1. A New Atomic Model
Section 4.1

2. Problems with Rutherford’s Model
Did not explain where the electrons were arranged
Did not explain why the negatively charged electrons did not get drawn into the positively charged nucleus

3. The Wave Description of Light
Electromagnetic radiation: a form of energy that exhibits wavelike behavior as it travels through space
Visible light is a form of electromagnetic radiation
Electromagnetic spectrum: all the forms of electromagnetic radiation

4. Electromagnetic Spectrum

5. More Facts about Light
Light travels at 3.00 x 108 m/s through a vacuum
Wavelength: (λ) is the distance between corresponding points on adjacent waves
Is a distance measurement (nm or m)
Frequency: (ν) is the number of waves that pass a given point in a specific time
Measured in waves/s which is a Hertz (Hz)

6. Wave (a) has a longer wavelength and a lower frequency than wave (b)

7. The Photoelectric Effect
Photoelectric effect: the emission of electrons from a metal when light shines on the metal
The frequency of the light striking the metal must be on a certain level or no electrons were emitted, even if the light was very bright

8. More about Light
The wave theory of light predicted that light of any frequency would supply enough energy to eject an electron
Max Planck, a German physicist, proposed that light must travel in small bundles of energy
Albert Einstein expanded on this theory by introducing the dual wave-particle theory

9. The Wave- Particle Description of Light
Quantum: (of energy) is the minimum quantity of energy that can be lost or gained by an atom
Photon: a particle of electromagnetic radiation having zero mass and carrying a quantum of energy
The energy of a particular photon depends on the frequency of the radiation

10. Einstein’s Explanation of the Photoelectric Effect
Electromagnetic radiation is absorbed by matter only in whole numbers of photons
There is a minimum amount of energy required to knock an electron loose
If the photon’s frequency is below the min, then the electrons stay put

11. More
Electrons in different metals require different minimum frequencies to exhibit the photoelectric effect

12. Ground State vs. Excited State
Ground state: the lowest energy state of an atom
Excited state: a state in which the atom has a higher potential energy than it has in the ground state
Excited atoms will return to the ground state
Only one ground state for an atom, many excited states

13. Emission-Line Spectrum
When an atom returns to its ground state, it emits light
Emission-line spectrum: a diagram or graph that indicates the degree to which a substance emits radiant energy with respect to wavelength

14. Hydrogen Atom Emission-Line Spectrum
Investigators passed an electric current through H2 at low pressure
There was a pinkish light emitted
When the light was passed through a prism, it was separated into 4 specific colors of visible light
This was the emission-line spectrum

15. Hydrogen Atom Emission-Line Spectrum

16. More about Hydrogen
There were other series of lines in the uV and IR regions of hydrogen’s emission-line spectrum
The series are called Lyman (UV), Balmer(visible), and Paschen (IR) after their discoverers

17. Hydrogen Atom Emission-Line Spectrum

18. Why did H2 only give off specific frequencies of light?
Continuous spectrum: the emission of a continuous range of frequencies, like a rainbow
Scientists had expected H2 to emit a continuous spectrum
The energy differences between the atoms’ energy states must be fixed

19. The Bohr Model Niels Bohr was a Danish physicist
He linked the atom’s electron to photon emission
According to his model, the electrons can circle the nucleus only in allowed paths, or orbits
The electron is in its lowest energy state when it is closest to the nucleus
Hydrogen only has one electron

20. Bohr’s Model of the Atom

21. More
The orbit is separated from the nucleus by an empty space where the electron cannot exist
When the atom is in the excited state, the electron is in a higher orbit farther from the nucleus
There are only certain orbits available, like the rungs of a ladder

22. When a hydrogen atom is in an excited state, the electron is in a higher orbit
When the atom returns to the ground state, it emits energy in the form of a photon
This is called emission
Energy must be added to the atom to move an electron to a higher level
This is called absorption

23. Visual of Absorption and Emission

24. More about Bohr’s Model
Bohr related the possible energy-level changes to the lines in the hydrogen emission-line spectrum
He calculated mathematically values for the lines
Unfortunately, Bohr’s model did not explain the emission-line spectrum of atoms with more than one electron

25. Emission-Line Spectra
The emission spectrum of each element is unique
Emission spectra are used to identify unknown samples
Also used to determine the relative composition of stars

26. Some Emission-Line Spectra