Discovering Radioactivity

• French scientist Henri Becquerel (in 1896)

discovered that a mineral he was testing was

fluorescent. It gave off energy without a light. He

concluded that it came from uranium in the mineral.

• This energy is called nuclear radiation (high

energy particles & rays that are emitted by the

nuclei of some atoms).

• Marie Curie (working with Becquerel) named the

process radioactivity (aka radioactive decay).

Kinds of Radioactive Decay

• During radioactive decay, an unstable

nucleus gives off particles and energy.

• 3 kinds are

– Alpha decay

– Beta decay

– Gamma decay

Alpha Decay

– The release of an alpha particle from a nucleus.

– The alpha particle is made up of 2 protons and 2

neutrons. It has an atomic number of 2 and a

charge of 2+.

– An alpha particle is the same as the nucleus of a

helium atom.

– Many large radioactive nuclei give off alpha

particles and become nuclei of atoms of different

elements.

Conservation in Alpha Decay

• The mass number is conserved.

– The sum of the mass numbers of the starting

materials is always equal to the sum of the mass

numbers of the products.

• Charge is conserved.

– The sum of the charges of the starting materials

is always equal to the sum of the charges of the

products.

Beta Decay

• It is the release of a beta particle from a

nucleus.

• A beta particle can be an electron (1- charge)

or a positron (1+ charge).

• Its mass number is 0 because it has no

protons or neutrons.

• Two Types: carbon -14 and carbon -11.

– Both the mass number and the charge are

conserved.

Gamma Decay

• Energy given off during alpha decay and

beta decay in the form of light that has a very

high energy is called gamma rays.

• The release of gamma rays from a nucleus is

called gamma decay.

• It happens as the particles in the nucleus

shift places.

• They have no mass or charge.

The Penetrating Power of Radiation

• The difference in particles is due to mass and charge.

Effects of Radiation on Matter

• Atoms that are hit by nuclear radiation can

give up electrons.

• Chemical bonds between atoms can break

when hit by nuclear radiation.

• Both of these things can cause damage to

living and nonliving matter.

Damage to Living Matter

• When an organism absorbs radiation, its

cells can be damaged.

– Radiation can cause burns.

– Radiation sickness has symptoms and results of

overexposure are: fatigue, loss of appetite, hair

loss, destruction of blood cells, can result in

death.

– Radiation can increase risk of cancer.

Damage to Nonliving Matter

• Metal can be weakened due to the loss of

electrons.

• This can cause damage to buildings.

• Radiation from the sun can cause damage to

spacecraft.

Finding a Date by Decay

• Carbon atoms are in all living things.

• A small percentage of these atoms is

radioactive carbon-14 atoms.

• Atoms are replaced that decay in a living

organism.

• When an organism dies, the carbon-14 is no

longer replaced; the levels of it drops due to

radioactive decay.

Finding a Date by Decay

• Scientists have found that for every 5,730 years, ½ of

the carbon-14 in a sample decays.

• The rate of decay is constant; not changed by

temperature or pressure.

• Each radioactive isotope has its own rate of decay,

called a half-life.

• A half-life is the amount of time it takes ½ of the nuclei

of a radioactive isotope to decay.

• Carbon-14 can be used to find the age of objects up to

50,000 years old.

Radioactive Decay & Half-Life

Examples of Half-Lives

Isotope Half-life Isotope Half-life

Uranium-238 4.5 billion years Polonium-210 138 days

Oxygen-21 3.4 s Nitrogen-13 10 min

Hydrogen-3 12.3 years Calcium-36 0.1 s

Calculating Decay

1. Determine number of half lives in given problem.

2. Divide the sample by two for each half life.

Examples:

• Paper tear demonstration

• Start = 100 kg

• First half life = 50 kg

• Second half life = 25 kg

• Third half life = 12.5 kg

• Fourth half life = 6.25 kg

Uses of Radioactivity

• Smoke detectors use a small amount of radioactive

material.

• Radioactive materials are used to treat cancer and

to sterilize food and healthcare products.

• Tracers are radioactive elements whose paths can

be followed through a process or reaction.

– Doctors use them to help diagnose medical

problems.

• Can help find defects in structures: thickness of

metal sheets as they are made; leaks in pipes.

Radioactivity Video

Nuclear Fission

• The process by which a large nucleus splits

into 2 small nuclei and releases energy.

• Some uranium atoms’ nuclei and other large

atoms can undergo nuclear fission naturally.

• Some large atoms can also be forced to

undergo fission by hitting the atoms with

neutrons.

Fission of a Uranium-235 Nucleus

Notice: the number of neutrons is the same

before and after the fission reaction.

Fission of Uranium-235

Notice: the number of neutrons is the same

before and after the fission reaction.

Energy from Matter

• Matter can be changed into energy.

• The total mass of the reactants is slightly

less than the total mass of the products.

Some of the matter was converted into

energy.

Nuclear Chain Reactions

A nuclear

chain

reaction

is a

continuous

series of

nuclear

fission

reactions.

Energy from a Chain Reaction

• In an uncontrolled chain reaction, huge

amounts of energy are given off very quickly.

– Ex. An atomic bomb

• Nuclear power plants use controlled chain

reactions to generate electricity.

How a Nuclear Power Plant Works

Fission

Advantages Disadvantages

• Nuclear power plants

often cost less to run than

fossil fuel plants.

• They also do not release

gases into the

atmosphere.

• They allow for fossil fuels

to last longer.

• The risk of an accident at a

nuclear power plant.

• Nuclear waste; the waste

will give off levels of

radiation for 1000s of

years. It has to be stored.

• Nuclear power plants cost

more to build than plants

that use fossil fuels.

• The supply of uranium is

limited.

Nuclear Fusion

• Nuclear Fusion is when two or more nuclei

that have small masses combine, or fuse, to

form a larger nucleus. This is another nuclear

reaction in which matter is converted into

energy.

Nuclear Fusion

• In order for fusion to happen, the repulsion between

positively charged nuclei must be overcome;

– needs temperatures more than 100,000,000OC;

– happens in the state of matter plasma

• (Plasma is made up of ions and electrons.)

– These temperatures are found on the Sun.

Fusion

Fusion

Advantages Disadvantages

• The concerns of an explosion if used as in a power plant are much lower. Very little radioactive material would be released.

• Fusion products are not radioactive.

• Products are much more common.

• It releases more energy per gram of fuel than a fission reaction does.

• A “cleaner” source of energy than fission.

• It cannot yet be generated to use in your homes.

• Scientists need more money for research to begin using this resource.

• Very high temperatures are needed.