Electrons in Atoms

•Niels Bohr in 1913 proposed a quantum model for the hydrogen atom which correctly predicted the frequencies of the lines (colors) in hydrogen’s atomic emissions spectrum.

Bohr Model

•His model gave atoms only certain allowable energy states.

•The lowest state is called the ground state.

•When an atom gains energy it is said to be in an excited state.

Bohr Model

• When elements are given energy from a variety of sources (heat, light, electricity) the electrons absorb that energy, & jump up to a higher energy level, the excited state. The electrons can return to ground state by giving off the energy as a color of light, called photons of light.

•Each element gives off a unique color called its atomic emission spectrum. The electrons are what give each element their unique color.

Light Energy

•Light gives us a clue to how electrons are arranged. Light behaves like a wave & like a particle.

•wave particle duality of nature – everything in the universe has properties of both waves and particles (ex. Duality of good vs. evil in man)

Light Energy

Light in wave form is called an electromagnetic wave and is part of the electromagnetic spectrum which includes x-rays, TV waves, microwaves, radio waves, UV waves & gamma rays.

Wave StructureTrough – low points Crest – high points

Wavelength – the shortest distance between points in which a wave repeats itself. (crest to crest or trough to trough) symbol - λ

Amplitude – max displacement from equilibrium (middle of wave).

crest

Rope demo

Parts of a Wave•Wavelength - of one wave, from a crest to a crest or trough to a trough•Symbol = λ ( ) •Measured in

•Frequency- number of waves that pass a point in 1 second•symbol = f or v ( )

•Measured in (Hz) or 1/s, s-1

length

lambdameters

nu

Hertz

Formula: c = λ v

or Formula: c = λ f

c is constant speed of light

ν or f is frequency

λ is wavelength

•All EM radiation travels at the speed of light, c = 3.00 x 108m/s in the vacuum of space

Calculations

Ex. 1) Light in the middle of the ultraviolet region of the electromagnetic radiation spectrum has a

frequency of 2.73x1016s-1. Find the UV light’s wavelength.

Ex. 2) Yellow light has a wavelength of 5.70x10-

7m. Calculate the frequency of the yellow light.

•Light as a particle is called a photon.

•Light is like a particle because there are only certain amounts of energy it can have.•The minimum amount of energy a photon can have is called a quantum.

Light Particles

•Max Planck found that this energy could be calculated with the formula E = h ν or E = h f

•Once the minimum amount of energy is calculated, the actual energy can be a multiple of the quantum amount. (1x E, or 2 x E, or 3 x E....)

•v or f is frequency (Hz or 1/s or s-1) same frequency as earlier

•h is Planck’s Constant = 6.626 x 10-

34 Js

•E is energy, measured in Joules (J)

Light Particles

Ex. 3 From examples 1 and 2, we have the frequency of UV light as 2.73x1016s-1 and the

frequency of yellow light as 5.26x1014Hz. Calculate the energy in joules, of an individual

photon of each.

Ex. 3 From examples 1 and 2, we have the frequency of UV light as 2.73x1016s-1 and the

frequency of yellow light as 5.26x1014Hz. Calculate the energy in joules, of an individual

photon of each.

UV light (6.626x10-34Js)(2.73x1016s-1) =1.81x10-17J

yellow light (6.626x10-34Js)(5.26x1014s-1) =3.49x10-19J

Which has more energy? UV light or yellow light?

Ex. 3 From examples 1 and 2, we have the frequency of UV light as 2.73x1016s-1 and the

frequency of yellow light as 5.26x1014Hz. Calculate the energy in joules, of an individual

photon of each.

UV light (6.626x10-34Js)(2.73x1016s-1) =1.81x10-17J

yellow light (6.626x10-34Js)(5.26x1014s-1) =3.49x10-19J

Comparing the two, UV light has more energy than yellow light

•Electrons (called photoelectrons) are emitted from a metal’s surface when light of a certain frequency shines on the surface.

•Example: calculators.•Photoelectric cells convert light into electric energy

Photoelectric Effect

In Summaryc = λ f or c = λ v

c = speed of light = 3.00 x 108 m/s

f (or v) = frequency = Hz, 1/s, s-1

λ = wavelength = m

E = h f (or E = h ν )

E = Energy = J

h = Planck’s Constant = 6.626 x 10-34 Js

Water drops in the air disperse the white light of the sun into a rainbow. What is the energy of a photon from the violet portion of the rainbow if it has a frequency of 7.23 x 1014 Hz?