Chapter 5: Electrons in Atoms

Section 3:Physics and the Quantum Mechanical Model

Draw and identify the features of a wave; Describe the relationship among the

frequency, wavelength, and energy of light; Identify the general ranges of

electromagnetic radiation and their relative wavelengths and frequencies;

Explain the source of atomic emission spectra;

Explain how the frequencies of emitted light are related to changes in electron energy

Students will be able to…

Light made of electromagnetic waves◦ Considered to be quanta of energy called photons◦ Has properties of particles and waves

I’ve seen the light!

Wavelength (λ) :distance between crests

Amplitude

Frequency (ν): number of wave cycles to pass a given point per unit of time◦ SI unit is Hertz (Hz), or s-1

Two are related by the formula: Since c is constant, λ and ν are inversely

proportional If one is big, the other is small

Light and the Universal Speed Limit

speed of light 3 x 108m/s

If you know either λ or ν, can solve for other Example: What is the wavelength of a

wave with frequency equal to 5.10 x 1014 Hz?

A little light math…

c

cRearrange

The Electromagnetic Spectrum

Energy of light is related to frequency◦ Higher the frequency, more energy

Visible light is small slice in the middle of spectrum◦ Red is lowest ν(longest λ) that the human eye “sees”

The Electromagnetic Spectrum

Ground state: electrons in lowest possible energy level

Excited state: atoms absorb energy and electrons move to higher energy levels ◦ Once energy source is removed, electrons

drop back down to ground state ◦ Emit quanta of energy in the form of… light

(sometimes visible)

So what does this have to do with electrons?

Atomic emission spectrum: pattern of discreet lines given off by electrons dropping to ground state◦ Energy levels are quantized◦ Each line represents a frequency (energy) that is

directly proportional to the energy lost by the electron

◦ No two elements have exactly the same spectrum

Reading between the lines

Each drop an electron makes relates to a specific frequency of energy

Not all is in visible range, but always at discreet frequencies (not continuous)

Reading between the lines

Draw and identify the features of a wave? Describe the relationship among the

frequency, wavelength, and energy of light? Identify the general ranges of

electromagnetic radiation and their relative wavelengths and frequencies?

Explain the source of atomic emission spectra?

Explain how the frequencies of emitted light are related to changes in electron energy?

Objective check: Can you...

Page 140 #14, 15 Page 146 #16, 17, 21

Homework