Light,
Electromagnetic
Spectrum, &
Atomic Spectra
Movement of Electrons
e
e
e*

An electron absorbs energy (in photons or
quanta) to jump to a higher energy level
known as the EXCITED STATE

An electron emits energy (in photons or
quanta) to fall back to original energy level
known as the GROUND STATE

Light is produced when electron EMITS
energy and falls back to its ground state.
Energy Absorbed = Energy Released = Energy of Light Produced
Then what was wrong with
Bohr’s Model?

Bohr’s Model DID help explain how
electrons moved around the nucleus.

Bohr’s Model DID NOT account for the
behavior of larger atoms with more than one
electron.

Bohr ONLY looked at the HYDROGEN
atom.
The Bohr Hydrogen Model
Reference Table
How to Read the Diagram

Shows the movement of
electrons around the nucleus
(solid black dot in center).

Electrons are represented by
little black dots on the rings.

Arrows from the electrons show
the electrons are JUMPING
DOWN (losing energy).

Light is only given off when
electrons move DOWN in
energy levels.
The Bohr Hydrogen Model
Reference Table
How to Read the Diagram

Each electron transition
corresponds to a specific
wavelength.

These electrons only show
wavelengths in the IR, Visible, and
UV spectrum.

Wavelengths from the Visible
spectrum correspond to a specific
color.

(Make sure to change UNITS!)
Example Problem:

When an electron falls from n=6
to n=2, what wavelength of light
is emitted?
 Wavelength = 410 nm

What is the region of the
electromagnetic spectrum that
corresponds to this wavelength?
 Visible Light
Example Problem:

When an electron falls from n=6
to n=2, what color of light will be
emitted?

Already know:

wavelength is 410 nm
 CHANGE UNITS!

410 nm = 4.1 x 10-7 m

So what area does this
number lie in?

Between 4.2 x 10-7 m &
4.0 x 10-7 m

Therefore, the color given
off is VIOLET
Emission Spectra

Emission Spectrum – the spectrum of light released from excited atoms of an
element.

No 2 elements have the same emission spectrum

The farther the electron falls, the greater the amount of energy released.

Ground State – the lowest energy level that an electron can occupy.
EMISSION SPECTRA (CONT’D)
The frequency (color) of light emitted corresponds
to the amount of energy released.
(Argon)
Atomic emission spectra provides the
evidence that there are energy levels.
Electromagnetic Spectrum

Electromagnetic radiation - energy waves that have both electrical and
magnetic properties.
Can travel through empty space at c = 3 x 108 m/s.

c = λν
E = hν
E = energy = J
h = Planck’s constant = 6.626 x 10-34 J/sec
c = speed of light = 3x108 m/s
λ = wavelength = m
v = frequency = Hz / s-1 <-same thing
What is the wavelength of an EM wave that
has a frequency of 7.8 x 106 Hz? What type of
wave is it?
c = λν
c = 3x108 m/s
v = 7.8x106 Hz or 7.8x106 s-1
λ=?
3x108 m/s = (7.8x106 s-1)(λ)
λ = 38.46 m = radiowaves
(1)What is the frequency of a wave that has a
wavelength of 4x10-7 m? (2)What color is this
wave? (3)How much energy is produced?
(1) c = λν
(2) violet
c = 3x108 m/s
v=?
λ = 4x10-7 m
(3) E=hv
E= ?
h= 6.6 x 10-34 J/sec
v= 7.5x1014 s-1
3x108 m/s = (v)(4x10-7 m)
v= 7.5x1014 s-1
E= (6.6 x 10-34 J/sec)(7.5x1014 s-1)
E= 4.95x10-19 J
Electromagnetic Spectrum
 Low Energy, Low Frequency, Long Wavelength
1. Radio Waves (AM, FM, TV)
2. Microwaves (cell phones, microwave ovens)
3. Infrared (given off as heat)
4. Visible Light (ROYGBIV: Red is the lowest energy, violet has the highest)
5. Ultraviolet (causes sunburns)
6. X-rays (can pass through body, except bones)
7. Gamma Rays (can pass through 3 meters of concrete)
 High Energy, High Frequency, Short Wavelength
LIGHT – QUICK REVIEW

Electrons absorb energy as they jump from a
lower energy level to a higher energy level

Electrons release energy in the form of light
when they lose energy and fall back down to a
lower energy level

The lowest possible energy level for an
electron is its ground state.

Electrons at higher energy levels than ground
state are said to be excited.

The light emitted by an electron moving from a
higher to a lower energy level has a frequency
directly proportional to the energy change of
the electrons.
LIGHT

The Quantum Mechanical Model was
developed out of the study of light.

Light travels in waves.

What are the parts of a wave?
Wavelength
Amplitude
Crest
Trough
Wavelength Verses Frequency

Wavelength (λ) – distance between the crests of two
different waves.

Frequency (ν) – the number of wave cycles to pass a
given point per unit of time.


Measured in cycles per second (Hertz)
Wavelength & Frequency are INVERSELY
PROPORTIONAL (i.e. when one goes up, the other
goes down)
Wavelength Verses
Frequency

High frequency, small wavelength

Low frequency, large wavelength
Reading Electromagnetic
Spectrum
 This spectrum only shows WAVELENGTH.
Remember:
Long Wavelength = Low Frequency
Short Wavelength = High Frequency
In terms of the Visible Spectrum:
VIOLET = High Frequency
RED = Low Frequency