E = hfE – energy of a quantum (Joules) h – Plank’s constant (6.626 x 10-34 J s)f – frequency of absorbed or emitted EMR

Wave-Particle Duality:The Beginnings of Quantum Mechanics

• Describe the photoelectric effect.

• Understand the basics of wave-particle duality.

• Understand the difference between quantum, photon and electron.

• Be able to explain how the Bohr model fits with knowledge of line spectra.

PHOTOELECTRIC EFFECT

Shining light on a metal surface will immediately eject electrons.

Electrons given enough energy (ionization) can escape the attraction of the nucleus.

*Light is acting like a “particle” in this experiment – collision.

Einstein (1905) - electromagnetic radiation is a stream of tiny bundles of energy called photons.

Photons have no mass but carry a quantum of energy.

One photon can remove one electron.

Light is an electromagnetic wave, yet it contains particle-like photons of energy.

Only high frequency light (> 1.14 x 10 15 Hz) will eject electrons - acting as particle.

Can only explain it if you think of it using photons in a collision.

Only more intense light (higher amplitude) will eject more electrons - acting as wave.

Can only explain it if you think of it as changing the size of the wave.

Compton (1922) – first experiment to show particle and wave properties of EMR simultaneously.

Incoming x-rays lost energy and scattered in a way that can be explained with physics of collisions.

Quantum Mechanical Model of the Atom

Helium nuclei – alpha (α) particles - fired at thin gold foil reflected strongly.

Discovered the nucleus – electrons just fly around.

Rutherford (1911) - “Gold foil ” experiment.

Bohr (1922) – explained unique line spectrums by restricting electrons to fixed orbits with different quantized energy levels.

1. Electron absorbs radiation and jumps fromground state (its resting state) to a higher unstable energy level (excited state).

2. Electron soon loses energy and drops back down to a lower energy level – emitting the absorbed EMR.

• Levels are discrete like quanta – no in between.

• Each jump/drop is associated with a specific frequency photon - same transition, same photon.

The size of nucleus will affect electron position around the atom.

Cl:

17 e-

Na:11 p+

11 e- 17 p+

*Each element has a unique line spectrum as each element has a unique atomic configuration.

Absorption spectrum – portion of visible light absorbed by an element – heating up.

Emission spectrum – portion of visible light emitted by that element – cooling down.

CAN YOU ? / HAVE YOU?

• Describe the photoelectric effect.

• Understand the basics of wave-particle duality.

• Understand the difference between quantum, photon and electron.

• Be able to explain how the Bohr model fits with knowledge of line spectra.