History of the Atomic Theory Chapters 3 and 11

Major Contributors Democritus – Greek philosopher who believed that matter was made up of small indivisible particles he called atoms Aristotle – also a Greek philosopher, disagreed with Democritus and said matter was made up of 4 elements: earth, wind, fire, and water (The idea of atomism died for a while. In the meantime, alchemists worked with matter, believing they could turn ordinary metals into gold)

John Dalton – compiled knowledge about matter into the first Atomic Theory. Dalton’s theory incorporated the works of other chemists, like Antoine Lavoisier (his work led to the Law of Conservation of Mass) and Joseph Proust (his work led to the Law of Definite Proportions)

(* we know today that these two are not true) Dalton’s Theory Each element is made of extremely small, indivisible particles called atoms. * All atoms of a given element are identical to each other, but different from those of another element.* Atoms are neither created nor destroyed in a chemical reaction. A given compound always has the same ratio of the atoms that make it up (* we know today that these two are not true)

J.J. Thomson – did experiments with the CRT (which was invented by William Crooks) The problem with Dalton’s theory is the idea that atoms are indivisible spheres Based on evidence from his experiments, Thomson proposed a new model. In this model, atoms were thought to be a sphere of positive charge with negative charges randomly embedded within the sphere. His model is called the Plum Pudding Model (or Raisin Bun Model)

Thomson experiments did not provide any direct evidence of individual negative particles versus a stream of energy with a negative charge. R.A. Millikan performed the Oil Drop experiment and was able to determine the mass (9.11 x 10-28 grams) and charge (1.60 x 10-19 Coulombs) of an individual electron, providing evidence that electrons are discrete particles.

Ernest Rutherford decided to test Thomson’s Plum Pudding Model, by bombarding gold atoms with alpha particles which are positively charged (the Gold Foil experiment). If Thomson’s model were correct, all of the particles should have passed straight through.

What Rutherford found was that 99% of alpha particles went straight through the gold foil, but 0.5% deflected, and 0.5% reflected! The ‘Plum Pudding’ model was wrong. He proposed that All of an atom’s positive charge is concentrated in a very small core at the atom’s center, which Rutherford called the nucleus. The negatively charged electrons move around the nucleus.

Henry Moseley – a student of Rutherford’s who determined the number of positive charges in the nuclei of most atoms known at the time; said that all atoms of a given element had the same number of positive charges (protons); rearranged Mendeleev’s periodic table

Niels Bohr, a theoretical physicist, addressed a fundamental problem with Rutherford’s model: This model couldn’t exist because electrons should lose energy and collapse into the nucleus. He added the concept of energy in quanta (small packets) to Rutherford’s Model.

Main Concepts of Bohr’s Model 1. Electrons assume only certain orbits around the nucleus. These orbits are stable and called "stationary" orbits. 2. Each orbit has an energy associated with it. For example the orbit closest to the nucleus has an energy E1, the next closest E2 and so on. 3. Light is emitted when an electron jumps from a higher orbit to a lower orbit and absorbed when it jumps from a lower to higher orbit. Only whole number jumps allowed.

Building on the work Neils Bohr (and other scientists like Max Planck, Albert Einstein,Louis deBrogelie, & Werner Heisenberg), Erwin Schroedinger developed a wave equation to describe regions outside the nucleus where electrons are likely to be found. These regions are called orbitals, and the model developed by Schroedinger is called the wave mechanical model.

James Chadwick was a coworker of Ernest Rutherford who discovered a second nuclear particle that was equal in mass to a proton but had a neutral charge. He called these particles neutrons.