1. The Development of a New Atomic Model

2. Thomson – Plum pudding model
Rutherford – planetary model

3. Problems with Rutherford’s model:
Since the electrons are negatively charged and the nucleus is positively charged, why aren’t the electrons drawn into the nucleus?
How exactly are the electrons distributed in the electron cloud?

4. A new atomic model came about as a result of investigations into the absorption and emission of light by matter.
Before we can understand what is going on with electrons, we need to look at some information about light.

5. Light Before 1900, scientists thought light behaved as a wave
Visible light is a kind of electromagnetic radiation
A form of energy that travels through space in waves

6. The electromagnetic spectrum includes ALL forms of electromagnetic radiation
Visible light is only a small part of the electromagnetic spectrum
Other parts include x-rays, UV light, infrared light, microwaves, and radio waves.

8. ALL forms of electromagnetic radiation move at a speed of 3
ALL forms of electromagnetic radiation move at a speed of 3.00 x 108 m/s through a vacuum.
Because air is mostly empty space, that is also light’s speed through air.
The symbol for the speed of light is c

9. Wave Measurements Wavelength Symbol is λ
Distance between corresponding points on adjacent waves
Unit is a distance unit (meters, centimeters, nanometers, etc.

10. Frequency
Symbol is ν
Number of waves that pass a given point in a certain amount of time (a second)
Units are waves/second which is called a Hertz (Hz)

11. For electromagnetic radiation, c = λν
c is the speed of light in m/s
λ is the wavelength in m
ν is the frequency in Hz (or s-1)
Because c is a constant, wavelength and frequency are inversely related.

12. Photoelectric Effect
In the early 20th century, experiments involving light and matter had results that could not be explained by the wave theory of light.
One of these experiments involved the photoelectric effect
The emission of electrons from a metal when light shines on the metal.

13. Light of different frequencies was allowed to strike different metals
Wave theory meant that no matter how low the frequency of the light, eventually enough waves would hit the metal that enough energy would be generated for electrons to be emitted.

14. BUT, scientists observed that if the light was not of a high enough frequency, no electrons were emitted. This threshold frequency was different for different metals.

15. German physicist Max Planck proposed that light does not just travel in waves, but that light exists in small packets of energy called quanta.
The relationship between the amount of energy in a quanta was related to the frequency of the light:
E = hν
h is Planck’s constant; x J∙s
E is energy in J

16. Einstein expanded on Planck’s theory
Electromagnetic radiation has a dual wave-particle nature.

17. Light is a stream of particles that travels through space in waves
Each particle of light carries a quantum of energy
Einstein called the particles photons
NO mass
The amount of energy in a photon depends on the frequency of the light: E = hν

18. Explanation of photoelectric effect:
Electromagnetic radiation is absorbed by matter in whole numbers of photons.
In order for an electron to be emitted from the surface of a metal, the electron has to be struck by a photon that has enough energy to knock it loose.

19. Ground state – lowest energy state
When an electric current is passed through a gas at low pressure, the potential energy of the atoms increases.
Ground state – lowest energy state
Like you asleep in bed
Excited state – atoms have higher potential energy
Like you in chemistry class 

20. When an excited atom returns to ground state, it gives off the extra energy in the form of electromagnetic radiation.
If the electromagnetic radiation is in the visible light spectrum, then the amount of energy given off is related to the color of the light
E = hν
c = λν
So λ = hc/E and each wavelength is a characteristic color

21. Scientists passed electric current through a tube containing hydrogen gas at low pressure
Observed a pinkish glow
They shined the pink light through a prism, and it separated into 4 specific colors of the visible spectrum
This is hydrogen’s line emission spectrum

22. There were also series of lines produced in the UV and infrared regions.
Each of the series of lines were named after the scientists that discovered them
Lyman series: ultraviolet
Balmer series: visible light
Paschen series: infrared

23. According to the Rutherford model of electrons orbiting around a compact nucleus, the excited electrons should have been anywhere in the electron cloud
If so, instead of just seeing a series of lines, scientists would have seen the whole continuous spectrum of light
The explanation of the line emission spectrum is what led to quantum theory

24. When an excited electron falls back to its ground state, it gives off a photon of radiation
The energy of that photon is equal to the difference in energy between the excited and ground state.
Remember that a certain amount of energy is associated with a specific color of visible light

25. Since scientists only observed 4 colored lines, the electrons could have only jumped to very specific energy levels when excited.

26. Our brains combine the different colors of the line emission spectrum and we just see the pinkish glow.
Each of the colors of the line emission spectrum are the result of electron transitions from higher energy levels to lower energy levels.

27. Bohr’s model shows that electrons can circle the nucleus in certain orbits
Each orbit is associated in with a specific amount of energy
When the hydrogen atom is in its ground state, its one electron is located in the lowest energy orbit, which is the one closest to the nucleus.

28. When in one orbit, an electron has a certain amount of energy
Electrons can only exist within the specific orbits; they can’t be in-between
When in one orbit, an electron has a certain amount of energy
When an electron gains energy, it jumps to a higher-energy orbit
Energy absorption
When an electron loses energy, it falls to a lower-energy orbit
Energy emission