Leucippus, democritus & the origin of atomic theory
- Leucippus and Democritus came up with the theory of "Matter is composed of tiny particles, yet it is unknown how they came up with the theory.
- They did not the think the particles were particularly special; they just thought that if you cut something in half enough times, eventually you will reach a particle that cannot be cut anymore.
- They called the particles "Atomos", meaning un-cuttable or indivisible.
- Also believed the particles were made up of themselves.
- The next major developments of atomic theory happened 2,300 years later.
- Antoine Lavoiser proposed 'The Law of Conservation of Mass' which states that "even if matter changes shape or form, its mass stays the same".
- James Dalton determined that "elements are discrete packets of matter".
Discharge tubes
- Discharge tubes are gas filled tubes with electrodes at each end which omit light when an electrical current passes through them. Eg. a Neon Light.
- Originally produced by a negative electrode or cathode ray.
- In 1886, a German physicist discovered the tubes also emitted light from positive electrode, meaning that there was also a positive charge in matter.
- Joseph John Thomson measured how much heat the cathode rays generated and how much could be bent by magnets. He therefore estimated the mass of rays (Approx. 1,000 times lighter than Hydrogen).
- JJ Thomson concluded that cathode rays weren't in fact rays but were very light and small charged particles. Noted as Corpuscles then, commonly known as Electrons now.
- Thomson knew the atom had an overall neutral charge. He therefore imagined the negatively charged electrons must be distributed randomly in a positively charged matrix. Commonly referred to as The Plum Pudding model.
- Positive Matrix = Cake
- Electrons = Fruit
Earnest rutherford & the nucleus
- Earnest Rutherford discovered the nucleus of an atom in 1907.
- In 1909 Rutherford designed an experiment using an extremely thin sheet of gold foil and a screen coated in Zinc Sulfide.
- His surprising result of it deflecting at large angles found the basis of his theory that the entire positive charge in an atom must be concentrated in a very small space. ie. The Nucleus.
- He then later went on to discover that if you put hydrogen with alpha particles, it creates a bunch of hydrogen ions. ie. Protons.
niels bohr model
- In 1913, Niels Bohr published the Bohr Model. Bohr came to realise that the mathematical principles could be applied to Rutherfords atomic model. The Bohr Model introduced the ideas that electrons orbit around the central nucleus.
- He discovered that the outer orbits could hold more electrons than the inner ones. It therefore made it possible to determine the chemical properties of an element.
- His model, The Bohr Model shows the electrons orbiting around the nucleus. It is also often referred to as the "Planetary Model".
heisenberg & QUANTUM theory
- Werner Heisenburg came up with the Quantum Theory. Heisenburg discovered that it is impossible to know the momentum of an electron (or any subatomic particle) and it's exact position. The more you know about of of two variables, the harder it gets to measure the other one.
- The Quantum Theory proposed that atoms weren't waves or particles, but were in fact a mix of both. Heinsenburg also discovered that you could never actually accurately guess the position or momentum of an electron. This then lead to the creation of the "Cloud Model".
example atom: sodium (11)
The element, Sodium, has a mass number of 11. Its mass number is 23 and contains 12 neutrons. It has a electron figuration of 2,8,1.
Reference
2013, The History of Atomic Chemistry: Crash Course #37, YouTube, https://www.youtube.com/watch?v=thnDxFdkzZs, Accessed 29th January 2015