Our Understanding of the Atom

Democritus and the Atom

Period: 460 BCE to 370 BCE

Theorized that everything consisted of small, indivisible particles called atoms; however, he never conducted experiments to test these theories.

John Dalton's Atomic Theory

1804

First known attempt to describe ALL matter in terms of atoms. Based his experiments with the conservation of mass.
Atoms are indivisible. Atoms of an element are identical in mass and properties.
Compounds are combinations of more than one type of element.
Chemical reactions are rearrangements of atoms.

Eugen Goldstein's Canal Rays

1886

Using a similar experiment to the one Thomson used several years later, Goldstein noticed that the different gases in the cathode ray chamber produced positively charged particles that moved opposite the electrons. The positively charged particles from hydrogen atoms were observed to be much more massive than the electrons.

J.J. Thomson and the Electron

1897

Cathode Rays from various elements were emitted through a magnetic field.
The particles were all attracted towards the positive end and they had substantially less mass than an atom.
This led him to conclude that all elements contained electrons but that they were fixed in a positively charged mass.

Robert Millikan's Oil Drop

1909

Negatively charged oil drops were suspended in an electric field.
Manipulating the electric field and using the variables of gravity and viscosity, he was able to measure how fast the droplets moved towards the positively charged plate.
Led him to an understanding that these negatively charged particles theorized by Thomson have specific amounts of charge.

Ernest Rutherford and the Nucleus

1911

Shooting a beam of alpha particles at a piece of gold foil, he noticed that most of these positively charged particles made it straight through the the gold foil. However, some particles were slightly deflected, and others were deflected straight back leading him to conclude that there was a very small positively charged area of an atom as opposed to a diffuse positive field as theorized by Thomson.

Niels Bohr's Energy Levels

1913

Observing that the current understanding of the atom following Rutherford's experiments did not explain changes to a substance's color with additional amounts of added heat/energy, Bohr was able to describe that the electron in the Hydrogen atom emitted specific amounts of energy as it moved from one energy level to another.

Sir James Chadwick and the Neutron

1932

Using alpha particles to bombard Beryllium, Chadwick noted that particles previously unexplained were released from the Beryllium. These particles displaced protons from wax but were not deflected by a magnetic field leading him to discover a new subatomic particle known now as the neutron.