1

Nuclear Changes

Physical Science

Chapter 10

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Radioactive decay

The spontaneous breaking down of a nucleus into a slightly lighter nucleus, accompanied by emission of nuclear radiation (particles, electromagnetic radiation, or both).

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Nuclear Radiation

Alpha particles, beta particles (positive or negative), and gamma rays.

Have different penetrating powers

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Alpha particles

Large mass (4 amu) and charge (+2). Can’t travel far in air Low penetrating power

Cannot penetrate skin Can be stopped by a sheet of paper

Harmful if ingested or inhaled

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Beta particles

Travel close to the speed of light Penetrate about 100 times as much as

alphas Can travel a few meters in air Can be stopped by lead or glass

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Gamma rays

Travel at the speed of light Greatest penetrating ability Can travel indefinitely through air or

empty space Can only be stopped by thick layers of

lead or concrete.

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Nuclear equations

The total of the atomic numbers and the total of the mass numbers must be equal on both sides of the equation.

Elements have atomic numbers 1 or greater

Neutrons have atomic numbers of 0 Electrons have atomic numbers of -1

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Examples

HeThU 42

23490

23892

PaeTh 23491

01

23490

RhePd 10045

01

10046

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Example

ClAr 3717

3718 _____

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You try

KrRb 8336

8337 _____

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You try

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You try

TlHe 20881

42_______

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Half-life

The amount of time it takes for half of a radioactive sample to decay

We can’t predict when an individual atom will decay, only the rate of decay for a large number of atoms.

There is a table on page 333.

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Radioactive dating

Determining the age of a substance based on the amount of radioactive nuclides present

Carbon-14 is used for organic materials up to 50 000 years old

Others used for older materials and minerals up to 4 billion years old

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Stability Protons repel each other through

electrostatic forces They attract each other (and also

neutrons) through nuclear forces – but only over small distances

More neutrons can increase attractive force without increasing repulsive force

Too many protons or neutrons makes the nucleus too big and unstable

Beyond bismuth (83), no stable nuclei exist

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Nuclear fission

A very heavy nucleus splits into more-stable nuclei

Mass of products is less than mass of reactants Releases enormous amounts of energy E=mc2

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Chain reaction

The material that starts the reaction is one of the products and can start another reaction.

Critical mass – minimum amount of nuclide that is needed to sustain a chain reaction

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Nuclear fusion

Light-mass nuclei combine to form a heavier, more stable nucleus

Releases more energy per gram of fuel than fission

Takes place in stars (including the sun) Hydrogen to helium

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Fusion requirements

High heat and pressure needed Right now, no known material can

withstand the initial temperatures (100 million K) needed for controllable manmade fusion.

Discuss

Section 2 review question #2 on page 342

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Background radiation

Natural radiation everyone is exposed to We are adapted to survive these low

levels of radiation

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rem

How radiation exposure is measured Up to 0.5 rem per year is considered

safe In the US, average yearly exposure is

0.1 rem.

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Radiation exposure damage

DNA mutations Cancer Genetic effects

Smoke detectors

Produce current by releasing alpha particles

Smoke particles reduce the current and set off the alarm

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Radioactive medicine

Used to treat cancer Used to detect cancer and other

diseases CT scans MRIs PET scans Radioactive tracers used to locate tumors

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Radioactive agriculture

Tracers can be used to determine water movement and fertilizer effectiveness

Radiation can be used to extend shelf life by killing bacteria and insects

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Nuclear waste containment

Waste can have a half life from a few months to thousands of years.

It must be contained to protect living organisms

Best sites for storage have low populations, little water at the surface or underground, and no earthquakes.

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Nuclear reactors

Use controlled fission chain reactions to produce energy.

Research is being done to try to build controlled fusion reactors.