“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.