START DATE: approx. 400 BC
Democritus was an ancient Greek philosopher who lived from 460 BC to 370 BC.
Democritus developed the idea of atoms first when he asked himself 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 make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms.
Democritus claimed that everything is made up of atoms. These atoms are physically, but not geometrically, indivisible; between atoms lies empty space; atoms are indestructible; have always been, and always will be, in motion; there are an infinite number of atoms and kinds of atoms, which differ in shape, and size.
He helped to propose the earliest views on the shapes and connectivity of atoms.
(http://www.universetoday.com/60058/democritus-atom/)
(http://www.nobeliefs.com/atom.htm)

Democritus first suggested the existence of the atom but it took almost two millennia before the atom was placed on a solid foothold as a fundamental chemical object by John Dalton . Although two centuries old, Dalton's atomic theory remains valid in modern chemical thought.
Dalton's Atomic Theory:
1. All matter is composed of extremely small particles called atoms.
2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.'
3. Atoms cannot be subdivided, created or destroyed.
4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
5. In chemical reactions atoms are combined separated or rearranged.
Dalton's theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights.
(http://www.rsc.org/chemsoc/timeline/pages/1803.html)
(http://www.iun.edu/~cpanhd/C101webnotes/composition/dalton.html)
(http://atomictimeline.net/index.php)
(http://wiki.answers.com/Q/What_are_the_parts_of_Dalton's_atomic_theory)

William Crookes was the first person to confirm the existence of cathode rays by displaying them, with his invention of the Crookes tube, a crude prototype for all future cathode ray tubes.
(http://inventors.about.com/od/cstartinventions/a/CathodeRayTube_2.htm)
(http://atomictimeline.net/index.php)

Physicists in the 19th century found out that if they constructed a glass tube with wires inserted in both ends, and pumped out as much of the air as they could, an electric charge passed across the tube from the wires would create a fluorescent glow. This cathode ray also became known as an ‘electron gun’.
Later and improved cathode ray experiments found that certain types of glass produced a fluorescent glow at the positive end of the tube. William Crookes discovered that a tube coated in a fluorescing material at the positive end, would produce a focused ‘dot’ when rays from the electron gun hit it. Crookes believed that the focused nature of the beam meant that they had to be negatively charged particles.
(http://explorable.com/cathode-ray-experiment)

In 1897, the British physicist J.J. Thomson was able to measure the ratio of the electrical charge to the mass of the electron using a cathode ray discharge tube.
He applied electrical and magnetic fields perpendicular to each other as well as to the path of the electrons in the cathode tube.
Thomson argued that the amount of deviation of the particles form their path in the presence of electrical and magnetic field depends upon:
a) The magnitude of the negative charge on the particle, greater the magnitude of the charge on the particle, greater is the interaction with the electric or magnetic field and thus greater is the deflection.
b) Lighter the mass of the particle, greater the deflection.
c) The deflection of electrons from its original path increases with the increase in the voltage across the electrodes or the strength of the magnetic field.

Thomson decided to try to work out the nature of the particles. They were too small to have their mass or charge calculated directly, but he attempted to deduce this from how much the particles were bent by electrical currents, of varying strengths.
Thomson found out that the charge to mass ratio was so large that the particles either carried a huge charge, or were a thousand times smaller than a hydrogen ion. He decided upon the latter and came up with the idea that the cathode rays were made of particles that emanated from within the atoms themselves, a very bold and innovative idea.
He determined the charge to mass ratio (e/m) of an electron to be 1.759 x 10⁸ coulombs/gram.
(http://www.tutorvista.com/physics/cathode-ray-experiment#)
(http://atomictimeline.net/index.php)
(http://explorable.com/cathode-ray-experiment)

J.J. Thomson originally believed that the hydrogen atom must be made up of more than two thousand electrons, to account for its mass. An atom made of thousands of electrons would have a very high, negative electric charge. This was not observed, as atoms are usually uncharged.
In 1906 Thomson suggested that atoms contained far fewer electrons, a number roughly equal to the atomic number. This is only one electron in the case of hydrogen, far fewer than the thousands originally suggested.
These electrons must have been balanced by some sort of positive charge. The distribution of charge and mass in the atom was unknown. Thomson proposed a 'plum pudding' model, with positive and negative charge filling a sphere only one ten billionth of a meter across.
This plum pudding model was generally accepted, until Thomson's student Rutherford proved him wrong later.
(http://www-outreach.phy.cam.ac.uk/camphy/nucleus/nucleus1_1.htm)

In 1907 Thomson began to investigate the rays that were moving towards the cathode. These rays were positively charged, and Thomson wanted to see if they were also composed of charged particles.
Thomson used a method which would show whether there were different types of carriers for the positive charge. He sent a narrow beam of positive rays between two brass plates and applied an electric field that bent the beam of charged particles in the XX' direction. He then put coils at the same point, generating a magnetic field to bend the beam YY' direction. Thomson's apparatus deflected the positive rays using an electric field, in the XX' direction, and a magnetic field, in the YY' direction, before allowing them to fall onto a photographic plate. The action of both fields makes ions of the same mass to charge ratio, m/e, fall onto the same parabolic curves. Fast ions are deflected less than slower ions, but this only affects their position on the curve, not which curve they'll fall on. If the gas in the discharge tube contains different sorts of ions with different specific mass, they will be split into separate parabola by the time they hit the photographic plate.
Thomson numbered the trails in relation to the mass to charge ratio of the hydrogen ion.
Thus he "discovered" the positive rays and the charge/mass ratio of positive ions.
(http://www-outreach.phy.cam.ac.uk/camphy/positiverays/positiverays1_1.htm)

R. A. Milikan's Oil drop experiment determined the charge (e=1.602 x 10 -19 coulomb) and the mass (m = 9.11 x 10 -28 gram) of an electron.
His experiment involved an atomizer, which helps to spray tiny droplets. By means of a short focal distance telescope, the droplets can be viewed. There are two plates, one positive and the other negative above and below the bottom chamber. DC supply is attached to the plates. Some of the oil drops fall through the hole in the upper plate.

Using X-rays the bottom chamber is illuminated causing the air to ionize. As the droplets traverses through the air, electrons accumulate over the droplets and negative charge is acquired. With the help of dc supply a voltage is applied. Speed of its motion can be controlled by altering the voltage applied on the plates. By adjusting the voltage applied, drop can be suspended in air. Millikan observed one drop after another, varying the voltage and noting the effect. After many repetitions he concluded that charge could assume only certain fixed values.

He repeated the experiment for many droplets and confirmed that the charges were all multiples of some fundamental value and calculated it to be 1.5924(17) ×10−19 C, within one percent of the currently accepted value of 1.602176487(40) ×10−19 C. He proposed that this was the charge of a single electron.
(http://amrita.vlab.co.in/?sub=1&brch=195&sim=357&cnt=1)
(http://atomictimeline.net/index.php)

Ernest Rutherford, using alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm) gold foil. This "Gold-Foil Experiment" involved the firing of radioactive particles through minutely thin metal foils (notably gold) and detecting them using screens coated with zinc sulfide (a scintillator). Rutherford found that although the vast majority of particles passed straight through the foil approximately 1 in 8000 were deflected leading him to his theory that most of the atom was made up of 'empty space'. He established that the nucleus was: very dense, very small and positively charged, and that the atom was mostly empty space. He also assumed that the negative electrons were located outside the nucleus.
(http://atomictimeline.net/index.php)
(http://www.rsc.org/chemsoc/timeline/pages/1911.html)

H.G.J. Moseley, using x-ray tubes, determined the charges on the nuclei of most atoms. He published the results of his measurements of the wavelengths of the X-ray spectral lines of a number of elements which showed that the ordering of the wavelengths of the X-ray emissions of the elements coincided with the ordering of the elements by atomic number. With the discovery of isotopes of the elements, it became apparent that atomic weight was not the significant player in the periodic law but rather the properties of the elements varied periodically with atomic number.
Because of Moseley's work, the modern periodic table is based on the atomic numbers of the elements. He wrote "The atomic number of an element is equal to the number of protons in the nucleus". This work was used to reorganize the periodic table based upon atomic number instead of atomic mass.
(http://www.chemistry.co.nz/henry_moseley.htm)
(http://atomictimeline.net/index.php)

In 1932, English Physicist James Chadwick conducted a experiment as follows:
A sample of Beryllium was bombarded with alpha particles, which causes it to emit this "mysterious radiation". It was then discovered by Irene Joliot-Curie and her husband Frederic Joliot-Curie that this radiation, upon striking a proton-rich surface, would discharge some of the protons, which could then be detected using a Geiger counter. Chadwick used that information as the premise to figure out that the mysterious radiation in question was neutral due to the fact that it was not affected by proximity to a magnetic field, and, unlike standard gamma radiation, did not invoke the photoelectric effect, but rather discharged protons, which meant that the particles had to be more massive than previously expected.
Thus, he discovered the neutron and added to the atomic model.
(http://suite101.com/article/the-discovery-of-the-neutron-a46060)