Chemistry 1027A/B Chapter Notes - Chapter 1: Matter Wave, Louis De Broglie, Emission Spectrum

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Published on 17 Aug 2012
School
Western University
Department
Chemistry
Course
Chemistry 1027A/B
Professor
Discovering Chemistry Chapter 1 September 14, 2011
Chapter 1 Atomic Theory
Atomic Structure
o What does an atom consist of?
Inside the nucleus: Protons (positive) and Neutrons (no charge)
Outside the nucleus: Electrons (negative)
Has some amount of energy
We can give it more energy
o “excited” state – higher energy level
Eventually it will return to “ground” state
o Loses energy it originally gained (emits energy in the form of
light)
History
o Started by looking at Hydrogen
o Wave Nature of Light
Light travels as a wave
Wavelength (lameda) is the distance between two peaks
Frequency, v, is the number of times a peak passes a given point in a given
amount of time
Speed of light is given by c: 3.0 x 10 (power 8) m/s
o Wave Interference
A) shows constructive interference; two waves “in phase” combine to form a
larger wave
B) shows destructive interference; two waves “out of phase” cancel
o Particle Nature of Light
Light interacts (may be either absorbed or emitted) as discrete particles with
discrete energies
Planck’s constant, h, is the link between wave properties of light and particle
properties of light and allows us to calculate the energy of the photon
o Hydrogen Emission
Wavelengths of visible light emitted by excited hydrogen are: 410, 434, 486, and
656 nm
See Balmer Equation in book
Light goes from energy level, n, to energy level 2
Energy level 2 is visible light
See Lyman Equation in book
Corresponds with UV Light
Light goes from energy level, n, to energy level 1
Energy level falls further, therefore there is more energy
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See Paschen Equation in book
Corresponds with IR light
Light goes from energy level, n, to energy level 3
o Summary of the Bohr Model
Successful at explaining the emission spectrum of hydrogen
Considered the electron as a particle orbiting the nucleus
No explanation for the splitting of emission lines
No explanation of intensities of emission lines
Didn’t work for atoms with more than one electron
o Quantum Mechanical Model
Louis de Broglie proposed that electrons (as well as light) could also behave as
both waves and particles
Quantum mechanical model takes a wave approach; when bound inside an
atom, electrons behave like waves
Bohr’s orbits are explained by standing waves; only certain wave patterns are
possible, resulting in only certain frequencies
Principal Quantum Number = # of Nodes + 1
Node: amplitude of the wave is zero
No possibility of finding an electron at a node
o What is a Wave Function
A wave function is a variable quantity that mathematically describes the wave
characteristics of a particle
By analogy with waves such as those of sound, a wave function, designated by
the Greek letter psi may be thought of as an expression for the amplitude of the
particle wave (or de Broglie wave), although for such waves amplitude has no
physical significance
The intensity of the wave, tells us the probability of finding the particle at that
point, and is proportional to psi squared
o The Schrodinger Equation
Mathematical representation of de Broglie’s waves
Solution of the Schrodinger Equation gives psi for the wave pattern of the
electron
Psi squared gives the probability of finding the electron 95% of the time
This probability is called an orbital (just a probability wave where we find
electron most of the time
o How is an Orbital Described?
Mathematics defines the wave functions of the orbitals as well as their shapes
and sizes
Solution of the Schrodinger Equation gives three numbers which specify the
orbital
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