Chapter 18

Physical Science

Why is it important? Radioactivity is everywhere because every

element on the periodic table has some atomic nuclei that are radioactive.

New Vocabulary that you will learn in this section:Strong forceradioactivity

Describe the structure of an atom and its nucleus

Explain what radioactivity is.

Contrast properties of radioactive and stable nuclei

Discuss the discovery of radioactivity.

Atoms are composed of protons, neutrons, and electrons.Nucleus occupies only a tiny fraction of the

space in the atom. Contains almost all the mass of the atom contains protons (+) and neutrons (neutral)Total amount of protons= atomic number (charge

in a nucleus)Electrons-located outside the nucleus (-) charge

How do you suppose protons and neutrons are held together so tightly in the nucleus?

The strong force causes protons and neutrons to be attracted to each other See Figure 2 on page 537

1 of the 4 basic forces in nature and is about 100 times stronger than the electric forceElectric force: long range force, so protons that

are far apart still are repelled by the electric force

The total force between two protons/neutrons depends on how far apart they are. The strong force is a short-range force: that

quickly becomes extremely weak as protons and neutrons get farther apart

If a nucleus has only a few protons and neutrons, they are all close enough together to be attracted to each other by the strong force.See Figure 4A

Protons and neutrons are held together less tightly in a large nuclei. (each protons and neutron is attracted to only a few neighbors by the strong force) See Figure 4BAll protons is a large nucleus exert a repulsive

electric force on each other. Thus, the electric repulsive force on a proton in a large nucleus is larger than it would be in a small nucleus

When the strong force is not large enough to hold a nucleus together tightly, the nucleus can decay and give off matter and energy. This process of nuclear decay is called Radioactivity.

Large nuclei tend to be unstable and can break apart of decay.All nuclei that contain more than 83 protons are

radioactive. See 2nd paragraph (read as a class)Almost all elements with more than 92 protons

do not exist naturally on Earth. Produced in labs (synthetic elements)

Atoms of the same element that have different numbers of neutrons but the same number of protonsEx). The elements Carbon

3 isotopes that occur naturally (Carbon nuclei can have 6,7, or 8 neutrons)

Look at figure 5 and identify the ratio of protons to neutrons in each isotope of helium

Answer: Helium-3 : 2 to 1; Helium-4 : 2 to 2

Brain Pop Video:http://glencoe.mcgraw-hill.com/sites/0078779626/student_view0/brainpop_movies.html

#

The ratio of neutrons to protons is related to the stability of the nucleus

Less massive elements are stable when:Ratio is 1 : 1

Heavier elements are stable when:Ratio is 3 : 2

Nuclei is any isotopes that differ much from these ratios are unstable. (whether the elements are light or heavy)Nuclei with too many or too few neutrons

compared to the number of protons are radioactive

Atomic #: # of protons in nucleusMass #: # of protons and neutrons

See page 539 at the bottom

15 silver=NEUTRONS13 green= protons2 red= 2 nuclei

Small Nucleus Model:Place 2 green protons and 3 silver neutrons

around a red nucleus so they touchLarge Nucleus Model:

Arrange the remaining candies around the other red nucleus so they are touching.

Compare the number of protons and neutrons touching a green protons in both models. (How many red and yellow are touching a

green)

Suppose the strong force on a green proton is due to protons and neutrons that touch it. Compare the strong force on a green proton in both models.

1. About the same number touch the green protons in the larger nucleus, so the strong force is about the same.

2. The total number of protons and neutrons increases. The number nearby stays the same. The electromagnetic force on a proton increases, but the strong force stays the same, so the nucleus is less stable.

1896 Henri Becquerel Uranium salt

1898 Marie and Pierre Curie 2 new elements: polonium and radium

1). Describe the properties of the strong force

2). Compare the strong force between protons and neutrons in a small nucleus and a large nucleus.

3). Explain why large nuclei are unstable.

Why it’s important?Nuclear decay produces nuclear radiation that

can both harm people and be useful

Compare and contrast alpha, beta, and gamma radiation

Define the half-life of a radioactive materialDescribe the process of radioactive dating

Occurs when an unstable nucleus decays, particles and energy called nuclear radiation are emitted from it.

3 types:AplhaBetaGamma

Alpha particle:Made of two protons and two neutrons is emitted

from the decaying nucleus.See Table 1

Compared to beta and gamma radiation, alpha particles are much more massive

They have the most electric charge (therefore, lose energy more quickly when they interact with matter than the other types of nuclear radiation do).

Can be dangerous if they are released by radioactive atoms inside the human body.

A single alpha particle can damage many fragile biological molecules.Can cause cells not to function properly,

leading to illness and disease.

Ex. Smoke detectors give off alpha particles that ionize the surrounding air.

The process of changing one element to another through nuclear decaySee Figure 8 on page 542

When an electron is emitted from the nucleus Beta decay is caused by another basic force

called the weak force.

Damage from Beta ParticlesBeta particles are much faster and more

penetrating than alpha particlesThey can pass through paper but are stopped

by a sheet of aluminum foilDamage cells when they are emitted by

radioactive nuclei inside the human body

Electromagnetic waves with the highest frequencies and the shortest wavelengths in the electromagnetic spectrum.Contain no mass and no chargeTravel at the speed of lightEmitted from a nucleus when alpha decay or

beta decay occurs.See Table 3 on Page 543

What stops gamma rays?Thick blocks of dense materials (lead and

concrete)They cause less damage to biological

molecules as they pass through living tissue.

Half-life: The amount of time it takes for half the nuclei in a sample of the isotope to decay.

Radioactive dating: Geologists, biologists, and archaeologists, among

others, are interested in the ages of rocks and fossils found on Earth.First: the amount of the radioactive isotope and its

daughter nucleus in a sample of material are measured.

Second, the number of half-lives that need to pass to give the measured amounts of the isotope and its daughter nucleus is calculated.

Third, the number of half-lives is the amount of time that has passed since the isotope began to decay

Carbon-14 often is used to estimate the ages of plant and animal remains.See page 545

Radioactive dating also can be used to estimate the ages of rocks.Some rocks contain uranium, which has two

radioactive isotopes with long half-lives.

1. Infer how the mass number and the atomic number of a nucleus change when it emits a beta particle.

2. Describe how each of the three types of radiation can be stopped.

What you will learn…Explain nuclear fission and how it can begin a

chain reactionDiscuss how nuclear fusion occurs in the Sun.Describe how radioactive tracers can be used

to diagnose medical problems.Discuss how nuclear reactions can help treat

cancer.

VOCABULARY: nuclear fission, chain reaction, critical mass, nuclear fusion, and tracer.

Nuclear fission:The process of splitting a nucleus into several

smaller nucleiThe word “fission” means to divideOnly large nuclei, such as the nuclei of uranium

and plutonium atoms, can undergo nuclear fission.Figure 16

The products of a fission reaction usually include several individual neutrons in addition to the smaller nuclei

Chain Reactions:The series of repeated fission reactions caused

by the release of neutrons in each reactionIf the chain reaction is uncontrolled, an enormous

amount of energy is released in an instant. However, it can be controlled by adding materials that absorb neutrons.

Critical Mass:The amount of material required so that each

fission reaction produces approximately one more fission reaction.If less than the critical mass of material is present, a

chain reaction will not occur

Nuclear Fusion:2 nuclei with low masses are combined to form

one nucleus of larger mass.Fusion: fuses atomic nuclei togetherFission: splits nuclei apart

For Fusion to occur: positively charged nuclei must get close to each other.

Example: The SunMost of the energy given off by the Sun is produced

by a process involving the fusion of hydrogen nuclei.

Tracer:A radioisotope that is used to find or keep track

of molecules in an organism.Scientist can use tracers to follow where a

particular molecule goes in your body or to study how a particular organ functions.

Also used in agriculture to monitor the uptake of nutrients and fertilizers.

Radiation can be used to stop some types of cancerous cells from growing.Cancer: a group of cells in a person’s body

grows out of control and can form a tumor

Nuclear FissionOccurs when a neutron strikes a nucleaus,

causing it to split into smaller nucleiA chain reaction requires a critical mass of

fissionable materialNuclear Fusion

Nuclear fusion occurs when two nuclei combine to form another nucleus

Nuclear fusion occurs at temperatures of millions of degrees which occur inside the Sun.

Explain how a chain reaction can be controlled.

Describe two properties of a tracer isotope used for monitoring the functioning of an organ in the body.