1803 - John Dalton - Atomic Theory

Matter is made up of indivisible atoms. All atoms of an element are identical. Atoms are neither created nor destroyed. Atoms of different elements have different weights and chemical properties. Atoms of different elements combine in simple whole numbers to form compounds.

1830 - Michael Faraday

• Set up a pair of metal plates sealed in a glass tube.  The tube was filled with a gas, and the metal plates were connected to a series of batteries.

As the pressure of the gas decreased, the gas began to glow. Julius Plucker (1858) noticed that only one end emitted light. He also changed the position of the patch of glass that glowed by bringing a magnet close to the tube. Conclusion:  The effect of the magnetic field as evidence that whatever produced this glow was electrically charged. Cathode - metal plate connected to the negative end Anode - metal plate connected to the positive end

1869 - Johannes Hittorf

• Found that when a solid object was placed between the cathode and anode, a shadow was cast on the end of the tube across from the cathode.

• Conclusion:  Some beam or ray is given off by the cathode - subsequently called the tubes cathode-ray tubes.

1879 - William Crookes

Developed a better vacuum pump that allowed him to produce cathode-ray tubes with a smaller residual gas pressure. Conclusion:  Cathode r0ays are negatively charged by studying deflection of cathode rays by magnetic fields.

1897 - J.J. Thompson

Found that cathode rays could be deflected by an electric field Showed that cathode "rays" were actually particles Found the charge to mass ratio of the particles to be approximately 108 Coulomb (C) per gram. Same charge to mass ratio regardless of metal used for cathode/anode or gas used to fill the tube. Conclusion:  Particles were a universal component of matter. Electron -  (originally called corpuscles by Thompson) particles given off by the cathode; fundamental unit of negative electricity Raisin Pudding Model - Matter is electrically neutral and electrons are much lighter than atoms. Conclusion:  There must be positively charged particles which also must carry the mass of the atom.  Dalton's model is now incorrect because atoms are divisible.

Raisin Pudding Model

1895 - William Conrad Roentgen

Discovered x-rays while using cathode-ray tubes.  Found that x-rays could pass through solid objects.

1899 - Ernst Rutherford

Studied absorption of radioactivity. Alpha radiation - positive charge - absorbed by a few hundredths of a cm or metal foil Beta radiation - negative charge - could pass through 100x as much foil before it was absorbed Gamma rays - no charge - could penetrate several cm of lead

1907-1911 - Rutherford updated Thomson's Raisin Pudding Model of the atom.

Studied the deflection of alpha particles as they were targeted at thin gold foil sheets. Most of the alpha particles penetrated straight through. However few were deflected at slight angles. Even fewer (only about 1 in 20,000) were deflected at angles over 90.

                 

• Conclusion:  The positive charge and mass of an atom were concentrated in the center and only made up a small fraction of the total volume.  He named this concentrated center the nucleus (Latin for little nut).

• Rutherford was also able to estimate the charge of an atom by studying the deflection of alpha particles.  He found that the positive charge on the atom was approximately half of the atomic weight.

1908-1917 - Robert Millikan - Oil-drop experiment

J.J. Thomson had previously hypothesized that the mass of a single electron was at least 1000 times smaller than that of the smallest atom. Millikan measured the charge on an electron with his oil-drop apparatus.

An "atomizer" from a perfume bottle sprayed oil or water droplets into the sample chamber.   Some of the droplets fell through the pinhole into an area between two plates (one positive and one negative).  This middle chamber was ionized by x-rays.  Particles that did not capture any electrons fell to the bottom plate due to gravity.  Particles that did capture one or more electrons were attracted to the positive upper plate and either floated upward or fell more slowly.

• Conclusion:  The charge on a drop was always a multiple of 1.59 x 10^-19 Coulombs.  He proved Thomson's hypothesis that the mass of an electron is at least 1000 times smaller than the smallest atom.

1913 - A. van den Broek

• Suggested that the positive charge on atoms should be compared to their atomic numbers, not their atomic weights.
  • At the time, atomic number (Z) only specified an element's location on the periodic table.  Today, the atomic number is, by definition, the number of protons in an atom.

1914 - H. G. J. Moseley

	Studied the frequencies of the x-rays given off by cathode-ray tubes when electrons strike the anode. Found that there was a relationship between the frequencies (v) of the x-rays given off by the cathode-ray tube and the atomic number of the metal used to form the anode:
Conclusion: He argued that the frequencies of the x-rays should depend on the charge on the nucleus emitting these x-rays.  Therefore, the atomic number was equal to the positive charge (charge on the nucleus) of an atom.

1920 - Rutherford proposed the name "proton" for the positively charged particles in the nucleus of an atom.  At the same time, he also proposed that the nucleus also contained electrically neutral particles which accounted for the remaining mass of the atom.  He called this yet unknown particle the "neutron".

1932 - James Chadwick

Proved that neutrons, neutral particles in the nucleus that made up approximately half the mass of an atom, did exist. Summary of Subatomic Particles Particle   Symbol   Charge   Mass Electron e- -1 0.0005486 amu Proton p+ +1 1.007276 amu Neutron n 0 1.008665 amu

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