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Transcript
Electrons in Atoms
Chapter 2 Section 2
Objectives
How does the electron relate to the modern
atomic theory?
How do electron energy levels in an atom
differ from one another?
How are Lewis electron dot diagrams used to
illustrate valence electrons?
Important Vocabulary
Electromagnetic spectrum
Emission spectrum
Energy level
Electron cloud
Valence electrons
Lewis dot diagrams
Wavelength
Frequency
Electrons Motion & Energy
Electrons have enough energy to keep them
in constant motion around the nucleus
This enables them to overcome the
attraction of the positive nucleus
Electrons occupy orbitals of only certain
amounts of energy
For them to move up a level of energy,
energy must be added
For them to move down a level of energy,
energy must be released in the form of
light or heat
Waves Transfer Energy
 Energy is the ability to exert a force over a certain
distance
 It is also the ability to do work
 Waves carry energy because they can do work
 For example:
Water waves can transfer energy to a leaf, to
a boat, or onto a beach
Sound waves can transfer energy to your
eardrum
Light waves can transfer energy to your eye
 The bigger the wave the _______ energy it carries
Electromagnetic Radiation
Electromagnetic radiation travels in
the form of waves that have both
electric and magnetic properties
Electromagnetic waves travel through
a vacuum at the speed of light
(300 million m/s)
Two properties of waves are
frequency and wavelength
Wavelength & Frequency
• Wavelength is the distance from one crest to the
next
• Frequency is the number of waves per second
• A low frequency results in a long wavelength and
a high frequency results in a shorter wavelength
Electromagnetic Spectrum
The electromagnetic spectrum consists
of electromagnetic radiation waves at all
possible energies, frequencies, and
wavelengths
The spectrum ranges from 103 m to 10-12
meters
Each part of the electromagnetic
spectrum has unique properties
Radio Waves
Heinrich Hertz proved the existence of radio
waves in the late 1880s
Radio waves have wavelengths that range from
200 to 600 m
Have the longest wavelengths and the lowest
energy
They are used as TV signals, AM and FM radio
signals, and for radar equipment
Radio telescopes view planets, comets, giant
clouds of gas and dust, stars, and galaxies
Microwaves
Are low energy, low frequency radiation waves
They are used in Doppler radar for weather
forecasting and to cook your food
Microwaves are also used to carry
telecommunication signals
Most mobile phones use microwaves to
transmit information, and space probes
transmit signals back to Earth with microwaves
Different wavelengths of microwaves (grouped
into "sub-bands") provide different
information to scientists.
Infrared Waves
In 1800, William Herschel discovered
them
Have less energy than visible light
Are given off by the human body and
other warm objects
We experience infrared rays as heat
from fires and electric heaters
Through night-vision goggles and
infrared thermal cameras we can see
infrared waves
Visible Spectrum
Cone-shaped cells in our eyes act as
receivers tuned to the wavelengths in this
narrow band of the spectrum
A typical human eye will respond to
wavelengths from about 380 to 750 nm
The spectrum does not contain all the colors
that the human eye and brain can distinguish
Unsaturated colors such as pink, and purple
colors such as magenta, are absent because
they can only be made by a mix of multiple
wavelengths
When white light shines through a prism,
the white light is broken apart into the
colors of the visible light spectrum
 Water vapor in the atmosphere can also
break apart wavelengths creating a rainbow
Ultraviolet Waves
In 1801, Johann Ritter discovered them
Have shorter wavelengths than visible light
UV waves are invisible to the human eye,
but some insects, such as bumblebees, can
see them.
The Sun is a source of the full spectrum of
ultraviolet radiation, which is commonly
subdivided into UV-A, UV-B, and UV-C
X-rays
 X-rays were first observed and documented in
1895 by German scientist Wilhelm Conrad
Roentgen
 X-rays have very small wavelengths, between 0.03
and 3 nanometers
 X-rays are used by doctors to see the internal
structures of the body
 X rays have very high energies, so they may kill
living cells or turn them into cancer cells when
exposed to too much of this type of radiation
Gamma Rays
Have the highest energy and shortest
wavelengths
They are produced by the hottest and
most energetic objects in the universe,
such as neutron stars and pulsars,
supernova explosions, and regions around
black holes
On Earth, gamma waves are generated by
nuclear explosions, lightning, and the less
dramatic activity of radioactive decay
Gamma rays can be used to treat cancer
by killing the diseased cells
Electrons and Light
When electrons become excited they give off
light
The spectrum of light released is called the
emission spectrum
Each element has a different emission
spectrum
So it serves as evidence of energy levels
within atoms
Energy Levels
There are seven levels of energy
available for electrons to occupy
Electrons can move between energy
levels like the rungs of a ladder
absorbing or releasing energy
The number of filled energy levels
depends on the number of electrons
Lower levels are filled first
Energy Levels
 1st energy level = 2 electrons
 2nd energy level = 8 electrons
 3rd energy level = 8 electrons
 4th energy level = 18 electrons
 5th energy level = 18 electrons
 6th energy level = 32 electrons
 7th energy level = 32 electrons
 For example: Na has 11 electrons
2 in level 1
8 in level 2
1 in level 3
Valence Electrons
 Every atom has between 1 and 8 valence electrons
 Valence electrons are electrons in the outermost
energy level of an atom
 They determine an atom’s chemical properties and
its ability to form bonds
 For example: Neon
Has 10 electrons
2 electrons in the lowest level
8 electrons in the 2nd level
Thus, it has 8 valence electrons!
Lewis Dot Diagrams
Valence electrons are usually the only
electrons used in chemical reactions
We represent valence electrons as Lewis
dot diagrams, which illustrate the valence
electrons of an element as dots
Normally, all the elements within a group
have the same Lewis dot structure with
the exception of Helium
Lewis Dot Diagrams