Chapter 9: Radioactivity and Nuclear Reactions

The last chapter we will study!

Discovery of Radioactivity

A French scientist named Henri Becquerel accidentally discovered radioactivity in 1896.

He left some uranium salt that he had been working with on top of some photographic plates in a desk drawer.

When he later developed his photographic plate, he found shadowed outlines of the uranium clumps he had been working with on the images on the photographs.

He hypothesized that uranium had given off some “invisible energy” (radiation) that had exposed the film.

The Strong Force

How can protons be held tightly together in the nucleus if like charges repel?

The strong force is a type of force that causes the protons in the nucleus to not repel each other.

The strong force is one of the four basic types of forces (gravitational, electromagnetic and weak forces are the others) and is about 100 times stronger than the electromagnetic force (causes repulsion).

The only limitation of the strong force is that it is a short-range force and to work, the protons and neutrons have to be very close together.

When protons are separated by distance, the electrical force takes over and causes like charges to repel.

The Strong Force

How can protons be held tightly together in the nucleus if like charges repel?

The strong force is a type of force that causes the protons in the nucleus to not repel each other.

The strong force is one of the four basic types of forces (gravitational, electromagnetic and weak forces are the others) and is about 100 times stronger than electric force.

The only limitation of the strong force is that it is a short-range force and to work, the protons and neutrons have to be very close together.

When protons are separated by distance, the electrical force takes over and causes like charges to repel.

Radioactivity

When the strong force is not great enough to hold the nucleus together, the nucleus can decay.

Decay means to give off matter and energy.

The process of nuclear decay is called radioactivity.

Nuclear Radiation

When an unstable nucleus decays, particles and energy are emitted.

Three types of nuclear radiation emitted are:

1. Alpha radiation

2. Beta radiation

3. Gamma radiation

Alpha Particles

When alpha radiation occurs, an alpha particle is emitted from a decaying nucleus.

An alpha particle is made up of two protons and two neutrons.

An alpha particle is the same as a helium atom without electrons.

An alpha particle has a charge of +2 as it has two protons.

Alpha Particle Effects

Alpha particles are massive compared to Beta and Gamma radiation.

They lose energy quickly due to this mass.They are the least penetrating form of

nuclear radiation as they can not even pass through a sheet of paper.

Alpha particles are dangerous to your body mainly if they are released internally as they can cause illness and disease.

Giving off Alpha Particles

Smoke detectors work because they release alpha particles inside the detector.

If smoke enters this device, the smoke absorbs the ions and electrons present.

Smoke causes a broken circuit (open) and causes an alarm to sound.

Transmutation

When an atom gives off an alpha particle, it undergoes a process called transmutation.

Transmutation is the process of changing from one element into another.

With alpha radiation, the original element has lost two protons, it now changes into the element on the periodic table that has an atomic number that has two less protons than the original atom.

For example, if Polonium-210 gives off an alpha particle, it will then become Lead-206 (plus the alpha particle emission).

Beta Particles

Beta decay involves an unstable nucleus where a neutron decays into a proton and thereby emits an electron.

Since neutrons are neutral, the process of changing into a proton involves giving off the negative charge.

Beta particles penetrate better than alpha particles, but will still not penetrate a sheet of aluminum foil.

Beta particles are primarily dangerous to us if they are emitted inside the body.

Beta Transmutation

Transmutation also occurs in Beta decay. Since a neutron changes into a proton, the

atomic number of the atom increases by one. So if Iodine-131 undergoes beta decay, it

becomes Xenon-131. Notice that the atomic mass did not change as the mass was not effected by a shift from a neutron to a proton.

Since beta particles are made up of electrons, they have a charge of -1.

Gamma Rays

Gamma rays are the most penetrating form of radiation.

It is not formed from protons, neutrons or electrons but comes from electromagnetic waves.

They have no mass or charge but travel at the speed of light.

It takes concrete or lead to stop the penetration of a gamma ray, but they are far less damaging to living tissue than alpha or beta particles.

Radioactive Half-Life

The measure of the time it takes for a radioactive material to decay is called its half-life.

A half-life is how long it takes for half of the nuclei in a radioactive sample to decay.

Half-lives vary greatly. Some materials may have a half-life of less than a second and some may last thousands of years.

The main thing about half-lives is that the radiation never fully goes away. There is always a residual radiation because only half can decay per half-life period.

Radioactive Dating

Carbon dating is a method of dating the age of an object that was once living. The half-life of Carbon-14 is 5,730 years, so we

can date things over many thousands of years.Uranium Dating is a method of dating the

age of rocks. Two different radioactive forms of Uranium are

checked in the calculation and rocks can be dated to be billions of years old.