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Waves & Particles Ch. 4 - Electrons in Atoms

Properties of Light Different types of electromagnetic radiation (x-rays, radio waves, microwaves, etc…) SEEM to be very different from one another. Yet they share certain fundamental characteristics. 3.0 x 108 m/sec is the speed of light!

A. Wavelength Wavelength – distance between identical points on successive waves Usually measured in nanometers

B. Frequency Frequency – the number of complete wave cycles that pass a given point in one second: the unit is cycles/second but is written as sec-1, or Hertz. One wave in one second equals one Hertz!

Amplitude Amplitude is the height of a wave. It is also measured in length units like nanometers, angstroms, etc.

 A A  A. Waves greater amplitude crest (intensity) origin trough
greater frequency (color)

c = f EM Spectrum c: speed of light (3.00  108 m/s)
Frequency & wavelength are inversely proportional c = f c: speed of light (3.00  108 m/s) : wavelength (m, nm, etc.) f: frequency (Hz)

EM Spectrum c = f Longer Wavelength means Lower Frequency
Shorter Wavelength means Higher Frequency Higher Frequency means Higher Energy Lower Frequency means Lower Energy

Range The range of visible light is from 400 to 700 nanometers. Low energy is colored red while high energy is colored violet. Violet:   nm Indigo:   nm Blue:   nm Green:   nm Yellow:   nm Orange:   nm Red:   nm

Ephoton = hf E : engery of a photon (J)
h: Plank’s constant (6.63 x J . sec) f: frequency (Hz or sec-1)

EM Spectrum HIGH ENERGY LOW ENERGY

EM Spectrum HIGH ENERGY LOW ENERGY R O Y G. B I V red orange yellow
green blue indigo violet

Examples 2. What is the wavelength of radiation whose frequency is 6.24 x l013 sec-1? Given: Work: f = 6.24 x l013 Hz c = 3.00  108 m/s λ = ? λ = c/f λ = 3.00  108 6.24 x l013

Answer 4.81 x 10-6 m

Examples 3. What is the frequency of radiation whose wavelength is 2.20 x l0-6 nm? (1 m = 1,000,000,000 nm) Given: Work: c = 3.00  108 m/s λ = 2.20 x 10-6 nm (you have to change nm to m) f = ? f = c/ λ f = 3.00  108 2.20 x 10-15

Answer 1.36 x 1023 Hz

Hydrogen Atom Line Emission Spectra
When investigators passed electric current through a vacuum tube containing hydrogen gas at low pressure, they observed the emission of a characteristic pinkish glow. When a narrow beam of the emitted light was shined through a prism, it was separated into a series of specific frequencies (and therefore specific wavelengths, c =) of visible light. The bands of light were part of what is known as hydrogen’s LINE-EMISSION SPECTRUM. (page 95)

The lowest energy state of an atom is its ground state.
A state in which an atom has a higher amount of energy is an excited state. When an excited atom returns to its ground state, it gives off energy.

B. Bohr Model e- exist only in orbits with specific amounts of energy called energy levels Therefore… e- can only gain or lose certain amounts of energy only certain photons are produced

B. Bohr Model 6 Energy of photon depends on the difference in energy levels Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom 5 4 3 2 1

C. Other Elements Helium
Each element has a unique bright-line emission spectrum. “Atomic Fingerprint” Helium Bohr’s calculations only worked for hydrogen! 

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