Transcript Ch. 5 Electrons in Atoms (WLC)

Chapter 5
Electrons in Atoms
The Bohr Model
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An electron is
found only in
specific circular
paths, or orbits,
around the
nucleus.
Each orbit has a
fixed energy. The
orbits are called
‘energy levels.’
Energy Levels
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Energy levels are like
the rungs of a ladder:
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You can move up or down by
going from rung to rung.
You can’t stand in-between
rungs.
•For an electron to change energy
levels it must gain or lose exactly
the right amount of energy.
A Quantum
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A quantum of energy is the amount needed to move an
electron from one energy level to another.
The energy of an electron is said to be “quantized.”
Energy levels in an atom are not all equally spaced.
An Airplane Propeller
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The blurry picture of an airplane propeller represents
the area where the actual propeller blade can be found.
• Similarly, the electron
cloud of an atom represents
the locations where an
electron is likely to be found.
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Comes from the mathematical solution to the
Schrodinger equation.
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Determines allowed energies an electron can have &
how likely it is to find the electron in various locations
around the nucleus.
Uses probability
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The Model
Quantum
Mechanical
Atomic Orbitals
A region in space in
which there is a high
probability of finding
an electron.
 Energy levels of
electrons are labeled
by principal
quantum numbers
(n)
n = 1, 2, 3, 4 …
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s Orbitals
are spherical
p Orbitals
are dumbbell- shaped
d Orbitals
4 out of the 5 d orbitals have clover leaf shapes
f Orbitals
are more complicated
Atomic Orbitals
The number and kinds of atomic orbitals depend on the energy sub level.
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N=1
N=2
N=3
N=4
has
has
has
has
1
2
3
4
sublevel called 1s
sublevels called 2s and 2p
sublevels called 3s, 3p, and 3d
sublevels 4s, 4p, 4d, and 4f
The maximum number of electrons that can occupy
a principle energy level is 2n2.
(n=principle quantum #)
Electron
Configurations
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Electrons in an atom try to make the
most stable arrangement possible
(lowest energy)
The Aufbau Principle, the Pauli
Exclusion Principle, and Hund’s Rule
are guidelines that govern electron
Aufbau Principle
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Electrons occupy the orbitals of lowest
energy first
Pauli Exclusion Principle
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An orbital can hold at most
2 electrons
Does it make sense that two
negatively charged
particles will ‘want’ to
share the same space?
This phenomenon is made
possible because electrons
possess a quantum
mechanical property called
spin
Electron Spin
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Spin may be thought of
as clockwise or counterclockwise
An arrow indicates an
electron and its direction
of spin
An orbital containing
paired electrons is written
Hund’s Rule
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When filling
orbitals of
equal energy,
one electron
enters each
orbital until all
the orbitals
contain one
electron with
similar spin
Hund’s Rule
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How would you put 2 electrons into a p
sublevel?
How would you put 7 electrons into a d
sublevel?
Light
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Now that we understand how electrons are
arranged in atoms, we can begin to look at
how the frequencies of emitted light are
related to changes in electron energies
Light
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Light waves properties:
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Amplitude – the wave’s height from zero to crest
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Wavelength – the distance between crests
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Frequency – the number of wave cycles to pass a
given point per unit of time (Usually Hz = 1/s)
Light
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Wavelength has the symbol (λ) lambda.
Frequency has the symbol (ν) nu.
The speed of light is a constant (c) = 3x108 m/s
c = λν
Light
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How are wavelength and frequency related?
They are inversely related. As one increases, the
other decreases
How long are the wavelengths that correspond to
visible light?
700-380 nanometers
Electromagnetic Spectrum
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Visible light is only a tiny portion of the
electromagnetic spectrum which also includes radio
waves, microwaves, infrared, visible light, ultra violet,
X-rays, and gamma rays.
If the entire electromagnetic spectrum was a strip of
professional 16 mm movie film stretching from Los
Angeles to Seattle, the portion of visible light would
be only ONE frame of film.
Atomic Spectra
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When atoms absorb energy, electrons move to higher
energy levels
Electrons then lose energy by emitting light as they
return to lower energy levels
Atoms emit only specific frequencies of light that
correspond to the energy levels in the atom
The frequencies of light emitted by an element
separate into discrete lines to give the atomic
emission spectrum of the element
Atomic Spectra
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An electron with its lowest possible energy is in its
ground state
The light emitted by an electron is directly
proportional to the energy change of the electron.
E = hν
Atomic spectra are like fingerprints: no two
are alike!