Radioactivity & Nuclear ReactionsChapter 9 Notes

How Does This Connect to Previous Chapters?

While chemical changes involve changes in the electrons (ex : bonding), nuclear reactions involve changes to the nucleus and involve much larger energies than chemical reactions.

Atoms are made of particles• Protons (+)• Neutrons (no charge)• Electrons (-)

The Strong Force Holds the Nucleus Together

Particles in the nucleus are attracted together by the strong force

• Causes protons and neutrons to “stick together” While the nuclear force is extremely weak at

most distances, over the very short distances present in the nucleus the force is greater than the repulsive electrical forces among protons.

• Protons all have positive charges which should repel If it is not strong enough to keep large nuclei

together = this is what causes radiation!!

Forces in a Large Nucleus When the attractive nuclear forces

and repulsive electrical forces in the nucleus are not balanced, the nucleus is unstable

Large nuclei are unstable and often decay! (aka, radiation)

If protons are too far apart to be attracted by the strong force, they are repelled by the electric force between them.

STOP and REVIEW!! What charge do protons have?? ________

What two particles are in the nucleus? ___________ and ____________

What is the force that holds the nucleus together? _______________________

Can the strong force hold together a large nuclei? ____________

Positive

Protons Neutrons

Strong Force

No!

Radioactivity Through radioactive decay, the

unstable nucleus releases radiation in the form of very fast-moving particles and energy to produce a new nucleus, and changes the identity of the element

• These elements are known as radioactive Uses of radioactive isotopes:

• Medical applications The radiation they release can be used to kill

undesired cells (ex: cancer cells) Radioisotopes can be introduced into the body to

show the flow of materials in biological processes.

Discovery of Radiation You can’t hear, touch, see, taste, or

smell radioactivity (small amounts are all around you though!!!)

Henri Becquerel discovered radioactivity accidentally in 1896 when he left a uranium salt in his desk drawer with a photographic plate.

When he removed the plate and developed it, he found the outline of the clumps of the uranium salt. He hypothesized that the uranium had given off some sort of invisible energy • He called it radiation

Half-Life: All nuclei break down over time

Half-Life - a measure of the amount of time it takes for half the nuclei in a sample of an isotope to decay• Remember… an isotope is an element with same

number protons and different numbers of neutrons in the nucleus

For any radioisotope, the half-life is unique and constant.

Graphs can be made that show the amount of a radioisotope that remains as a function of time and can be interpreted to determine the value of the half-life

Each element has a different half-life

Example of Half-Life: Carbon Dating

Carbon-14 is used to find the age of once-living things.

When animals eat plants, they take in carbon-14. Carbon-14 decays into nitrogen-14

The half-life of carbon-14 is 5,730 years The amount of carbon-14 stays constant

in a living organism. Once the organism dies, the amount of carbon-14 decreases at a regular rate.

Half-Life Example

Carbon-14 has a half-life of 5,730 years. How much of a 100 g sample of carbon-14 will remain after:• 5,730 years? _____________• 11,460 years? ____________• 17,190 years? ____________• 22,920 years? ____________

50 g25 g12.5 g6.25 g

Nuclear Fission Nuclear Fission – process of

splitting a nucleus into two nuclei with smaller masses, releasing large amounts of energy

• Fission = divide Only atomic #s above 90 can

undergo nuclear fission Released energy can be used for

electricity or nuclear weapons

Chain Reaction A chain reaction is an ongoing series

of fission reactions This is how nuclear power plants

generate electricity

Nuclear Fusion Nuclear Fusion – 2 nuclei with low

masses are combined to form one nucleus of larger mass

• Fusion = together To occur, positively charged nuclei

must move very fast to overcome alike charge repulsion

• More kinetic energy = more temperature Must occur at very high

temperatures

Fusion is Found on Stars Found in the center of stars and the

sun• As the sun ages, there is less hydrogen

and more helium because of hydrogen combining

STOP and REVIEW! Which type of nuclear energy

combines elements together at very high temperatures? _______________________

Which type of reaction is shown below?

Nuclear Fusion

Nuclear Fission