1. Section 3: Atomic Emission Spectra & Quantum Mechanics
General Need to Know – units
Joule (J)
Base SI unit of energy
1000J = 1kJ

2. Section 3: Atomic Emission Spectra & Quantum Mechanics
calorie (cal)
Scientific calorie
1 cal = 4.18 J
1 cal = energy required to raise 1g 1oC
1000 cal (1 kcal) = 1 Cal (nutritional calorie)
1 cal
4.18J
1g 1oC

3. Section 3: Atomic Emission Spectra & Quantum Mechanics
Light and Atomic Emission Spectra
Before 1900, scientists thought visible light behaved solely as a wave.
Visible light is a kind of electromagnetic radiation.
A form of energy that exhibits wavelike behavior as it travels through space.
Also known as electromagnetic energy.

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According to the wave model, light consists of electromagnetic waves.
All forms of electromagnetic radiation interact with matter.
Ability to penetrate matter is a measure of the energy of the waves
Visible light spectrum
Decreasing in   ROYGBIV

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Other forms of electromagnetic radiation.
X rays
Ultraviolet light
Infrared light
Microwaves
Radio waves

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All the forms of electromagnetic radiation make up the electromagnetic spectrum.
All forms of electromagnetic radiation move at approximately the same constant speed.
Speed of light (c) through air
2.998 x 108m/sec = x 108ms-1
Use 3.0 x 108m/sec = 3.0 x 108ms-1

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Wavelength ()
The distance between 2 corresponding points on adjacent waves or crests.
Often measured in meters, centimeters, or nanometers.
1 nm = 10-9m

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Frequency ()
The number of waves (wave cycles) that pass a given point in a specific time.
Usually measured in seconds.
Hertz (Hz)
Unit of 
1 Hz = 1 wave/sec
A hertz can also be expressed as a reciprocal second (s-1)
v = c/
 is inversely proportional to 
As  increases,  decreases.
As a waves frequency increases so does its energy.

10. Section 3: Atomic Emission Spectra & Quantum Mechanics
When an electric current is passed through a gaseous element or through the vapor of a liquid or solid element, the electrons of the atom of the gas or vapor are energized.
This energy causes them to emit light.
When atoms absorb energy, their electrons move to higher energy levels.

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These electrons lose energy by emitting light when they return to lower energy levels.
The energy absorbed by an electron for it to move from its current energy level to a higher energy level is identical to the energy of the light emitted by the electron as it drops back to its original energy level.
When the light emitted by the energized electrons of a gaseous element is passed through a prism, the spectrum consists of a limited number of narrow lines of light.
The wavelengths of these spectral lines are characteristic of the element, and they make up the atomic emission spectrum of the element.

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Each spectral line in an atomic spectrum of an element corresponds to exactly one wavelength of light emitted by the electrons of that element.
The atomic emission spectrum of each element is like a person’s fingerprint.
No two elements have the same atomic emission spectrum.
Much of the knowledge about the composition of the universe comes from studying atomic emission spectra of the stars, which are hot glowing bodies of gases.

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Experiments in the early 1900’s involved this phenomena.
German physicist Max Planck’s experiment and theory – 1900
Max Planck (1900):
Proposed that amounts of energy are quantized 
only certain values are allowed

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Proposed that a hot object does not emit electromagnetic radiation continuously, as would be expected from waves.
Suggested that the object emits energy in small, specific amounts called quanta.
Quantum – the minimum quantity of energy that can be lost or gained by an atom.

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A quantum leap from n=5 to n=1 is associated with the greatest amount of light.
When an electron moves from a lower to a higher energy level, the electron absorbs a quanta of energy.

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Planck’s equation for this energy (E)
E = h = hc/ or Δ E = h Δ 
Planck’s Constant (h)
6.626 x J.s (Joules x second)
E = energy carried by 1 photon (a particle of energy).

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Albert Einstein – 1905
Introduced the theory that electromagnetic radiation has a dual wave-particle nature.
While light exhibits many wavelike particles, it can also be thought of as a stream of particles.
Photons
A particle of electromagnetic radiation having zero rest mass and carrying a quantum of energy.
A quanta of light

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Proposed that electromagnetic radiation is absorbed by matter only in whole numbers of photons.
In order for an electron to be ejected from a metal surface, the electron must be struck by a single photon possessing at least the minimum energy required to knock the electron loose.
Corresponds to a minimum frequency.
If a photon’s frequency is below minimum, the electron remains bound to the metal surface.
In his special theory of relativity the main significance of the equation E = mc2 is that energy has mass.

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Problems
Magnetic Resonance Imaging (MRI) is a powerful diagnostic tool used in medicine. The imagers used in hospitals operate with a wavelength of 7.5 x 108 nm. Calculate: 
The frequency in MHz (1 MHz = 106 Hz)
The energy in joules/photon
The energy in kJ/mol
Determine the frequency of light with a wavelength of x 10-7 cm.
Determine the energy in joules of a photon whose frequency is x 1017 Hz.

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Hydrogen – Atom Line-Emission Spectrum
Emission Spectrum – the spectrum of light released from excited atoms of an element.
When current is passed through a gas at low pressure, the potential energy of some of the gas atoms increases.
Ground State
The lowest energy state of an atom.
Excited State
A state in which an atom has a higher potential energy than it has in its ground state.
Therefore a higher frequency.

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When an excited atom returns to its ground state, it gives off the energy it gained in the form of electromagnetic radiation.
Neon lights.

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Experiments with hydrogen
Emitted a characteristic pinkish glow when an electric current passes through a vacuum tube containing hydrogen gas at low pressure.
When the visible portion of the emitted light is passed through a prism
It is separated into specific wavelengths that are part of hydrogen’s line emission spectrum.
Suggest that the electron of a hydrogen atom exists only in very specific energy states.

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Bohr Model of the Hydrogen atom –revisited
Proposed a model of the hydrogen atom that linked the atom’s electron with photon emission.
The electron can circle the nucleus only in allowed paths or orbits.
Hydrogen atoms exist in only specified energy states.
The atom has a definite, fixed energy, when the electron is in one of these orbits.

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Hydrogen atoms can absorb only certain amounts of energy.
Therefore, the electron (and the hydrogen atom) is in its lowest energy state when it is in the orbit closest to the nucleus.
The energy of the electron is higher when it is in orbits that are successively farther from the nucleus.
The energy required to remove the electron from an Hydrogen atom in its ground state is x 10-8 J

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There is empty space between the electron orbital and the nucleus.
Electrons can be in one orbit or another, but not in between.
While in orbit, the electron can neither gain nor lose energy.
The electron can move to a higher energy orbit by gaining an amount of energy equal to the difference in energy between the higher energy orbit and the initial lower energy orbit.

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When the hydrogen atom is excited, it is in the higher energy orbit.
Outer orbit is the excited states (unstable e-‘s)
Emission
When the hydrogen atom falls back from the excited state, the electron drops down to a lower energy orbit.
A photon is emitted that has an energy equal to the energy difference between the initial higher energy orbit and the final lower energy orbit.

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Electrons as Waves
Louis de Broglie’s role in the development of the quantum model of the atom.
Pointed out that in many ways the behavior of Bohr’s quantized electron orbits was similar to the known behavior of waves.
Any wave confined to a space can have only certain frequencies.
These frequencies correspond to specific energies.
The quantized energies of Bohr’s orbits.

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Experiments based on Broglie’s hypothesis demonstrated that electrons, like waves can be bent, or diffracted.

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The Heisenberg Uncertainty Principle.
It is impossible to determine simultaneously both the position and velocity of an electron or any other particle.
Because photons have about the same energy as electrons, any attempt to locate a specific electron with a photon knocks the electron off its course.
However, the position of a very tiny particle can be determined by analyzing its interactions with another particle.