Chapter 18

Radioactivity and Nuclear Reactions

What do you think of when you hear the word “radioactivity” or “radiation”?

How Did It All Happen? Radioactivity 4.16

To understand the structure and arrangement of atomic particles;

To distinguish between nuclear fission and nuclear fusion.

The Objectives:

Classroom Activity Personal Radiation DoseWe live in a radioactive world and always have. Radiation is all around us as a part of our natural environment. It is measured in millirems (mrems). The annual average dose per person from all sources is about 620 mrems, but it is not uncommon for any of us to receive more than that in a given year (largely due to medical procedures).

To find your average annual dose (mrems), use the interactive Personal Annual Radiation Dose Calculator handout.

We can’t get away from it!

How does radiation affect me?

Background radiation cannot be eliminated. Each breath we take, we inhale about 3 billion carbon-14 atoms.

Our Objective: To understand the structure and arrangement of atomic particles;

The Vocabulary

Become the expert!Find the word in section 1 or 4. Read its meaning and any other info given.

Nuclear DecayStrong Force (1)Radioactivity (1)Nuclear Fission (4)Chain Reaction (4)Critical Mass (4)Nuclear Fusion (4)

Let’s Review Describe the structure

of an atom. How many electrons will

an atom have? How can you know how

many protons an atom has?

Let’s build one more model of a non-radioactive element.

Argon◦ ______ Protons◦ ______ Neutrons◦ ______ Electrons

_____ Level 1 _____ Level 2 _____ Last Level

To understand the structure and arrangement of atomic particles;

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Four Forces in Nature

GravityElectromagnetic ForceWeak Nuclear ForceStrong Nuclear Force

The Goal: To understand the structure and arrangement of atomic particles.

The strong force causes protons and neutrons to be attracted to each other. The strong force is one of the four basic forces in nature and is 100 times greater than the electric force.

What makes an atom radioactive?Definition strong force

The Goal: To understand the structure and arrangement of atomic particles;

The attractive force between all the protons and neutrons in a nucleus keep the nucleus together. However, The strong force is powerful only when neutrons and protons are very close together.

What makes an atom radioactive?

The Goal: To distinguish between nuclear fission and nuclear fusion.

The strong force is a short-range force that quickly becomes extremely weak as protons and neutrons get farther apart. The electric force is a long-range force so protons that are far apart are repelled by the electric force.

Alpha particle is 2 protons and 2 neutrons that are emitted from a decaying atom. Alpha particles can easily penetrate a sheet of paper.

What makes an atom radioactive?Definition 1

Radioactivity process when a nucleus decays and

emits radioactive particles and energy

• The protons and neutrons in an atomic nucleus are held together by the strong force.

Isotopes are atoms of the same element with varying numbers of neutrons.

The strong force is the strongest force in nature.

Larger atoms mean that most of the protons and neutrons have less strong force holding them to each other allowing the electrical force to dominate and pull the atom apart. The atom is unstable.

Nuclear Fission

The Goal: To distinguish between nuclear fission and nuclear fusion.

Chain Reaction Radioactive Uranium

The Goal: To distinguish between nuclear fission and nuclear fusion.

In periodic tables the atomic mass of an element is usually not written as a whole number. For example, silicon is shown as having an atomic mass of 28.086. The atomic mass on the periodic table is the average of the masses of all the isotopes of an element weighted by the relative occurrence of each isotope.

Radioactivity

A. The nucleus of an atom is composed of protons and neutrons which comprise most of the atom’s mass.

Radioactivity

The Goal: To distinguish between nuclear fission and nuclear fusion.

B. The strong force causes protons and neutrons to be attracted to each other in the nucleus.

Radioactivity

1. The strong force is powerful only when neutrons and protons are very close to each other.

2. The protons and neutrons in a large nucleus are held together less tightly by the strong force than protons and neutrons in a small nucleus.

Radioactivity

C. Radioactivity means that nuclear decay happens when the strong force is not large enough to hold the nucleus together; the nucleus gives off matter and energy.

Radioactivity

1. Isotopes are atoms of the same element with varying numbers of neutrons.

2. A nucleus with too many or too few neutrons compared to protons is radioactive.

Radioactivity

3. Atomic number is the number of protons in an atom.

4. Mass number is the number of protons and neutrons in a nucleus.

Radioactivity

D. Uranium radioactivity was discovered in 1896 by Henrie Becquerel; Marie and Pierre Curie discovered the radioactive elements polonium and radium in 1898.

Radioactivity

DID YOU GET IT? Do you understand the structure and arrangement of atomic particles?

The Object……

A. Nuclear fission—process of splitting a nucleus into two nuclei with smaller masses; a large amount of energy is released.

Nuclear Reactions

Radioactivity• The protons and neutrons in an atomic

nucleus are held together by the strong force.

• The ratio of protons to neutrons indicates whether a nucleus will be stable or unstable. Large nuclei tend to be unstable.

• Radioactivity is the emission of energy or particles from an unstable nucleus.

• Radioactivity was discovered accidentally by Henri Becquerel about 100 years ago.

Let's take a look at uranium-238 to illustrate the decay chain.

• Half-life is the amount of time in which half of the nuclei of a radioactive isotope will decay.

1. Chain reaction—ongoing series of fission reactions.

2. Critical mass—amount of fissionable material required to continue a reaction at a constant rate.

Nuclear Reactions

B. Nuclear fusion—two nuclei with low masses are combined to form one nucleus of larger mass.

1. Nuclear fusion can happen only when nuclei are moving fast enough to get close to each other.

2. Temperatures in stars are high enough for fusion to occur.

Nuclear Reactions

Nuclear Fusion

C. Nuclear reactions have medical uses.

1. Radioisotopes are used as tracers to find or keep tract of molecules in an organism.

2. Cancer cells can be killed with carefully measured doses of radiation.

Nuclear Reactions

DID YOU GET IT? Can you distinguish between nuclear fission and nuclear fusion?

The Object……

Research is underway to develop reactors that use nuclear fusion to generate electricity. Unlike fission reactors, a fusion reactor would generate very little long-lived radioactive material. However, extremely high temperatures are needed for the fusion reactions to occur and the plasma created is difficult to confine under these conditions. One way to contain the plasma is to use strong magnetic fields that prevent the plasma from touching the inside surfaces of the reactor.

Nuclear Reactions

Energy from the Atom

Did You Get It?

Do you understand the structure and arrangement of atomic particles?

Can you distinguish between nuclear fission and nuclear fusion?

The Objectives:

Seconds from Disaster-Meltdown in Chernobyl