2006-7 Quantum Theory Slides Lecture 1

Quantum theory and atomic spectroscopy Dr Ian Lane Room G18B Lab B38D

Course outline • Experimental evidence for atomic structure • Old quantum theory – making it up as you go along • Description of atoms in terms of FOUR quantum numbers • Quantum mechanics – logical description of nature BUT it’s mathematical not intuitive!

The Standard Model • Explains the myriad of particles observed • Chemistry concerned with just three fundamental particles • But it gives us no idea why it is the true picture!

The Periodic table

• But why can we arrange elements like this? • Quantum theory tells us!

The atom today • A compact nucleus surrounded by electrons • Quark diameter one ten-billionth of the atom’s

The Cathode Ray tube • Evacuated glass tube with electrodes attached • You see a current flow! • It is bent by magnets

Discovery of electron • Lenard- the Lenard window • Stoney – coins the name electron (1894) • JJ Thomson – shows cathode ray affected by electric field and measures its mass (1897)

Geiger and Marsden

• Rutherford explained this and grabbed all the glory! • Atom has tiny NUCLEUS

Optical spectroscopy

• White light is composed of many wavelengths • But the white light from the sun is different from that of a lamp – why? • Wollaston – dark lines from the sun!

Early atomic spectroscopy

• Fraunhofer analyses the sun for better prisms! • Bunsen and Kirchoff develop spectroscope • Discover caesium

Spectra of atoms

• Different lines for different elements – spectral analysis (Brewster)

What use is this? Spectroscopy led to a number of HUGE discoveries including: The sun is just a STAR! (Huggins 1864) The sun has new elements! (Lockyer, 1868)

X-ray scattering • High energy x-rays scatter from atoms • 1906- each atom has unique spectra • 1911- signal depends on number of electrons • 1913- Soddy finds isotopes

Is an atom stable? • NO! Classically, there are two reasons to argue an atom is “impossible” • A static arrangement of charges will simply collapse • When charged particles accelerate they release radiation, so cannot achieve a stable “orbit”

Wait a minute then…… • How are electrons arrange- why don’t atoms collapse? • What do the lines in an atomic spectra actually mean?