Unit 2: Structure of Matter

Content Outline: History of the Atomic Model (2.1)

  1. Democritus (400 B. C.)
  1. He was a Greek philosopher of science.
  2. First to use the term “atom” to describe the basic particle of nature.
  1. “atom” means “indivisible”
  2. Atom – the smallest particle of an element that still retains the chemical properties of that element.
  1. John Dalton (1808)
  1. He was an English schoolteacher.
  2. He was the first to propose an “Atomic theory” that states the 5 following statements:
  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.
  1. This has since been modified based on Isotopes and ions.
  1. Atoms cannot be subdivided, created, or destroyed.

a. This has since been modified based upon current studies in quantum physics. Such examples include muons and quarks.

4. Atoms of different elements combine in simple whole-number ratios to form chemical

compounds.

  1. In chemical reactions, atoms are combined, separated, or rearranged.
  1. Joseph John Thomson (1897)
  1. He was an English Physicist.
  2. He worked with glass gas-filled tubes referred to as Cathode-Ray tubes.
  1. The glass tubes were filled with a gaseous element under low pressure.
  2. He then passed an electrical current using a battery and wires.
  1. The electrical current caused the gas within the tube to glow with a beam (“ray”) within the tube to intensely glow within the tube.
  1. Magnets could make the “ray” move/deflect in various directions.
  2. The ray is being deflected by the negative charge of the magnet.
  3. Negative charge repels/deflects like negative charges.
  4. The ray is made of negatively charged Thompson called electrons (since they were associated with the electrical current.)
  1. The electrical current came into the chamber (by a wire) at the cathodeend. (The end where electricity enters the tube.)
  2. The electrical current left the tube on the anode end. (The end where the electricity goes back into the wire.)
  3. Hence the term Cathode Ray tubes.
  1. Further investigations using different elements in Cathode-Ray tubes confirmed that every elements atoms possess electrons.
  2. He proposes the “Plum Pudding” model of atoms.
  1. It states that negatively charged electrons are evenly placed inside a positively charged mass.
  1. Robert A. Milliken (1909)
  1. He was an American Physicist.
  2. He was the first to measure the charge and mass of an electron.

The symbol for an electron is: e-

  1. Electron charge = 1.602 x 10-19 Coulombs.
  1. This is an extremely small quantity of energy.
  1. Electron mass = 9.11 x 10-31 kg
  1. Electrons are 1/1837th the mass of a single proton or neutron.
  2. This is a very, very, very small amount and size.
  1. Milliken’s experiments allow for 2 inferences (conclusions based upon evidence and reasoning) to be made:
  1. Because atoms, in the natural state, are electricallyneutral, they must also contain an equal amount of positively charged particles.
  2. Because electrons have so little mass, atoms must contain other particles with much greater mass (protons & neutrons).
  1. Ernest Rutherford, Hans Geiger, and Ernest Marsden (1911)
  1. Geiger and Marsden were students of the New Zealand Physicist.
  2. They performed the Gold Foil Experiment.
  3. They used high-energyalpha particle radiation( 2 protons & 2 neutrons ejected from a decomposing, radioactive element) to bombard a piece of gold foil that is surrounded by a fluorescent screen.
  1. As alpha particles struck the fluorescent screen, they would produce a small detectable burst of light.
  2. As the experiment was running, they detected light burst mainly behind the gold foil, but also occasionally all around the ring.
  1. These bursts of light around the ring were because of the positively charged alpha particles been deflected by positively charged particles in the atoms of the foil.
  2. The particles became known as protons.
  1. Just as with the electrons, positive charges repel/deflect like positive charges.
  1. As most of the bursts of light occurred behind the gold foil, they concluded that the majority of space in an atom is “empty space”. The alpha particles travelled through and never hit anything.
  1. Rutherford proposes the idea of the neutrally charged neutron particle in 1920.
  1. Niels Bohr (1913)
  1. He was also a student of Rutherford’s.
  2. He proposes the Bohr model of an atom.
  1. The electrons move in a circular pattern around the positively charged center. (Much like the planets revolve around the sun.)
  1. Dmitri Ivanenko & Victor Ambartsumian (1930)
  1. These gentlemen were Russian Physicists.
  2. They propose a model of the nucleus of an atom being composed of positively charged protons and neutral charged particles (neutrons).
  1. James Chadwick (1932)
  1. He was an English Physicist.
  2. He proves that the nucleus is definitely composed of protons and neutrons through his experiments with betaparticle radiation.(This is stronger radiation than alpha.)