Chemistry 1027A/B Lecture Notes - Ruthenium, Lead, Noble Gas

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22 Jan 2013
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UNIT 2 STRUCTURE & PROPERTIES
Origins of Quantum Theory
Kirchhoff’s work on “blackbodies” was start of quantum theory but
Planck was credited with starting quantum theory
“blackbody” used to describe an ideal perfectly black object i.e.
does not reflect any light & emits various forms of light as a result
of temperature
Plancks Quantum Hypothesis
when an object is heated it was observed that it will glow
initially it will glow red, then white, these changes do not depend
on the composition of the solid
if we look at the intensity of the different colours we get a bell
shaped curve
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scientists couldn’t explain why the bell curve didn’t follow what
was expected “the classical theory” until Planck came up with an
equation to explain this (1900)
Planck’s theory was that the energies of the atoms in the solids
were multiples of small quantities of energy i.e. energy was not
continuous
Einstein continued this work by pointing out that Planck’s
hypothesis led to the conclusion that light emitted by a hot solid is
also “quantized” i.e. sent out as bursts of energy
Each of these bursts were referred to as a
quantum
of energy
(read money analogy in text on page 169)
lead to thinking that as temperature increased, the proportion of
each larger quantum becomes greater
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The Photoelectric Effect
Maxwell proposed that light was an electromagnetic wave
composed of electric and magnetic fields that could exert forces
on charged particles (classical theory of light)
Hertz discovered photoelectric effect which involved the effect of
electromagnetic radiation or light on substances
classical
theory said that
the intensity
(brightness)
shining on the
metal would
determine the
kinetic energy
of the freed
electrons i.e.
the brighter the
light, the
greater the
energy of the emitted electrons
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