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

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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

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