1. Unit 3: Electrons in the Atom

2. Recall the Evolution of the Atom

3. He had a question: Why don’t the electrons fall into the nucleus?
The Electrons move like planets around the sun
In circular orbits at different levels
Amounts of energy separate one level from another
“Planetary Model”
Niels Bohr’s Model

4. Bohr’s Planetary Model
Energy levels of an electron
Levels are like rungs on a ladder
An electron cannot exist between energy levels
A Quantum of energy is required to move an electron to the next highest level
Bohr’s Planetary Model

5. Bohr’s Absorbance and Emission of Quantum Energy

6. The study of light led to the development of the quantum mechanical model
Light is a “wave phenomenon” or a kind of electromagnetic radiation
Electromagnetic radiation includes many kinds of waves
All waves travel in a vacuum at 3.00X10^8 m/s=c
What is Light?

7. Parts of a wave Origin- the base line of the energy
Crest- high point on a wave
Trough- low point on a wave
Amplitude- distance from origin to crest
Wavelength- distance from crest to crest (λ)
Parts of a wave

8. Frequency Number of waves that pass a given point per second
Units: Cycles/sec or hertz (hz or sec^-1)
Abbreviated by greek letter nu =(ν)
c=νλ
Frequency

9. Frequency and Wavelength
Different wavelengths (frequencies of light) are different colors of light
Whole range of frequencies is called the Electromagnetic Spectrum
Frequency and Wavelength

11. Nanometers
1nm=10^-9m

12. According to Bohr Model
Electrons can change orbits, accompanied by the
Absorption (electrons on the way up) or
Emission (electrons on the way down) of a photon of a specific color of light
According to Bohr Model

13. Features of Light Why is the cup red? Red λ is reflected
All other wavelengths are absorbed
Features of Light

14. Features of Light When does a sample absorb a given color?
A sample absorbs a given color when
That color’s frequency is a match to
Cause the excitement of an electron
To a higher state
Features of Light

15. White Light through a Prism
White light is made up of all colors of the visible spectrum
Passing it through a prism separates it
Visible light represents a continuous spectrum
White Light through a Prism

16. If the Light is not White
By heating a gas at low pressure with electricity we can get it to EMIT LIGHT
Passing this light through a prism does something different.
Each element gives off its own characteristic colors
Can be used to identify an atom
If the Light is not White

17. Atomic Emission Spectrum
These are called discontinuous spectra, or line spectra
Unique to each element
Each line corresponds to a specific amount of energy being emitted
Atomic Emission Spectrum

18. Continuous vs. Emission Spectra
Continuous spectra are produced by solids, liquids, and dense gases under high pressure
Emission spectra are produced by heating a gas at low pressure
Continuous vs. Emission Spectra

19. Recapping the Wave: Frequency, Wavelength, and Energy of Light

20. Exciting the Electron of Hydrogen
Heat or electricity or light can move the electron up energy levels (“excited”)
Exciting the Electron of Hydrogen

21. Exciting Electron of Hydrogen
As the electron falls back to ground state, it gives the energy back as light
Exciting Electron of Hydrogen

22. Emission Spectrum of Hydrogen using the Bohr Model
Lyman series: UV Emissions down to n=1 (Longest Transition)
Balmer Series: Visible emissions down to n=2
Paschen Series: Infrared emissions down to n=3; (Shortest Transition)
Emission Spectrum of Hydrogen using the Bohr Model

23. Simplified Bohr Model Simplified model
Orbitals also have different energies inside energy levels
All the electrons can move around
Bohr model can’t show all the possible transitions for more complex elements
Simplified Bohr Model

24. Revisiting Atomic Emission Spectrum
Each line of color represents an emission of light from an excited state  Ground state e- transition
The more e- are in an atom, the more transitions are allowed, and the more complex the emission line spectrum will be
Revisiting Atomic Emission Spectrum

25. Is Light a wave or a particle?
Max Planck pointed out that energy is quantized
Light is energy
Light must be quantized
These smallest pieces of light are called photons
Is Light a wave or a particle?

26. The particle (Photon) Nature of light helps explain the previously mysterious Photoelectric Effect
In the Photoelectric
Effect, metals eject
electrons when light
Shines on them
Dual Nature of Light

27. Einstein discovers that light can be quantized via
𝑬=𝒉𝝂
In order for the photoelectric effect
To occur, the incident light has to reach a threshold frequency, which corresponds to a threshold photon of energy via 𝑬=𝒉𝝂
Dual Nature of Light

28. 𝑬=𝒉𝝂 Energy and Frequency E is the energy of the photon
ν is the frequency
h is Planck’s constant
h=6.626 x 10^-34 Joules/sec
Joule is the metric unit of energy
Energy and Frequency

29. Frequency, Wavelength, and Energy of Light Problems
𝑬=𝒉𝝂 and c=λν
What is the frequency of red light with a wavelength of 4.2 x 10^-5 m?
ν=𝑐÷λ
ν=(3.0𝑥10^8 m/s)/4.2x10^-5m
ν=7.14x10^12 sec^-1
What is the energy of a photon of this light?
ν=7.14𝑥 sec −1
𝐸=ℎν=6.626𝑥 10 −34𝐽𝑠𝑒𝑐 𝑥 7.14𝑥 𝐻𝑧
𝐸=4.7𝑥 10 −21 𝐽
Frequency, Wavelength, and Energy of Light Problems