1 Structure of the Atom Learning Outcomes Describe

1 Structure of the Atom – Learning Outcomes Describe Rutherford’s experiment. Simulate Rutherford’s experiment. Discuss the Bohr model of the atom. Discuss energy levels. Discuss emission line spectra. Solve problems about emission spectra. Demonstrate line spectra and continuous spectra. Discuss the use of spectroscopy. Discuss the operation of lasers.

2 Rutherford Experiment Ernest Rutherford determined the structure of the atom in the early 1900 s. He fired alpha particles (positive charge, more details later) at a thin sheet of gold foil.

3 Rutherford Experiment Results Conclusions Most alpha particles went The atom is mostly empty space. straight through the foil. At the centre of the atom is a Some alpha particles small positive centre called the were deflected slightly. nucleus. A very small number of alpha particles were reflected backwards. The electrons must be very low mass and do not exist in the nucleus, but orbit it.

4 Rutherford Experiment By Kurzon – CC-BY-SA-3. 0

5 Bohr Model Rutherford’s orbit idea didn’t hold up to scrutiny. Accelerating charges emit photons, so electrons in orbits (experiencing centripetal acceleration) would inevitably lost all their kinetic energy and spiral into the nucleus. Bohr proposed that electrons inhabited orbitals.

6 Bohr Model Orbitals are discrete (separate, distinct) energy levels. Electrons can jump to a higher energy level if it absorbs enough energy. of light they emit.

7 Emission Spectra (Line) These levels are different for each atom, so atoms can be identified by the light they emit. This light can be split into its component colours by dispersion. The resulting pattern is called a line spectrum. Hydrogen Iron By Merikano, Adrignola – public domain By nilda – public domain

8 Emission Spectra (Continuous) In some cases (e. g. hot objects, metals), electrons or atoms are not bound to specific energies and may emit light of any wavelength when they accelerate. The emission spectrum in this case is called a continuous spectrum. By stkl – public domain

9 To Demonstrate an Emission Spectrum 1. Pass light from a source through the collimator of a spectrometer. 2. Place a triangular prism on the turntable. 3. Look through the telescope and note the light observed. 4. Depending on the source, a line or continuous spectrum will be observed.

10 Uses of Spectroscopy Since each element has a unique line spectrum, spectroscopy is used to identify which elements are in a given material. The intensity of the lines can also indicate concentration. It is used to determine compounds in food samples. It is used to determine the composition of stars, giving information about their age and classification. It can detect chemical changes in materials over time, e. g. wood degrading in poor weather.

Lasers 11

12 Lasers (Light Amplification by Stimulated Emission of Radiation) provide energy to put a lot of electrons in higher levels (“pumping”) They then use stimulated emission to make lots of them fall, creating an intense beam of coherent light.