Download Development of a New Atomic Model

Properties of Light
Before 1900, scientists thought light
behaved solely as a wave.
We know know that light has a dual
nature
It behaves as a wave AND as a particle
Wave Properties of Light
Light is energy!
A form of energy that exhibits wavelike
behavior as it travels through space is
known as electromagnetic radiation
Forms of electromagnetic radiation: Gamma
rays, X rays, ultraviolet, visible, Infrared,
microwaves, and radio waves
Electromagnetic Spectum
Wave Properties of Light
All forms of light move at a constant
speed
speed of light = c
c = 3.00 x 108 m/s or 3.00 x 1010 cm/s
Wave Properties of light
The significant feature of wave motion is
its repetitive nature
Measurable properties
Wavelength – (l) the distance between
corresponding points on adjacent waves
Units: meter or some fraction of a meter such as
nanometer or centimeter,
Wave Properties of Light
Frequency- (n) the number of wave cycles that pass a
given point in a given amount of time, usually one
second.
Unit: waves/second
One wave/second is called a Hertz (Hz)
1/sec
Wave Properties of Light
Wavelength and Frequency are mathematically
related to each other
c = wavelength x frequency
c=l n
l=c/n
n=c / l
Wave Properties of Light
Long l
Low n
Low Energy
Short l
High n
High Energy
The Photoelectric Effect
Two experiments involving the interaction
between light and matter that could not be
explained by the wave theory of light.
Photoelectric effect refers to the emission
of electrons from a metal when light shines
on the metal.
Only light of a specific frequency could
knock loose electrons from a metal.
The Particle Description
of Light
Max Plank studied the emission of light by
hot objects
Proposed that hot objects do not emit
electromagnetic radiation continuously
as would be expected if the energy
emitted were in waves
The Particle Description
of Light
Objects emit energy in small, discrete
packets of energy called quanta
A quantum of energy is the minimum
quantity of energy that that can be lost or
gained by an atom.
The Particle Description
of Light
Energy and frequency are mathematically
related
Energy = planck’s constant x frequency
E=h n
h = 6.626 x 10-34 J.s
The Particle Description
of Light
1905 Albert Einstein used Planck’s work
to describe how light has a dual-nature
[particle and wave]
Light can be thought of as a stream of
particles that carry a quantum of energy
Einstein called these particles photons
The Particle Description
of Light
A photon is a particle of electromagnetic
radiation having zero mass and carrying a
quantum of energy.
Ephoton = h n
The Particle Description
of Light
Electromagnetic radiation is absorbed by matter
only in whole numbers of photons.
In order for an electron to be ejected from the
metal surface, it must be struck by a single
photon possessing the minimum amount of
energy required to knock the electron loose.
This minimum energy relates to the minimum
frequency.
Photoelectric Effect
Photoelectric Effect Explained
The Hydrogen-Atom LineEmission Spectrum
Classical theory predicted that the hydrogen atoms
would be excited by any amount of energy
Expected the emission of a continuous range of
frequencies of electromagnetic radiation, a rainbow of
colors.
Observations led to a new atomic theory: Quantum
theory
Bohr's bright-line spectra
Bohr Model of the Atom
An electron will
absorb a specific
amount of energy
and move from the
ground state to the
excited state.
When it falls back
down to the ground
state it releases a
photon of light
equal to the energy
difference between
the energy levels.
Bohr Model of the Atom
The greater the energy
difference the higher
the frequency of light
being emitted.
Ex: n = 6 - n = 2 emits a
violet light because of
the large energy
difference
n = 3 - n = 2 has a small
energy difference thus
a red light is emitted.