Ch. 9: RADIOACTIVITY AND NUCLEAR REACTIONS

Section 1--RADIOACTIVITY

What is an atom? An atom is the smallest piece of

matter.

Ex. The element silver is composed of only silver atoms. The element hydrogen is composed of only hydrogen atoms.

Atoms are composed of protons, neutrons, and electrons.

Nucleus =

protons + charge

neutrons no charge (neutral)

Outside the nucleus =

electrons - charge

Atomic Number = # of protonsAtomic Mass = # of protons & # of neutrons

Element

Atomic #

Symbol

Atomic Mass

Uranium92

U

238.029

PROTONS The total amount of charge in a nucleus is

determined by the number of protons, which also is called the atomic number.

Atomic number = number of protons

ELECTRONS Negatively charged electrons are

electrically attracted to the positively charged nucleus and swarm around it.

An electron has a charge that is equal, but opposite to a proton’s charge.

Atoms usually contain the same number of protons as electrons.

# of protons = # of electrons

PROTONS AND NEUTRONS IN THE NUCLEUS

Protons and neutrons

are packed together tightly in the nucleus.

The region outside the nucleus in which the electrons are located is large compared to the size of the nucleus.

Nucleus/Atom VS. Marble/StadiumThe size of a

nucleus in an atom can be compared to a marble sitting in the middle of an empty college football stadium.

Where is the majority of an atom’s mass?

The nucleus contains almost all the mass of the atom, because the mass of one proton or neutron is

almost 2,000 times greater than the mass of an electron.

MASSESPARTICLE RELATIVE

CHARGERELATIVE MASS

ACTUAL MASS

ELECTRON 1- 11836

9.11 X 10-28

PROTON 1+ 1 1.674 X 10-24

NEUTRON 0 1 1.675 X 10-24

HOW ARE PROTONS AND NEUTRONS HELD TOGETHER SO TIGHTLY IN THE NUCLEUS?

Usually only positive (+) and negative (-) charges attract, but another force, called the STRONG FORCE, causes protons and neutrons to be attracted to each other.

(STRONG FORCE HOLDS NUCLEUS TOGETHER)

FOUR BASIC FORCES 1) GRAVITY

2) ELECTROMAGNETIC

3) STRONG (NUCLEAR) The strong force is one of the four basic forces and is about 100 times stronger than the electric force.

4) WEAK (NUCLEAR)

SHORT-RANGE FORCE Protons and neutrons have to be close together, like

they are in the nucleus, to be attracted by the strong force (SHORT-RANGE FORCE)

(ONLY WORKS OVER SHORT DISTANCES—small atoms)

LONG-RANGE FORCE The electric force is a long-range force, so protons

that are far apart still are repelled by the electric force.

(ONLY WORKS OVER LONG DISTANCES—LARGE ATOMS)

STRONG FORCE

FORCESSTRONG VS. ELECTRIC

RADIOACTIVITY SMALL ATOMS

SF >(greater) EF

STABLE

LARGE ATOMS

EF >(greater) SF

Unstable

Particles repel; give off matter and energy.

This process of nuclear decay is called radioactivity.

LARGE ATOMS LARGE nuclei tend to be UNSTABLE and can break

apart or decay.

ALL nuclei that contain more than 83 protons ARE radioactive (ATOMIC NUMBER >greater than 83)

ISOTOPES Nuclei that have the same number of protons, but

different number of neutrons are called isotopes.

EXAMPLE—

U-235, U-238

Same element (URANIUM) At. # = 92

Different # of neutrons (U-235 = 143 n)best

(U-238 = 146 n)

CHAPTER 9: RADIOACTIVITY AND NUCLEAR REACTIONS

Section 2—NUCLEAR DECAY

WHAT IS NUCLEAR RADIATION?

When an UNSTABLE nucleus decays, particles and energy are emitted from the decaying nucleus.

These particles and energy are called NUCLEAR RADIATION. (Also known as NUCLEAR DECAY AND RADIOACTIVITY)

WHAT ARE THE 3 TYPES OF NUCLEAR RADIATION?

The 3 types of nuclear radiation are alpha, beta, and gamma.

Alpha and beta radiation are particles (matter)

Gamma radiation is not a particle; it behaves like a wave; similar to light; electromagnetic radiation; most dangerous

ALPHA PARTICLES Made of protons and

neutrons

Cannot pass through low density materials, such as a sheet of paper

SMOKE DETECTORS some give off alpha particles; smoke disrupts process and alarm goes off (Americium-241)

What happens when an atom no longer has the same number of protons?

It is no longer the same element. (The number of protons identifies the element as a particular element)

Transmutation is the process of changing one element to another through nuclear decay.

TRANSMUTATION

BETA PARTICLESMade of protons, neutrons, and

electrons

Caused by another basic force—the WEAK FORCE

Much faster and can pass through low density materials, such as paper, but are stopped by aluminum foil

GAMMA RAYSThey are NOT made of protons,

neutrons, or electrons

They are a form of radiation called electromagnetic waves

They are able to travel through low and medium density materials, such as paper and aluminum foil; but are blocked by high density materials, such as lead and concrete

Is RADIATION harmful? Damage from alpha and beta particles and

gamma rays can cause cells in your body to quit functioning properly, which can lead to illness and disease.

How does someone protect themselves?

1. TIME (around material)

2. DISTANCE (from material)

3. SHIELDING (safety equipment)

4. ROUTINE MONITORING

RADIOACTIVE HALF-LIFEThe half-life of a radioactive isotope

is the amount of time it takes for half of the nuclei in a sample to decay.

Some decay to stable atoms in less than a second.

Some require millions of years to decay.

RADIOACTIVE DATING Some geologists, biologists, and

archaeologists are interested in the ages of ROCKS and FOSSILS found on Earth.

CARBON DATING –carbon-14 is often used to find the ages of objects once living. (Also, potassium-40)

URANIUM DATING –uranium-235 can be used to estimate the ages of rocks.

CHAPTER 9: RADIOACTIVITY AND NUCLEAR REACTIONS

Section 3: DETECTING RADIOACTIVITY

Because you can’t see or feel alpha and beta particles or gamma rays, you must use instruments to detect their presence.

A Geiger counter is a device that measures radioactivity by producing an electric current when radiation is present. The intensity of radiation present is determined by the number of clicks or flashes of light per second.

BACKGROUND RADIATIONThis type of radiation is not

produced by humans. It comes mainly from Earth’s rocks, soils, and atmosphere (building materials—bricks, wood, and stones; food, water, and air)

RADON GAS The largest source of

background radiation comes from radon gas. Radon is produced in Earth’s crust by the decay of Uranium-238. Radon gas can seep into houses and basements from the surrounding soil and rocks.

CHAPTER 9: Radioactivity and Nuclear Reactions Section 4: NUCLEAR REACTIONS

What is the first step in a game of pool? A similar result occurs

when a neutron is shot into the large nucleus of a uranium-235 atom. The nucleus is split.

The process of splitting 1 nucleus into 2 nuclei with smaller masses is called nuclear fission. The word “fission” means to divide.

FISSION RHYMES WITH DIVISION

FISSION

The total mass of the products is slightly less than the mass of the original nucleus and the neutron. The small amount of missing mass is converted into a tremendous amount of energy.

CHAIN REACTION

A chain reaction is an ongoing series of fission reactions.(Billions of reactions can occur each second)

↑ REACTIONS = ↑ ENERGY

Nuclear fission is used to generate electricity and is used in nuclear weapons.

The critical mass is the amount of fissionable material required so that each fission reaction produces approximately 1 more fission reaction (keeps chain reaction going—so won’t die out or grow out of control).

NUCLEAR FISSION

U-235 VS. DYNAMITE U-235 (1 nucleus) Dynamite (1 molecule)

U-235 30 million times more energy

U-235 VS. COAL1 gram of U-235 2 tons of COAL

NUCLEAR FUSION Even more energy

can be released in another type of nuclear reaction called nuclear fusion.

In nuclear fusion, 2 nuclei with low masses are combined to form one nucleus of larger mass.

NUCLEAR FUSION of the Sun

Extremely high temperatures are found in the center of stars, including the Sun.

These high temps (millions of degrees Celsius) are related to nuclear fusion.

NUCLEAR FUSION of the Sun

A small amount of mass is changed into an enormous amount of ENERGY.

Earth receives a small amount of this energy as HEAT and LIGHT.

As the Sun ages, HYDROGEN nuclei are converted to HELIUM. (ONLY 1% OF SUN’S MASS HAS BEEN CONVERTED TO ENERGY—ENOUGH FOR 5 BILLION MORE YEARS)

Nuclear fission—splits nuclei apart (RHYMES WITH DIVISION)

FUSION VS. FISSIONNUCLEAR FUSION—FUSES NUCLEI TOGETHER (SUN)