James Chadwick discovered the neutron in 1932. Neutrons were proposed to explain discrepancies between atomic mass and charge. Ernest Rutherford published his atomic theory in 1911 describing the atom as a small, dense nucleus surrounded by orbiting electrons. Most of an atom's mass is in the nucleus, with the rest being empty space. Robert Millikan's 1909 oil drop experiment helped establish the quantized nature of electric charge and supported Thomson's discovery of the electron as a fundamental subatomic particle with a discrete negative charge.

1. James Chadwick1932 Discovers the neutron. As they studied atomic disintegration, they kept seeing that the atomic number (number of protons in the nucleus, equivalent to the positive charge of the atom) was less than the atomic mass (average mass of the atom). For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, it seemed that something besides the protons in the nucleus were adding to the mass. One leading explanation was that there were electrons and additional protons in the nucleus as well -- the protons still contributed their mass but their positive charge was canceled out by the negatively charged electrons. So in the helium example, there would be four protons and two electrons in the nucleus to yield a mass of 4 but a charge of only 2. Rutherford also put out the idea that there could be a particle with mass but no charge. He called it a neutron, and imagined it as a paired proton and electron. There was no evidence for any of these ideas.Ernest Rutherfordpublishes his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons. This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space. Rutherford came to this conclusion following the results of his famous gold foil experiment. This 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'.Robert Millikan(1909)The scheme of the experiment is as follows: An atomizer sprayed a fine mist of oil droplets into the upper chamber. Some of these tiny droplets fell through a hole in the upper floor into the lower chamber of the apparatus. Millikan first let them fall until they reached terminal velocity due to air resistance. Using the microscope, he measured their terminal velocity, and by use of a formula, calculated the mass of each oil drop. Next, Millikan applied a charge to the falling drops by irradiating the bottom chamber with x-rays. This caused the air to become ionized, which basically means that the air particles lost electrons. A part of the oil droplets captured one or more of those extra electrons and became negatively charged.J.J ThomsonIn 1897 J. J. Thomson, an English physicist, conducted a series of experiments on cathode rays and after observing that the beam of light in the cathode ray tube is attracted to a positive charge and repelled by a negative charge he concluded that the rays consist of a stream of small, electrically negatively charged particles which have a mass over a thousand times less than that of a hydrogen atom. Thomson has discovered the electron. From this point onward, it becomes increasingly clear that atoms are not fundamental particles, but in fact are made up of smaller particles.<br />John DaltonJohn Dalton (1766-1844) developed the first useful atomic theory of matter around 1803. Atoms of an element cannot be created, destroyed, broken into smaller parts or transformed into atoms of another element. Dalton based this hypothesis on the law of conservation of mass and on centuries of experimental evidence. Dalton's postulates can be used to estimate atomic weights from experimental data, and how they explain three basic laws of chemistry.<br />John Dalton<br />1 atom C @ 12 u each = 12 u C2 atoms O @ 16 u each = 32 u O } 12/32 = 0.375 u C / u O 4 atoms C @ 12 u each = 48 u C8 atoms O @ 16 u each = 128 u O } 48/128 = 0.375 u C / u O 8 atoms C @ 12 u each = 96 u C16 atoms O @ 16 u each = 256 u O } 96/256 = 0.375 u C / u O<br />6797675878840James chadwivck3729355412750-1480032127690-658495403860robert millikkan Ernest<br />