History of Atomic Structure

Democritus Aristotle

Ancient Philosophy Who: Aristotle, Democritus When: More than 2000 years ago Where: Greece What: Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”.

Alchemists Who: European Scientists When: 800 – 900 years ago Where: Europe What: Their work developed into what is now modern chemistry. Why: Trying to change ordinary materials into gold.

Alchemic Symbols

John Dalton

Particle Theory Who: John Dalton When: 1808 Where: England What: Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom. The atom was a solid sphere.

J. J. Thompson

Discovery of Electrons Who: J. J. Thompson When: 1897 Where: England What: Thompson discovered that electrons were smaller particles of an atom and were negatively charged. Blueberry muffin model where the cake is positive and the electrons are embedded. Why: Thompson knew atoms were neutrally charged, but couldn’t find the positive particle.

Cathode rays J.J. Thomson studied was the conduction of electricity through gases. One subject which interested Thomson was cathode rays. These rays are emitted at the cathode, or negative terminal, in a discharge tube. I The nature of the cathode rays was controversial. Although Thomson thought the rays must be particles, many Europeans thought they were an 'etherial disturbance', like light. In Germany Hertz had observed the rays passing through thin sheets of gold. It seemed impossible that particles could pass through solid matter.

Cathode rays being deflected by a magnetic field, whose direction is into the paper. The magnetic field produces a force which deflects the cathode rays. In his tube, Thomson positioned the coils so that the deflection was in the opposite direction to the deflection produced by the electric field. By adjusting the strengths of the fields the rays could be deflected, in one direction by the electric field, and back an equal amount by the magnetic field. The forces were balanced.

Ernest Rutherford

Atomic Structure I Who: Ernest Rutherford When: 1911 Where: England What: Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core. Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.

He fired Alpha particles at an extremely thin gold foil. He expected them to go straight through with perhaps a minor deflection. Most did go straight through, but to his surprise some particles bounced directly off the gold sheet!   What did this mean?  Rutherford hypothesized that the positive alpha particles had hit a concentrated mass of positive particles, which he termed the nucleus. Most of the atom is empty space with a small dense positive nucleus. Electrons were very small and occupy the space outside the nucleus.

Rutherford’s Lab in Cambridge

Atomic Structure II Who: Niels Bohr When: 1913 Where: England What: Proposed that electrons traveled in fixed energy levels around the nucleus based on atomic spectra experiments. Why: Bohr was trying to show why the negative electrons were not sucked into the nucleus of the atom.

Modern Synthesis Electrons do not travel around the nucleus in orbits, rather are most likely to occupy 3D areas called orbitals. Electrons travel so fast, they appear to form a “cloud” around the nucleus.

Electron Cloud Model

Neils Bohr

Rays or Particles? Curie-Joliots in Paris, had bombarded beryllium with alpha-particles, and found that very penetrating gamma rays were emitted. Cockcroft and Walton thought that similar rays should be observed when they bombarded light elements with protons. Unfortunately, the Curie-Joliots were mistaken in identifying the 'radiation'. In January 1932 James Chadwick repeated their experiments and found that the reaction produced neutral particles, neutrons, that Rutherford had predicted in 1920. A few months after Chadwick's discovery Rutherford came to Cockcroft and Walton and told them they 'ought to put in a fluorescent screen and get on with the job'. Rutherford was clearly hoping to see alpha-particles, and a fluorescent screen would be the best way to detect them. On 14 April 1932 Walton set up the tube and bombarded lithium with high energy protons. He then crawled into the little observation cabin set up under the apparatus and immediately saw scintillations of the fluorescent screen. The reaction was giving off alpha-particles.