Download and electrons - Purdue Physics

This Week
 Atoms and nuclei
What are we made of?
 The periodic table
Why does it stop?
How were the elements made?
 Radioactive decay
Useful but can be toxic
 Discovery of X Rays: Cathode Rays and TV sets
 Medical imaging
Limit your dose
4/30/2017
Physics 214 Fall 2010
1
The basic building blocks
The basic building blocks that make up
atoms are
charge
mass
Proton 1.6 x 10-19
1.672 x 10-27kg
Neutron
0
1.675 x 10-27kg
Electron -1.6 x 10-19 9.1 x 10-31kg
Photon
0 E = hf p = h/λ
h = Planck’s constant = 6.626 x 10-34J.s
4/30/2017
Physics 214 Fall 2010
2
The nucleus
The nucleus was found by
Rutherford in scattering
charged particles from a
gold foil.
He observed that
occasionally the charged
particle was deflected
through a large angle
indicating it had “hit
something hard”
4/30/2017
Physics 214 Fall 2010
3
Atoms
Atoms are made up with a central nucleus of protons and neutrons
surrounded by a number of electrons equal to the number of protons.
The notation we use is 2He4
2 is the atomic number = number of protons (and electrons)
4 is the mass number = number of protons + neutrons
Note atomic mass is the actual mass of the nucleus in atomic mass units
with Carbon set to be 12 units and atomic mass in general is not an
integer
H2O gives the number of atoms required to make a molecule
water = 2 Hydrogen plus 1 Oxygen
In a nuclear reaction energy, charge, and atomic mass are conserved.
Atoms can decay
Atoms can join together to form new atoms or molecules
Chemistry is determined by the electrons
4/30/2017
Physics 214 Fall 2010
4
Isotopes
For a given number of protons there is a nucleus that
is most stable for a particular number of neutrons.
Isotopes are when for the same number of protons
the number of neutrons is different from the most
stable configuration.
Since the number of electrons is the same the
chemical properties are “identical” but the nucleus
can be unstable.
The larger the difference between ideal and the
isotope the more unstable the nucleus becomes.
Nuclei have been made artificially that do not exist in
nature.
4/30/2017
Physics 214 Fall 2010
5
Radioactive decay
The general rule is that if a lower
energy configuration exists for
the same set of particles then they
will try to change into that
configuration.
Each radioactive element has a
characteristic half life, that is the
time for half the sample to decay.
Radioactive elements are clocks
which tick at a specific rate.
The amount remaining after N half
lives is ½ x ½ x ½ ………..
Half lives vary from billions of
years to a fraction of a second.
Those in the range of greater
than thousands of years are
used to date objects on earth
back to it’s formation 4.6
billion years ago
C14 5730 years I129 1.6 million
U238 4.47 billion
http://www.physics.purdue.edu/class/applets/phe/lawdecay.htm
4/30/2017
Physics 214 Fall 2010
6
Types of nuclear decays
A proton can change into a neutron and emit a positron (anti
particle of electron)
0
e
1
Atomic number decreases by 1 mass number is the same
A neutron can change into a proton and emit an electron
0
-1e
Atomic number increases by 1
mass number is the same
A nucleus can emit an alpha particle which is 2 neutrons and 2
protons (actually a helium nucleus)
4
2He
Atomic number decreases by 2 mass number decreases by 4
Photons can be emitted
No change in Atomic number or mass number.
4/30/2017
Physics 214 Fall 2010
7
What is the daughter?
When an element decays it generally changes to a new
element.
Remember the notation we use is 3Li7 where 3 is the
number of protons, called the atomic number, and 7 is
the number of protons plus neutrons, called the mass
number.
Any decay or nuclear reaction conserves charge and
mass number.
4
2He
+ 7N14 = 8O17 + 1H1
226 =
222 + He4
88Ra
86Rn
2
4/30/2017
nuclear reaction
nuclear decay
Physics 214 Fall 2010
8
Energy and atoms
When we make atoms from the constituents energy is released.
The sun’s energy comes from fusion E = Mc2
When we make molecules by joining atoms energy is released.
Burning is Carbon plus oxygen making CO2
Electrons in atoms have specific energy levels
When an electron is removed from an atom the atom will capture
an electron and emit photons of specific energies
When many atoms are emitting photons we get spectral lines,
that is electromagnetic radiation of specific wavelengths and
colors. Each atom has it’s own “fingerprint”
4/30/2017
Physics 214 Fall 2010
9
Waves and particles
Particles have both discrete properties such
as charge and energy but also wave
properties.
λ = h/p
and f = E/h
The theory which explains behavior at small
distances is called quantum mechanics.
Electron orbits in atoms are given by a three
dimensional probability distribution.
Particle beams can interfere just like the
light beams in the two slit experiment. The
answer to the question “which slit did the
particle go through?” is “ it went through
both!!”
4/30/2017
Physics 214 Fall 2010
10
Periodic table
If we start with hydrogen we make new
atoms by adding protons and neutrons and
adding electrons to be equal to the protons.
In principle we can make an atom with any
mixture of protons and neutrons but if the
balance between the forces is not correct the
nucleus will be unstable and decay.
There are three numbers that specify an
electron and no two electrons in an atom can
have the same set of numbers.
The three numbers are n which defines an
energy shell, s which defines the spin and
has only two values and L which defines the
angular momentum which is a positive
integer from 0 to n-1 in each shell.
Most chemistry involves the behavior of the
electrons. When a shell is full the atom is
chemically inert e.g. noble gases. When a
shell has just one or a few electrons it is
chemically very active
4/30/2017
n
1
2
3
Physics 214 Fall 2010
# electrons
(2L+1) x 2 = 2
(2L+1) x 2 = 8
(2L+1) x 2 = 18
11
Cathode rays and TV sets
Using a high electric field one can
strip electrons from atoms and then
accelerate them.
Heating a filament increases the flow
of electrons.
Magnetic and electric fields can then
be used to shape and steer the beam.
The original observation was called
cathode rays because the beam
came from the negative terminal.
Today a CRT TV set produces an
electron beam which is raster
scanned over a surface containing
strips of phosphors which when hit
with the electrons produce the colors
that form the image.
4/30/2017
The accelerating voltage is
typically 30,000 volts and
X rays are produced as the
beam hits the screen
Physics 214 Fall 2010
12
X rays
On 8 November 1895, Wilhelm Conrad
Roentgen (1845-1923) was experimenting on
cathode rays
when an object across the room began to
glow.
First x ray picture
Soon there were X ray portrait studios and X
ray machines were used by magicians and
you could by X ray machines from catalogs
for home use
4/30/2017
Physics 214 Fall 2010
13
X ray imaging
An electron beam is accelerated through 50,000
volts and collides with a tungsten target. The
deceleration produces X rays and X rays also
come from inner shell electrons being ejected.
A CAT scan uses X rays to image a thin slice of
the human body. Images are taken of
successive slices and computers are used
to produce 3 D rotatable images
X rays are absorbed the denser the material so
they are very good for imaging bones
4/30/2017
Physics 214 Fall 2010
14