Chapter 18: Radioactivity

& Nuclear Reactions

Radioactivity

Types of Radiation

Nuclear Reactions

Half Life

Radioactivity

The word radioactivity

was first used in the 1890s by

Marie Curie.

She used the word radioactivity to describe

the property of certain substances to give

off invisible “radiations” that could be

detected by films.

Radioactivity

We now know that

radioactivity comes from the

nucleus of the atom.

If the nucleus has too many

neutrons, or is unstable for

any other reason, the atom

undergoes radioactive

decay.

Radioactivity

Unstable atoms will continue to undergo

radioactive decay until they form stable atoms.

(Change into atoms of another element.)

The word decay means to "break down.”

Radioactive decay gives off energy.

Nuclear Reactions and Energy

A nuclear reaction is any process that changes

the nucleus of an atom.

Radioactive decay is one form of nuclear

reaction.

Types of Radiation

Scientists quickly learned that

there were 3 different kinds of

radiation given off by radioactive

materials.

The scientists called them “rays”

because the radiation carried

energy and moved in straight lines,

like light rays.

Types of Radiation In Alpha decay, the nucleus ejects two protons and

two neutrons.

Beta decay occurs when a neutron in the nucleus

splits into a proton and an electron.

Gamma decay is not truly a decay reaction in the

sense that the nucleus becomes something different.

Alpha Radiation

It decayed to an alpha

particle and U-235. This is the original

“unstable” atom.

Number of Protons Number of Protons

An alpha particle is

equivalent to a helium-4

nucleus

This is an

example

of a nuclear

equation.

Fusion Reactions

A fusion reaction is a

nuclear reaction that

combines, or fuses, two

smaller nuclei into a larger

nucleus.

It is difficult to make fusion

reactions occur because

positively charged nuclei

repel each other.

Fission reactions

A fission reaction splits up

a large nucleus into smaller

pieces.

A fission reaction typically

happens when a neutron

hits a nucleus with enough

energy to make the nucleus

unstable.

Examples of fusion and fission.

FISSION Fission is used to produce energy for nuclear bombs, but we also use fission peacefully everyday to produce energy in nuclear power plants

FUSION Fusion is the process that takes place in stars like our Sun. Whenever we feel the warmth of the Sun and see by its light, we are observing the products of fusion.

Half-Life • The “half-life” (h) is the time it takes for

half the atoms of a radioactive substance to decay.

• For example, suppose we had 20,000 atoms of a radioactive substance. If the half-life is 1 hour, how many atoms of that substance would be left ….

10,000 (50%)

5,000 (25%)

2,500 (12.5%)

1 hour (one lifetime) ?

2 hours (two lifetimes) ?

3 hours (three lifetimes) ?

Time #atoms

remaining

% of atoms

remaining

1. What is the percentage of radioactive

nuclei left after 3 half-lives pass?

Half-Life Practice

2. The half-life of Rn-222 is 3.823 day. If the original mass of a sample of this isotope is 0.20 g, how much of it remains after 7.646 days?

Notice how the decays are fast and furious at the beginning and slow down over time

Half-Life Graph

What is the half life of this isotope?

Decay Chain