CHEM 217 Lecture Notes - Lecture 7: Atomic Orbital, Ionic Bonding, Emission Spectrum
Unit 7
7 . 1 Q U A N T U M M EC H A N I C S : B EH A V IO U R O F T H E A B SO L UT E LY
S M A L L
oQuantum mechanics changed deterministic thinking
oInability to observe electrons without disturbing them
oDoes not behave the same when being observed
oQuantum-mechanical model of the atom
7 . 2 N A T U R E O F LI G HT
oWave-particle duality – certain properties are best described as wave/particle
oWave nature
oLight is electromagnetic radiation: a type of energy embodied in oscillating electric & magnetic
fields
Electric field: region of space where electrically charged particle experiences force
Ex. a proton has a field around it – another charged particle will be
repulsed/attracted
Magnetic field: a region of space where a magnetic particle experiences a force
Ex. magnets & around them
oElectromagnetic radiation can be described as a wave composed of oscillating, mutually
perpendicular electric and magnetic fields, propagating through space
oMove fast (in vacuum) @ 3.00x108 m s-1
oCan characterize wave by amplitude and wavelength
oAmplitude: vertical height of a crest or depth of a trough
The amplitude of the electricand magnetic field waves in light determine
intensity/brightness
oWavelength (λ)
Distance between adjacent crests (or any two analogous points) in m, mirometers
or nanometers
oBoth amplitude and λ are related to quantity of energy carried in a wave
oMost energetic have high amplitude and low λ
oLight also characterized by its frequency (v): the # of cycles that pass through a point at a
given time
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Units: cycle s-1 or s-1 (same thing)
oFrequency is inversely proportional to λ.
V= C/ λ
C is speed of light
. λ is wavelength
oboth expressed in some unit of distance
odefferent ways to specify same info
VISIBLE LIGHT
odetermines colour:
ored: ~750nm : longest λ of visible light
oViolet: ~400nm : shortest
ELECTROMAGNETIC SPECTRUM
oVisible light is a tiny portion
oShortest λ is gamma ray (y) – produced by sun, stars, certain unstable atomic nuclei
oX-rays: longer than gamma and pass through many substances that block visible light but
sufficient energy to damage biological molecules
obetween x-rays and gamma rays there is UV rays – radiation
oThen visible
oThen infrared radiation (IR). The ehat when you place your hand near a hot object
oBeyon that, microwaves -> the longest are radiowaves
INTERFERENCE/DIFFRACTION
oWaves interact with interference: cancel each other out or build each other up depending on alignment
upon interaction
oIF two waves of equal amplitude are ‘in phase’ when they interact – if they align with overlapping crests –
a wave with twice the amplitude results
oConstructive interferences
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find more resources at oneclass.com
oIf two waves are completely out of phase, i.e. crests from one overlap w/trough of other – they cancel out
oDestructive interference
oDiffraction:
oWave encounters an obstacle or slit comparable in size to λ, it bends or diffracts around it
oDiffraction of light through two slits,
separated by a distance comparable to λ of
light, coupled with inferference produces
interference pattern
oEach slit acts as a new wave source and
interaction causes bright spots (in phase)
and dark spots (out of phase)
PARTICLE NATURE OF LIGHT
oPhotoelectric effect – observation that metals can emit
electrons when light shines upon them
oFirst thought that this was due to transfer of light to the electrons but predicts that emission was
proportional to wavelength and frequency – this didn’t hold up
oFound that in high frequency-low intensity, light produced electrons with no lag time and light
exhibited a threshold frequency below which no electrons were emitted
Low frequency, long wavelengths would not eject; high frequency, short wavelength
would even eject with low intensity
oBlackbody radiation
oEmissions from heated objects
Predicted that density of radiation from a heated body would increase to infinity at high
frequencies
‘ultraviolet catastrophe’ because theory breaks down at high frequencies
oNew theory 1900: could obtain agreement with experimental blackbody radiation curve only if it
was assumed that energy was emitted in discrete packets called quanta
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
S m a l l: quantum mechanics changed deterministic thinking o. Inability to observe electrons without disturbing them: does not behave the same when being observed, quantum-mechanical model of the atom. 2 n a t u r e o f l i g h t: wave-particle duality certain properties are best described as wave/particle, wave nature o. Light is electromagnetic radiation: a type of energy embodied in oscillating electric & magnetic fields. Electric field: region of space where electrically charged particle experiences force. Ex. a proton has a field around it another charged particle will be repulsed/attracted. Magnetic field: a region of space where a magnetic particle experiences a force. Electromagnetic radiation can be described as a wave composed of oscillating, mutually perpendicular electric and magnetic fields, propagating through space: move fast (in vacuum) @ 3. 00x108 m s-1 o. Can characterize wave by amplitude and wavelength o. Amplitude: vertical height of a crest or depth of a trough.
