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. 1

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

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

Rutherford Experiment By Kurzon – CC-BY-SA-3.0 4

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. 5

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. 6

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. 7 Hydrogen Iron By nilda – public domain By Merikano, Adrignola – public domain

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. 8 By stkl – public domain

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. 9

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. 10

Lasers 11

Lasers  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. 12