A history of atomic structure AH Chemistry, Unit 1(a)

The key players: part 1 John Dalton J..J. Thompson Eugene Goldstein Max Planck Proposed atomic theory Discovered the electron Discovered the proton Quantization of energy 1808 1897 1900 1900 Albert Einstein Ernst Rutherford Niels Bohr James Chadwick Photoelectric effect Discovered the nucleus Devised the quantum model Discovered the neutron 1905 1911 1913 1932

One of the best ways of probing the structure of an atom is by studying their interaction of electromagnetic radiation.

Electromagnetic spectrum

Wave structure

Heating objects What colour do objects turn when they are heated? Red Orange Yellow White Blue

Quantisation In 1900, Max Planck proposed that electromagnetic radiation could only be emitted or absorbed in small packets (quanta). These were later called photons.

The photoelectric effect UV radiation No electrons emitted until a particular frequency is reached (threshold) which is specific to the metal The higher the intensity, the more electrons that are ejected

Why does it happen? UV radiation No electrons emitted until a particular frequency is reached (threshold) which is specific to the metal The higher the intensity, the more electrons that are ejected

Energy of photons Proportional to their frequency E = hv (for a single photon) E = Lhv (for one mole of photons)

SQA 2009 paper

So light is made up of particles? Only when we want it to be!

The single slit experiment: light

Diffraction

The double slit experiment: light

Here, the evidence is that light is behaving as waves. Light has wave-particle duality.

What about if you do it with electrons?

This does not happen!

Light and electrons both have dual wave-particle properties. The electrons appear to be behaving as waves and producing an interference pattern. In 1906, J.J. Thompson won the Nobel Prize for showing that electrons were particles. In 1937, G.P. Thompson (his son) won the Nobel Prize for showing that electrons where waves. Light and electrons both have dual wave-particle properties. Does anything else?

Everything! It’s just that to appreciate wave properties, you have to be in the realms of the very, very small

What about if only one electron is fired at a time, so there can be no interference?

…but this does not happen if only one slit it present!

A possible explanation is that the electron splits when it is fired at a double slit, one part passing through one slit and one part passing through the other. But how did the electron know that the other slit was there?

If it does split, behaving as waves, this would allow for the constructive and destructive interference observed. This theory can be tested further using particle detectors…

Particle detectors

What happens? Only one green detector ever registers an electron at any one time, never both simultaneously. None of the pink detectors detect an electron. The electron has stopped behaving as a wave when the particle detectors are introduced!

The paradox With no particle detectors, it seems to go through both and behave as a wave; with particle detectors to goes through one and behaves as a particle. Knowledge of the detectors seems to be influencing the event. Where has this knowledge come from? The only place it can have come from is the mind of the observer.

In other words, simply by observing, the observer influences the outcome.

Explore it further…