Electrons in Atoms Bohr Model Niels Bohr in

Electrons in Atoms

Bohr Model • 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.

Light Energy • 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 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 Structure Trough – low points Crest – high points Amplitude – max displacement from equilibrium (middle of wave). Wavelength – the shortest distance between points in which a wave repeats itself. (crest to crest or trough to trough) symbol - λ crest

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Parts of a Wave length • Wavelength crest to a crest or trough to a trough lambda • Symbol = λ ( ) • Measured in meters of one wave, from a • Frequency- number of waves that pass a point in 1 second • symbol = f or v ( nu ) • Measured in Hertz (Hz) or 1/s, s-1

Calculations • All EM radiation travels at the speed of light, c = 3. 00 x 108 m/s in the vacuum of space Formula: c = λ v or Formula: c = λ f c is constant speed of light ν or f is frequency λ is wavelength

Ex. 1) Light in the middle of the ultraviolet region of the electromagnetic radiation spectrum has a frequency of 2. 73 x 1016 s-1. Find the UV light’s wavelength.

Ex. 2) Yellow light has a wavelength of 5. 70 x 10 -7 m. Calculate the frequency of the yellow light.

Light Particles • 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 • E is energy, measured in Joules (J) • h is Planck’s Constant = 6. 626 x 10 -34 Js • v or f is frequency (Hz or 1/s or s-1) same frequency as earlier • Once the minimum amount of energy is calculated, the actual energy can be a multiple of the quantum amount. (1 x E, or 2 x E, or 3 x E. . )

Ex. 3 From examples 1 and 2, we have the frequency of UV light as 2. 73 x 1016 s-1 and the frequency of yellow light as 5. 26 x 1014 Hz. 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. 73 x 1016 s-1 and the frequency of yellow light as 5. 26 x 1014 Hz. Calculate the energy in joules, of an individual photon of each. UV light (6. 626 x 10 -34 Js)(2. 73 x 1016 s-1) =1. 81 x 10 -17 J yellow light (6. 626 x 10 -34 Js)(5. 26 x 1014 s-1) =3. 49 x 10 -19 J 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. 73 x 1016 s-1 and the frequency of yellow light as 5. 26 x 1014 Hz. Calculate the energy in joules, of an individual photon of each. UV light (6. 626 x 10 -34 Js)(2. 73 x 1016 s-1) =1. 81 x 10 -17 J yellow light (6. 626 x 10 -34 Js)(5. 26 x 1014 s-1) =3. 49 x 10 -19 J Comparing the two, UV light has more energy than yellow light

Photoelectric Effect • 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

In Summary c = 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?