Modern & Nuclear Physics
The Basics
Ordinary matter is made up of collections of atoms. Around 90 different types of atoms can be found on
Earth. Each of the different types is called an element. Elements are substances that cannot be broken down
into other substances.
Each atom has a nucleus, which contains most of its mass. In this nucleus are the nucleons – the protons
and neutrons. Surrounding the nucleus is the electron cloud. There is one electron for every proton in a
neutral atom. When the number of electrons and protons is different, there is a net charge, and the atom is
called an ion.
The atomic number is the number of protons in an atom. This information can be easily found from the
periodic table. Z is the symbol for the atomic number.
The mass number is the number of nucleons in an atom – the number of protons plus the number of
neutrons. We distinguish atoms by mass number because atoms of the same element can have different
numbers of neutrons. For example some atoms of carbon (atomic number 6) have 6 neutrons while others
might have 8. Atoms that have different mass numbers are called isotopes. Isotopes of an element behave
pretty much the same way, but they have a slight difference in mass. A is the symbol for the mass number.
The atomic mass on the periodic table is the average of mass numbers for atoms of that element on Earth.
N is the number of neutrons.
Isotopes are identified by their mass numbers. There are several ways to do this. Let’s take as our example an
isotope of uranium. We could call it:
Uranium – 235
Name and mass number
U – 235
Chemical symbol and mass number
Chemical symbol and mass number
Chemical symbol, atomic number, and mass number
Exercise: How many protons, electrons, and neutrons are in
Z = 92 - this is by definition the number of protons.
Number of electrons = 92 (because # electrons = # protons)
Number of neutrons = atomic mass – number of protons = 146
?
Radioactivity: Certain isotopes are not, for various reasons, stable. The nuclei tend to break apart. We
call such elements radioactive isotopes. Radioactivity has to do with the weak nuclear force and the
combination of protons and neutrons. Without the right arrangement of neutrons and protons, the geometry
of the nucleus can’t support itself. Some combinations are more stable than others. Radioactivity is defined
as the spontaneous breakdown of an unstable atomic nucleus with emission of particles and rays.
Characteristics of radioactive isotopes:
Radioactive emissions affect photographic film.
Radiation ionizes surrounding air molecules.
Radiation makes certain compounds fluoresce (give off electromagnetic radiation).
Radiation has physical effects on living organisms; it can kill or damage tissue.
Radiation alters atomic nuclei, producing a new elements or elements from old ones.
Why Are Some Elements Radioactive?
The mechanism of radioactivity appears to be related to the interaction among protons and neutrons in the
nucleus. The normal isotope of hydrogen has only one proton in its nucleus - no neutrons. Most helium atoms
have two protons and two neutrons. The neutrons are required, in some way not fully understood, to
"cement" the protons together to form a nucleus. Protons should repel each other because of their like
charges, but this does not happen in the nucleus. As the number of protons increases, the number of neutrons
increases. Large nuclei have more neutrons than protons. Certain combinations of neutrons and protons are
more stable than others. For example, C-12 is stable, but C-14 is radioactive (even though it has more
neutrons). The force between nucleons that keeps the nucleus together is called the nuclear force, and it is
many orders of magnitude stronger than Coulomb repulsion. The nuclear force is effective over a very small
distance, about the radius of a proton. The nuclear force is enormously stronger than the electromagnetic
force, but it has a much smaller effective range. Neutrons are required, but we don’t know why, exactly.
Half-life:
Half life is the time for half of a radioactive sample to decay.
Radium-226 has a half-life of 1620 years. So after 1620 years, a 100lb sample of radium-266 would only have
50lbs of radium-266. The other 50lbs decay into other things.
Types of Radioactivity: There are three major types of radiation: alpha, beta, and gamma. Alpha
radiation consists of “alpha particles” which are just helium nuclei. Beta radiation is also made up of particles –
electrons. Gamma radiation is made up of very short wavelength electromagnetic waves. The reason for the
odd names is a simple one; the types of radiation were discovered before the particles were.
Alpha particles. The symbol for the alpha particle is α.
α particles are helium nuclei. Each alpha consists of 2 protons and 2 neutrons.
α's have a positive charge (+2).
α's are only slightly deflected by a magnetic field (because of their large mass).
They are stopped easily by a sheet of paper.
Beta particles. The symbol for the beta particle is β.
β's are electrons, so they have a have a negative charge (-1).
They can be greatly deflected by a magnetic field (because of their small mass and negative charge).
β's penetrate matter a greater distance than α particles, but they still aren’t very penetrating.
They can be stopped by a layer of metal foil or several sheets of paper.
Gamma rays. The symbol for gamma rays is γ.
γ’s are very short wavelength, high frequency photons.
γ’s have no charge (they’re light waves)
They are not deflected by magnetic fields (again, they have no charge).
They are the most penetrating form of radiation.
Stopping γ’s requires great thicknesses of heavy materials such as lead or concrete.
Exercise
You have three radioactive samples
One emits α particles, one β, and one γ.
You must hold one, put one in your jacket pocket, and eat the other.
What do you do?
Hold the α; your skin will stop the particles.
Your jacket pocket will stop the β.
Nothing but a few feet of lead will stop the γ,
so eating it is no more deadly than the other options.
Symbols Used For Particles In Nuclear Reactions:
Neutron
Proton
Alpha particle
Gamma ray
Electron
Beta particle
Nuclear Reactions:
In nuclear reactions, the atomic number and the mass number.
Balancing the atomic number balances the charge of the reactant and product.
Alpha decay: In alpha decay, an unstable nucleus produces a daughter nucleus and releases an α particle.
U-238 decays to produce Th-234 and an alpha particle.
During alpha decay, the mass number decreases by 4 and the atomic number decreases by 2.
The Th-234 is called a daughter or daughter product.
Beta decay: In β decay, a neutron becomes a proton and ejects a β particle (an electron).
Here, Thorium-234 has one of its neutrons become a proton, increasing the atomic number by one.
This does not affect the mass number. A beta particle is also produced.
Note that the atomic number on the left is equal to the total atomic number on the right.
Gamma Decay : Many nuclear reactions also produce γ rays. Alpha decay does this frequently.
Exercises
Identify what type of reaction this is, and then complete it.
Obviously, this is a beta decay because one of the products is an electron.
Oxygen-15 loses a beta particle and becomes an element with a Z=9, which is fluorine.
There is no change to the mass number, so it stays at 15. Now we can fill everything in.
Balance this reaction.
The mass number on the right side goes to 11.
The Z total on the right side is five.
The neutron on the left side has a mass number 1,
so the mass number of the decaying nucleus must be ten.
The neutron has no effect on the atomic number of the decaying nucleus,
so it must be five.
The decaying nucleus must be boron.