Chapter 4, Section 1 Notes

The Development of a New Atomic Model

• A wave is a disturbance moving through a medium

• The medium is whatever the wave moves through

• Can either be longitudinal or transverse

• Longitudinal – the disturbance is parallel to the motion

• Sound waves

• Transverse – the disturbance is perpendicular to the motion

• Electromagnetic radiation – a form of energy the exhibits wavelike behavior as it travels through space

Waves

• The forms of electromagnetic spectrum:

• Radio waves

• Microwaves

• Infrared

• Visible light

• Ultraviolet

• X-rays

• Gamma Rays

Electromagnetic Spectrum

Longitudinal Wave Anatomy

Transverse Wave Anatomy

• Wavelength (λ) = distance between corresponding points on adjacent waves

• Frequency (ν) = the number of waves that pass a given point in a specific time.

• It’s measured in Hertz (Hz), which is one wave/second.

• The speed for electromagnetic waves is c=3.00x108 m/s.

• For electromagnetic waves, c= λν

Wave Variables and Equation

• What is the wavelength of EMR with a frequency of 4.23x1010 Hz?

• λ =c

ν=

3.00×108m

s

4.23×1010Hz= 0.00709 m

• What is the frequency of EMR with a wavelength of 134 m?

• λ =c

ν=

3.00×108m

s

134m= 2.24 x 106 Hz

Examples

• Interference – waves that overlap and combine into larger or smaller waves

• Constructive interference –waves adding into larger waves

• Destructive interference –waves combining into smaller waves

Wave Interactions

• A vibration of a system in which some particular points remain fixed while others between them vibrate with the maximum amplitude.

• Nodes are the places with zero displacement along the wave, and antinodes are the places with maximum displacement.

Standing Waves

• Vibrations at the natural frequency of an object cause the object to vibrate along with the external vibrations.

Resonance

• When a wave passes through a small opening in a wall, it spreads out in a circular pattern on the other side of the wall.

• Waves also diffract when they pass by an edge.

Diffraction

• A wave passing from one medium into another medium will refract, or bend, at the interface.

Refraction

• A wave that returns from a surface is reflected.

Reflection

• Experiments in the early 1900s showed things that the wave theory of light could not explain, such as the photoelectric effect.

• When light hits a piece of metal, an electron is emitted from the metal.

• The light has a minimum frequency for the electron to be released, no matter the intensity of the light.

Photoelectric Effect

• Light waves should not need a certain frequency to make the electrons come out, meaning light is behaving more like a particle than a wave.

• The particles of light are called photons.

• The particle has to carry a quantum of energy with it. A quantum is the minimum quantity of energy that can be lost or gained by an atom.

• Energy is the ability to do work and is measured in joules (J).

• The energy and frequency of a photon are related by Planck’s equation: E = hν.

• Planck’s constant: h = 6.626x10-34 J∙s

Photons

•What is the energy of a photon with a frequency of 4.24x1012 Hz?

•E= hν = 6.626x10-34 J∙s × 4.24x1012 Hz

=2.81x10-21 J

Example

Wave-Particle Duality of Light

• The Rutherford model had a few parts missing:• How were the electrons

arranged around the nucleus?

• Why didn’t the electrons merge into the nucleus?

• Studies into light in the early 20th century led to a new model of the atom.

Shortcomings in Previous Atomic Model

• Electrons exist only in very specific energy states for every atom of each element.

• The ground state is the lowest energy state of an electron. This is where it will normally be found.

• The excited state is when it has more potential energy. This extra energy comes from an external source.

• The excited electrons can release the extra energy as visible light.

Electrons and Energy

• Each element has a unique combination of quanta for its electrons, meaning each one has a unique combination of photons it can release.

• This means elements with excited atoms will glow with different colors. You will get to see this in tomorrow’s lab!

Emission-line Spectrum

• The electron can only orbit in levels that correspond to the quanta of energy found on the emission-line spectra.

• The electron can jump to a higher level when it gains energy, and drop back down when the photon is released.

• Works very well for hydrogen, but not for atoms with multiple electrons.

Bohr Model