chapter 20 chemistry grades 5 8

The Atomic Nucleus

20-1 Radiation• Objectives:• Identify the three types of radiation• Describe how radiation is produced.

History of radioactivity• French physicist, Antoine Henri Becquerel discovered that

uranium compounds gave off invisible rays. • He told Marie Curie, he asked her to isolate , or separate

the new element. • Currie and her husband Pierre, began experiments in their

home laboratory. After a few months, the isolated an element they called polonium. Several months later they isolated radium.

• They discovered that these two elements give off invisible particles, or rays – a quality they named radioactivity.

• Later, Ernest Rutherford studied radium, and found that the radiation, or energy released from radioactive substances exists in three forms: alpha particles, beta particles, and gamma rays.

Alpha Particles• Alpha particles are charged atomic particles. • Each particle is made up of two protons and two neutrons

from the nucleus. • Relatively large, so it does not penetrate materials very

far. • It can be stopped by a sheet of paper or aluminum foil or

5cm of air.• When an alpha particle leaves an atom, the atom changes

into another type of atom.

Beta Particles• Beta particles are negatively charged particles

from the atom.• They are smaller than alpha particles and have only one

negative charge.• Beta particles have 100 times more penetrating ability

than alpha particles. • Require 2 in of aluminum foil to stop them• The loss of beta particles also changes the first atom into

another type of atom

Gamma Rays• Gamma rays are not charged particles. • They are just very high-energy X-rays. • Gamma rays have more penetrating ability than alpha or

beta particles. • The can pass through a foot of concrete or through the

human body.• Over exposure to gamma rays is harmful to living things

because the rays can hit cells inside the body of an organism and destroy them.

20-2 Half-life• Objective• Explain half-life

Half-life• The nucleus of each radioactive element gives off a

random number of particles, or rays, in a fixed amount of time.

• This process is called radioactive decay.• This time it takes for half the atoms of a sample to break

down or decay is called half-life.• EXAMPLE:• It takes 20 minutes for half the atoms of Carbon 11 to break

down.• Carbon 11 has a half-life of 20 minutes. There are only 4 grams of

Carbon 11 left. It has changed into another type or atom (boron II)• This change has occurred because the atoms have lost alpha and

beta particles. • Different radioisotopes have different half-lives.

20-3 Fission and Fusion• Law of Conservation of Mass-Energy• States that matter can change into energy, and energy

can change into matter.• During these changes any amount of matter lost it

changed into energy, and any energy lost is changed into matter• First proposed by Albert Einstein, E=mc²

Fission• In 1939 scientists discovered that the nuclei of uranium

atoms could be split into smaller nuclei in a process called fission

• Fission is accomplished by hitting the nuclei with slow-moving neutrons.

• There is no change in the total number of protons and neutrons, there is a change in the total mass

• The small nuclear mass that is missing changes into energy, called nuclear energy.

• A chain reaction is a continuous series of nuclear fissions• Controlled chain reactions in power plants allow us to

capture the energy given off during nuclear reactions without an explosion

Fusion• Fusion puts atoms together, or fusing them. • A small loss of mass, which comes out of the

reaction as a large amount of energy. • Very high temperatures of about 10 million degree

Celsius are necessary for fusion to take place.

20-4 Nuclear Energy• Objectives:• Identify the dangers associated with nuclear energy• Identify the benefits of nuclear energy

Nuclear Energy• Nuclear Reactors – produce large amounts of

energy using small amounts of fuel.• Businesses use nuclear energy to check structures

and manufactured goods for flaws or weak points. • Scientific researchers in the fields of biology and

paleontology use nuclear energy to learn more about what they study.

• Medicine uses nuclear energy to diagnose and treat a wide variety of problems and illnesses.

A proton accelerator uses nuclear particles to bombard cancerous tissue and destroy it.