1. 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
9/13/2018
Physics 214 Fall 2011

2. The basic building blocks
The basic building blocks that make up atoms are
charge mass
Proton x x 10-27kg
Neutron x 10-27kg
Electron -1.6 x x 10-31kg
Photon E = hf p = h/λ
h = Planck’s constant = x 10-34J.s
9/13/2018
Physics 214 Fall 2011

3. 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”
9/13/2018
Physics 214 Fall 2011

4. 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
9/13/2018
Physics 214 Fall 2011

5. 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.
9/13/2018
Physics 214 Fall 2011

6. 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
C years I million
U billion
9/13/2018
Physics 214 Fall 2011

7. Types of nuclear decays
A proton can change into a neutron and emit a positron (anti particle of electron)
1e0
Atomic number decreases by mass number is the same
A neutron can change into a proton and emit an electron
-1e0
Atomic number increases by mass number is the same
A nucleus can emit an alpha particle which is 2 neutrons and 2 protons (actually a helium nucleus)
2He4
Atomic number decreases by 2 mass number decreases by 4
Photons can be emitted
No change in Atomic number or mass number.
9/13/2018
Physics 214 Fall 2011

8. 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.
2He4 + 7N14 = 8O17 + 1H nuclear reaction
88Ra226 = 86Rn He nuclear decay
9/13/2018
Physics 214 Fall 2011

9. 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”
9/13/2018
Physics 214 Fall 2011

10. 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!!”
9/13/2018
Physics 214 Fall 2011

11. Periodic table n # electrons (2L+1) x 2 = 2 L = 0
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
n # electrons
(2L+1) x 2 = 2 L = 0
(2L+1) x 2 = 8 L = 0 or 1
(2L+1) x 2 = 18 L = 0 or 1 or 2
9/13/2018
Physics 214 Fall 2011

12. 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.
The accelerating voltage is typically 30,000 volts and
X rays are produced as the beam hits the screen
9/13/2018
Physics 214 Fall 2011

13. X rays
On 8 November 1895, Wilhelm Conrad Roentgen ( ) was experimenting on cathode rays
when an object across the room began to glow.
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
First x ray picture
9/13/2018
Physics 214 Fall 2011

14. 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
9/13/2018
Physics 214 Fall 2011