Electromagnetic Radiationand Light

I. Models of the Atom

• Many different models: – Dalton-billiard ball model (1803)– Thompson – plum-pudding model (1897)– Rutherford – Nuclear model of the atom (1911)– Bohr – uses quantized energy of the atom (1913)– Quantum Mechanical Model of the Atom (1926)

• Each new model contributed to the model we use today.

• Even our current model, does not give us an exact model of how electrons behave.

A. The Bohr Model• Bohr used the simplest element, hydrogen, for his model• Proposed electron is found in specific circular paths, or orbits around

the nucleus• Each electron orbit was thought to have a fixed energy level.• Lowest level-ground state• Any Higher Level- excited state

The Bohr Model cont.• One electron is capable of many different

excited states (e- jumping to higher level)

• Quantum: specific amount of energy an e- can gain or lose when moving energy levels

• You can excite an e- with energy like electricity, the sun, or magnets

Electron dropping from higher level to lower-releases energy

energy

B. Problems with the Bohr Model• OOPS!- Model only works

with hydrogen• Did not account for the

chemical behavior of atoms• WRONG: Electrons do not

move around the nucleus in circular orbits

• Still very helpful!!

II. How do Neon Signs work?

• They have “excited” gases in them.

Explanation

Step 1

Step 2Step 1: an electron absorbs

energy and moves to a higher energy level

Step 2: e- drops back down to a lower energy level

•During drop it gives off energy called a “photon”

•Sometimes this energy is visible light (ROYGBIV)

• When a photon is emitted, energy is released. We can calculate the energy released using the equation: E = h ν

Application: Atomic Emission Spectrum• Used to determine which elements are present in a

sample• Used to determine which elements are present in a

star (because stars are gases) • Each element has a unique spectrum• Only certain colors are emitted because the energy

released relates to specific frequency

Spectroscope

• A spectroscope is needed to see the atomic emission spectra, which acts similar to a prism, separating different frequencies of light

Electromagnetic Spectrum

• Electromagnetic spectrum is the range of all energies emitted from photons acting like waves.

Electromagnetic Spectrum with Visible Light Spectrum

Lab: Atomic

Emission Spectra

of Several Gases

Light

• Behaves like a particle• Behaves like a wave

Characteristics of a Wave

Am

plitu

de

(Wavelength)

(Wavelength)

• Wavelength (lambda) – shortest distance between equivalent points on a continuous wave [Unit = meters]

• Frequency (nu) – the number of waves that pass a given point per second [Unit = 1/second = s-1 = Hertz (Hz)]

• Crest – Highest point of a wave• Trough – Lowest point of a wave• Amplitude (a)– height from its origin to its crest (highest point) or trough

(lowest point) [Unit = meters]

Wavelength and Frequency• Wavelength () and frequency () are related• As wavelength goes up, frequency goes down• As wavelength goes down, frequency goes up• This relationship is inversely proportional

Wavelength and Frequency cont.

c = Speed of light (c) = 3 x 108 m/s

c

wavelength frequency

Speed of lightc =

= c / = c /

Question Time

• Calculate the wavelength () of yellow light if its frequency () is 5.10 x 1014 Hz.

c

Question Time

• What is the frequency () of radiation with a wavelength () of 5.00 x 10-8 m? What region of the electromagnetic spectrum is this radiation?

c

How Much Energy Does a Wave Have?

• Energy of a wave can be calculated • Use the formula E= h• E= Energy• = frequency• h = Planck’s constant = 6.626 x 10-34 Joule . Sec• Joule is a unit for energy (J)• Energy and frequency are directly proportional,

as frequency increases, energy increases

E

h Planck’s constant frequency

Energy

Question Time• Remember that energy of a photon given off by

an electron is E =h• How much energy does a wave have with a

frequency of 2.0 x 108 Hz? ( h = 6.626 x 10-34 J.s)

E = 1.3 x 10-34 Joule

Visible Light, Frequency, and Energy• Red: longest wavelength (),

smallest frequency ()

• Red: frequency smallest (), least amount of energy (E)

• Violet: smallest wavelength (), largest frequency ()

• Violet: frequency largest (),greatest amount of energy (E)

Flame Test• The flame test is a way to determine the element

present in a sample• When placed in a flame, each element gives off a

different color

• Operates same as neon signs; electrons excited by heat and fall back down and give off different colors.