“No familiar conceptions can be woven around the electron. Something unknown is doing we don’t know what.” -Sir Arthur Eddington The Nature of the Physical World (1934) The ELECTRON: Wave – Particle Duality
“No familiar conceptions can be woven around the electron. Something unknown is doing we don’t know what.”
-Sir Arthur Eddington The Nature of the Physical World (1934)
The ELECTRON: Wave – Particle Duality
The Dilemma of the Atom
• Electrons outside the nucleus are attracted to the protons in the nucleus
• Charged particles moving in curved paths lose energy (so they should fall inward)
• What keeps the atom from collapsing?
The Bohr Model of the Atom
Neils Bohr
I pictured electrons orbiting the nucleus much like planets orbiting the sun. But I was wrong! They’re more like bees around a hive.
Quantum Mechanical or Electron Cloud Schrödinger
Mathematical laws identify the areas outside the nucleus where e- are found.
The model says:
1) there is a 90% probability of finding the e- in this area called an ORBITAL.
2) the e- travel randomly in the space and they act like waves.(more later)
Wave-Particle Duality JJ Thomson won the Nobel prize for describing the electron as a particle. His son, George Thomson won the Nobel prize for describing the wave-like nature of the electron.
The electron is a particle!
The electron is an energy
wave!
Wave- particle duality says that all energy and matter exhibits both wave-like and particle-like properties…so e- behave like waves & like particles.
The Wave-like Electron
Louis deBroglie
The electron propagates through space as an energy wave. To understand the
atom, one must understand the behavior of
electromagnetic waves.
https://youtu.be/MFPKwu5vugg
Electromagnetic Radiation energy (e-) traveling through space
Small wavelengths= more dangerous -the waves have more energy
Big wavelengths= less dangerous- the waves have less energy
These waves travel at the speed of light (c) =3.0 x 108 m/s
Electromagnetic spectra- radiation over a broad range of wavelengths. These all travel at the speed of light.
Electromagnetic Spectrum
Light is composed of a small section of the Electromagnetic Spectrum
Light can be broken up into a spectrum of colors
Red – orange- yellow – green – blue –indigo- violet
Violet is the highest energy
Red is the lowest energy
Wave Properties • Amplitude-height of the wave from origin to crest • Wavelength (λ)- the distance from crest to crest
Red light • Big wavelength • Low frequency
Blue light • Small wavelength • High frequency
Frequency (v) - number of waves to pass a point
(cycles/sec – hertz)
Wavelength and frequency are inversely proportional, if one goes up the other goes down.
• c = speed of light in vacuum = 3.00 x108 m/s • λ = wavelength in meter (m) • v = frequency in or hertz (Hz) or sec-1 They are inversely proportional, as one goes up the
other goes down.
c = λ v
The relationship of wavelength & frequency
Calculate the wavelength of the yellow light emitted by a sodium lamp if the frequency of radiation is 5.10 x1014 Hz
c = λ v c = 3x108 m/s v = 5.10 x1014 Hz 3x108 m/s = λ 5.10 x1014 Hz λ = 5.10 x1014 Hz 3x108 m/s λ = 5.88 x 10-7 m units--hertz (Hz) or sec-1
Example
LET’S DO A FEW PROBLEMS!
How does an electron act like a particle????
The Photoelectric Effect – EVIDENCE!
The light heats the metal the metal ejects e- Really? The metal only ejected e- if a high frequency of light was used, but the wave theory of light predicted that any frequency should cause e- to be ejected.
LIGHT
The Photoelectric Effect
•Plank solved the problem by explaining that the hot material behaves like particles not waves • if it was a wave it would emit the e- continuous, but it doesn’t, it emits energy in small packs called quanta. •A quantum of energy is the minimum quantity of energy that can be gained or lost by an atom.
LIGHT
• Changes color
Why? • Energy excites the electrons, when they release
energy they can emit light • Energy is absorbed or emitted in quanta, finite
amounts of energy!
What happens to a metal when it is heated?
• Planck explained that relationship with E = hv
• E= energy • h= Planck’s constant = 6.6262 x 10-34 J•s • v= frequency in 1/s or hertz (Hz)
What happens to a metal when it is heated?
• Calculate the energy of a quantum of radiant energy (the energy of a photon) with a frequency of 5.00 x 1015 Hz
E = hv • h= 6.6262 x 10-34 Js • v= 5.00 x 1015 Hz
• E= (6.6262 x 10-34 Js) (5.00 x 1015 Hz) • E = 3.31 x 10-18 J (units- hertz (Hz) or 1/s)
Example
Work on next part of worksheet!
Return and finish PowerPoint before going to the lab!!
c = λ v
• When describing a wave
• When you are given the wavelength
E = hv
• When you are describing the energy absorbed or released
When to use the equations
Conversion • When solving problems you must be sure the
units are appropriate!
• This means that you may have to convert your units!
• Standard Conversions for light & energy
– 100cm = 1m – 1km = 1000m – 1nm = 10-9m
Answering the Dilemma of the Atom
• Treat electrons as waves • As the electron moves toward the
nucleus, the wavelength shortens • Shorter wavelength = higher energy • Higher energy = greater distance
from the nucleus
This produces bands of light with definite wavelengths.
Electron transitions involve jumps of definite amounts of energy.
…produces a “bright line” spectrum
Spectroscopic analysis of the hydrogen spectrum…
Flame Tests
strontium sodium lithium potassium copper
Many elements give off characteristic light which can be used to help identify them.