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Electrons
Date your notes
10/31/2012 or 11/1/2012
How many electrons
do atoms have?
• The number of protons and the number of
electrons determine the charge of an atom
• Unless you are given a charge, assume
that your atom is neutral
• Therefore, the number of electrons can be
found…
Niels Bohr
Danish physicist, 1913
Worked with Ernest
Rutherford
Studied the emission
spectrum of Hydrogen to
learn about placement of
electrons in atoms
Emission Spectra
• When electricity is passed through a gas, the
atoms will emit lines of different frequencies,
which serves as its “fingerprint”
• This is very similar to when
light is passed through a prism.
Bohr’s Theory
Electrons exist in
orbits that contain
a certain amount
of energy (energy
levels)
Similar to planets
orbiting the sun
Bohr’s model
nucleus
Energy
levels/electron
orbits get larger
as you move
farther away
from the nucleus
Electrons will exist in the lowest possible
energy level—we call this “ground state”
Bohr’s model
The first energy
level is the smallest.
It can only hold 2
electrons
Bohr’s model
The second energy
level is larger.
It can hold up to 8
electrons
Bohr’s model
The third energy
level is slightly
larger.
It can hold up to 8
electrons
Bohr’s model
The fourth energy
level is much larger.
It can hold up to 18
electrons
Summary of Energy Levels
1st energy level holds up to 2 electrons
2nd energy level holds up to 8 electrons
3rd energy level holds up to 8 electrons
4th energy level holds up to 18 electrons
5th energy level holds up to 18 electrons
6th energy level holds up to 32 electrons
7th energy level holds up to 32 electrons
Radon
Valence Electrons
Energy levels do not have to be filled.
If an atom has only 7 electrons:
2 go on the 1st energy level
5 go on the 2nd energy level.
The electrons on the outermost energy level are
called valence electrons
Sulfur
6 valence
electrons
Practice Bohr Models – make a Bohr model
for each element and identify how many
valence electrons the atom has.
1.
2.
3.
4.
Oxygen
Aluminum
Potassium
Calcium
Energy and Electrons
When electrons
are in the lowest
possible energy
level, they are in
their ground state
Electrons can
absorb a quantum
of energy and
move to a higher
energy level. This
is called the
excited state.
Electrons can
absorb a quantum
of energy and
move to a higher
energy level. This
is called the
excited state.
A quantum is a unit of energy. It is the
amount needed to send an electron to the
next higher energy level
This is unstable,
and electrons
quickly return to
ground state
When electrons return to the ground state,
they release the quantum of energy they
absorbed. It is released as a photon of
visible light.
This is unstable,
and electrons
quickly return to
ground state
When electrons return to the ground state,
they release the quantum of energy they
absorbed. It is released as a photon of
visible light.
Electrons can
absorb a larger
quantum of
energy and move
to an even higher
energy level to a
higher excited
state.
Electrons can
absorb a larger
quantum of
energy and move
to an even higher
energy level to a
higher excited
state.
This is unstable,
and electrons
quickly return to
ground state
When electrons return to the ground state,
they release the quantum of energy they
absorbed. It is released as a photon of
visible light.
This is unstable,
and electrons
quickly return to
ground state
This electron absorbed a greater amount
of energy, so it releases a photon of light
with higher energy. It will be a different
color.
Flame Test
• We can see the result of electrons moving
back down from an excited state to a
ground state, when we provide the energy
to move them to an excited state.
• What you will do:
http://www.teachertube.com/viewVideo.php?video_id=138154&title=Flame_Test_Lab
• You will not finish today—test two or three.
We will finish on Tuesday.
Your Turn
Brainstorm for 2 minutes –
what do you know about light?
Properties of Light
• Light travels as a wave
• Light travels at the speed of light
3.00x108 m/s
• Light is electromagnetic radiation
• Visible light is a very small part of the EMR
spectrum
• ROYGBIV
• Blue light has the most energy; red light
has the least energy
Properties of Light
• Light is made of photons
– It is a particle
– It travels as a wave of energy
– It is both at the same time—wave-particle duality
• Speed of light = wavelength of light x frequency
of light
c = fλ
or
c = λν (lambda x nu)
c = 3.00x108 m/s
• Energy of a photon = Planck’s constant x
frequency of light
Ephoton = hν
or
Ephoton = hf
h=Planck’s constant = 6.63x10-34 J*s
Light and Electrons
• Electrons can absorb varying sized quanta and
“jump” to varying excited states
• They also “drop” from excited state to ground
state and release a different amount of energy
and a different color of light
• In any given sample of an element, all possible
jumps and drops are taking place
• Not all photons are in the visible light range—UV
and infrared
Practice
Radiation from outer space is detected that
has a frequency of 94.7 MHz. What is the
wavelength of this radiation, and what type
is it?
Practice
What is the energy of a photon of red light
that has a frequency of 4.48 x 1014 Hz?
Practice
A student has both a red laser pointer and a
green laser pointer. Which has the greater
amount of energy?
Red light wavelength = 670 nm
Green light wavelength = 530 nm