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 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 The electron propagates through space as an energy wave. To understand the atom, one must understand the behavior of electromagnetic waves. Louis debroglie https://youtu.be/mfpkw u5vugg
Electromagnetic Radiation energy (e-) traveling through space Small wavelengths= more dangerous -the waves have more energy These waves travel at the speed of light (c) =3.0 x 10 8 m/s Big wavelengths= less dangerous- the waves have less energy
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
Frequency (v) - number of waves to pass a point Red light Big wavelength Low frequency (cycles/sec hertz) Blue light Small wavelength High frequency Wavelength and frequency are inversely proportional, if one goes up the other goes down.
The relationship of wavelength & frequency c = λ v c = speed of light in vacuum = 3.00 x10 8 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.
Example Calculate the wavelength of the yellow light emitted by a sodium lamp if the frequency of radiation is 5.10 x10 14 Hz c = λ v c = 3x10 8 m/s v = 5.10 x10 14 Hz 3x10 8 m/s = λ 5.10 x10 14 Hz λ = 5.10 x10 14 Hz 3x10 8 m/s λ = 5.88 x 10-7 m units--hertz (Hz) or sec -1
LET S DO A FEW PROBLEMS!
How does an electron act like a particle????
The Photoelectric Effect EVIDENCE! LIGHT 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.
What happens to a metal when it is heated? 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)
Example Calculate the energy of a quantum of radiant energy (the energy of a photon) with a frequency of 5.00 x 10 15 Hz E = hv h= 6.6262 x 10-34 Js v= 5.00 x 10 15 Hz E= (6.6262 x 10-34 Js) (5.00 x 10 15 Hz) E = 3.31 x 10-18 J (units- hertz (Hz) or 1/s)
Work on next part of worksheet! Return and finish PowerPoint before going to the lab!!
When to use the equations c = λ v E = hv When describing a wave When you are given the wavelength When you are describing the energy absorbed or released
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 = 10-9 m
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
Electron transitions involve jumps of definite amounts of energy. This produces bands of light with definite wavelengths.
Spectroscopic analysis of the hydrogen spectrum produces a bright line spectrum
Flame Tests Many elements give off characteristic light which can be used to help identify them. strontium sodium lithium potassium copper