History of Atom Timeline
By: Mary Ziperman
600 B.C
At about this time, Thales of Miletus mad a discovery that after rubbing a piece of amber with fur it attracts bits of hair, feathers, other lightweight objects. He suspected this force came from amber.
460 B.C
Democritus, a Greek philosopher, created the idea of atoms. He asked this question, "If you break a piece of matter in half, and then break it in half again, how many breaks will you have to break before you can break it no further?" Democritus thought it ended at some point, a smallest bit of matter, which he called atoms.
1800's
John Dalton, an English chemist, performed many different experiments with many different chemicals that showed matter seemed to consist of lumpy particles, or atoms.
1897
J.J Thomson, who was an English physicist, discovered the electron and suggested a model for the structure of the atom. He knew that electrons had a negative charge, and thought that matter had a positive charge.
1900
Max Planck, a professor of theoretical physics, showed that if you vibrate atoms strong enough you can measure the energy in discrete units called a quanta.
1905
Albert Einstein wrote a paper explaining that light absorption can release electrons from atoms, a phenomeon called the "photoelectric effect".
1911
Ernest Rutherford conducted an experiment that could research the inside of an atom. He used the Radium as a source of alpha particles and shinned them onto the atoms in gold foil and then observed the alpha particles impact. The results from his experiment showed most alpha particles smoothly going through the foil, where as the occasional alpha particle would deviate from its path. He though negative electrons orbited a positive center.
1912
A Danish physicist, Niels Bohr, created a theory. He said that electrons don't spiral into the nucleus and came up with some rules for what really does happen. Rule 1 being "Electrons can orbit only at certain allowed distances from the nucleus" Rule 2 being "Atoms radiate energy when an electron jumps from a higher energy orbit to a lower energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low energy orbit to a high energy orbit." The electron can only exist in only one of the orbits.
1919
Rutherford officially identified the particles of the nucleus as discrete positive charges of matter. He also found a proton's mass is 1836 times as great as the mass of the elctron.
1920's
Further experiments were conducted showing issues in Niels Bohr's theory. A German physicist Arnold Sommerfeld and Bohr expanded the model. Their new model showed electrons moved in certain orbits, but the orbits have different shapes and tilt in the presence of the magnetic field.
1924
An austrian physicist, Wolfgang Pauli predicted that an electron should spin while it orbits around the nucleus. It can spin in either of two directions.
1924
A frenchman, Louis de Broglie, thought if light can exist as both particles and waves, why couldn't atom particles also behave like waves? He showed what matter waves would behave like if they existed at all.
1926
Austrian physicist, Erwin Schrodinger, had the idea: Why not go all the way with particle waves and try to form a model on that basis? His wave mechanics did not question the makeup of the waves but he had to call it something so he gave it a symbol, the psi symbol.
1927
Heisenberg formulated the idea that no experiment can measure the position and momentum of a quantum particle simultaneously.
1932
James Chadwick discovered the neutron. He found it to measure slightly more than the proton with a mass of 1840 protons and a neutral charge.
1935
Hideki Yukawa suggested that exchange forces might also describe the strong forces between nucleons. Virtual photons do not have enough strength for this force, so he thought there must exist a new kind of virtual particle. Later in 1947, Cecil F Powell detected this particle and called it the pion.
1960
Murray Gell Mann and Yuval Ne'man proposed a method for classifying all particles that were known, this method was named the periodic table.