was much higher than the combined mass of the protons and electrons. Chadwick then observed the neutral particles radiating off of a metal bombarded with alpha particles.

Werner Heisenberg

1939

Werner Heisenberg formulates the quantum theory of ferromagnetism and the Heisenberg Uncertainty Principle which states that the speed and the position of an electron cannot be known at the same time

Louis De Broglie

1940

Louis De Broglie discover electron energy levels and that waves can act like particles and that particles can act like waves. Broglie describes that particles should be described as waves and that in order to describe energy you need to look at the wavelength.