Nuclear Changes

Section 1: What is Radioactivity?

Vocabulary

Radioactivity Nuclear radiation Alpha particle Beta particle Gamma Ray Neutron emission Half-life

Nuclear Radiation

To discuss radiation, we need to define radioactivity

Radioactivity is a PROCESS by which an unstable nucleus emits one or more particles or energy in the form of electromagnetic radiation

This means two things can happen because of radioactivity

1. The nucleus of an atom gives off some particle

2. Energy (electromagnetic energy) is given off by the atom

Changes in Nucleus

Radioactive materials have unstable nuclei These nuclei undergo changes by emitting particles

or radiation After the changes in the nucleus, the element CAN

transform into a new element entirely But doesn’t always

This process of changing the nucleus is called nuclear decay

The released energy and matter from nuclear decay is called nuclear radiation CAN cause damage to living tissue

Types of Nuclear Radiation

There are four types of nuclear radiation Alpha particles Beta particles Gamma rays Neutron emission

After a radioactive atom decays, the nuclear radiation leaves the nucleus of the atom

This nuclear radiation then interacts with nearby matter The result of this interaction depends on the nuclear

radiation itself

Alpha Particles,

Alpha particles are positively charged and more massive than any other type of nuclear radiation Made of 2 protons and 2 neutrons from the unstable nucleus Is effectively the nucleus of a helium atom

Alpha particle don’t travel far through other materials Will barely pass through a sheet of paper This is because it’s so massive

Because they have a positive charge, they remove electrons (ionize) matter as they pass through As they ionize material they lose energy and slow down

even more

He42

Beta Particles, Beta particles are fast moving electrons

Travels farther through matter than alpha particles Still comes from the nucleus How this works

A neutron (neutral) decays to form a proton and an electron The electron is then ejected from the nucleus The proton stays in the nucleus (making this atom a new

element) Will penetrate paper, but can be stopped by 3mm of aluminum

foil or 10 mm of wood. Like alpha particles, will also ionize matter they pass through And as they ionize matter, they lose energy and slow down

e01

Gamma Rays Discovered by Marie Curie in 1898 Gamma rays are high energy electromagnetic

radiation emitted by a nucleus during radioactive decay. Much more penetrating than beta particles Not made of matter like alpha and beta particles

Does not possess a electrical charge Because of no electrical charge, does not easily

ionize matter it passes through Damages materials due to high energy, not due to

ionization Because they don’t ionize matter, not easily stopped Can penetrate up to 60 cm of aluminum or 7 cm of lead

Neutron Emission

Neutron emission is the release of high-energy neutrons by some neutron-rich nuclei during radioactive decay Scientists actually discovered the neutron by neutron

emission Because neutrons have no charge, they do not ionize

matter like alpha and beta particles Since they don’t ionize, they don’t waste their energy ionizing So neutrons will travel farther through matter than either

alpha or beta particles A block of lead about 15 cm thick is required to stop most

fast moving neutrons during radioactive decay.

n10

Nuclear Decay

When an unstable nucleus emits and alpha or beta particle, the number of protons and neutrons changes. Example: radium-226 changes into radon-222 by

emitting an alpha particle

Alpha Decay

A nucleus will give up two protons and two neutrons during alpha decay We can write the nuclear decay process like a

chemical equation The nucleus before the decay is like a reactant

Placed on the left side of the equation The nucleus after the decay is like a product

Placed on the right side of the equation The particle emitted also treated like a product

Example Equation

Radium-226 Radon-222

Hint on reading these Number on top is the mass number

Number of protons + neutrons Number on bottom is the atomic number

Number of protons

Has mass been conserved here? Yes

HeRn Ra 42

22286

22688

Summary of Alpha Decay

During alpha decay The mass number (top #) goes down by 4 The atomic number (bottom #) goes down by 2 Will always have the alpha particle as part of the

products.

Beta Decay

During beta decay, a nucleus GAINS a proton and loses a neutron This means that in total, the mass number (top #) stays

the same Atomic number (bottom #) increases by one

e01147

146 NC

Gamma Ray Decay

When a nucleus undergoes nuclear decay by gamma rays, there is no change to the atomic number

Only the energy of the nucleus changes

Nuclear Decay Practice-Determine: A, Z, X and type of decay

XAZ CB 126

125

XAZ FrAc 22187

22589

XAZ eNi 01-

6328

XAZ HeBi 42

21283

Radioactive Decay Rates

It is impossible to predict the moment when any particular nucleus will decay But it is possible to predict the time it takes for half

of the nuclei in a given sample to decay The time it takes for half of a sample of

radioactive nuclei to decay is called the sample’s half-life.

Half-Life

Half-life is just a certain amount of time Each substance has a unique half-life

After the first half life of a sample has passed, half of the sample will remain unchanged

After the second half life of a sample has passed, half of the half decays leaving only a ¼ of the original sample unchanged

After 3rd half-life has passed, half of the ¼ remains (or 1/8) and so on and so forth

Half-Life is a Measure of How Quickly a Substance Decays

Different radioactive isotopes have different half-lives Half-lives can be as small as nanoseconds to

billions of years (like Uranium-238) The length of the half-life depends on the stability

of the nucleus The more stable the nucleus is, the longer the

half-life

Use of Half-Life

If you know how much of a particular radioactive isotope has present at the start, we can predict how old the object is Geologists know the half-life of long-lasting

isotopes, like potassium-40 They use the half-lives of these isotopes to

calculate the age of rocks Potassium-40 decays into argon-40 So the more argon-40 there is compared to the

potassium-40, the older the rock is

Archaeologists us the half-life of carbon-14 to date more recent materials Remains of animal or fibers from recent clothing Can only be used to date once-living things The ratio of carbon-14 (radioactive) to carbon-12

(stable) decreases with time So the more carbon-14 to carbon-12 there is, the

newer the once-living organism is

Nuclear Changes

Section 2 – Nuclear Fission and Fusion

Vocabulary

Strong nuclear force Fission Nuclear chain reaction Critical mass Fusion

Nuclear Forces

Protons and neutrons are tightly packed into the tiny nucleus of an atom Certain nuclei are unstable, and undergo decay Elements can have stable and unstable isotopes

Carbon-12 is stable while carbon-14 is not The stability of an nucleus depends on the

nuclear forces holding the nucleus together. These forces act between the protons and the

neutrons

Nuclei are held together by a special force

We know that like charges repel Took scientists a while to determine how so

many positively charged protons fit into an atomic nucleus without flying apart

Answer is the strong nuclear force The strong nuclear force is the interaction that

binds protons and neutrons together in a nucleus This attraction is MUCH stronger than the

repulsion between the protons But force only occurs over very short distances

(about the width of 3 protons)

Neutrons contribute to nuclear stability

Due to the strong nuclear force, neutrons and protons in a nucleus attract other protons and neutrons

Because neutrons have no charge, they don’t repel anything Whereas the protons, having charge, repel other

protons In a stable nuclei, the attractive forces are

stronger than the repulsive forces

Too many neutrons or protons

While neutrons help hold a nucleus together, too many neutrons makes a nucleus unstable Nuclei with more than 83 protons are always

unstable, no matter how many neutrons they have These nuclei will always decay Will always release large amounts of energy and

nuclear radiation

Nuclear Fission

The process of the production of lighter nuclei from heavier nuclei is called fission During fission, a nucleus splits into two or more smaller

nuclei, releasing neutrons and energy

Note that this fission reaction was started by firing 1 neutron at the Uranium-235 nucleus

Also note we produced more neutrons, energy, and 2 different, lighter nuclei

energyn15KrBanU 10

8436

13756

10

23592

Energy is released during nuclear fission

Each dividing nucleus releases about 3.2x10-

11 J of energy Compare this with TNT, which releases only

4.8x10-18J per molecule During fission reactions, a small amount of

mass is lost after the reaction That mass was converted into energy E = mc2

Neutrons released by fission can start a chain reaction

A nucleus that splits when it is struck by a neutron forms smaller nuclei Smaller nuclei need fewer neutrons, so excess

neutrons are emitted Excess neutrons can collide with another

large nucleus Triggering another nuclear reaction Which releases more neutrons, and so on and so

on

Generally each nucleus split will generate about 3 neutrons So each nucleus that fissions will cause 3 other

nuclei to fission This generates a nuclear chain reaction

A series of fission processes in which the neutrons emitted by a dividing nucleus cause the division of other nuclei.

The more nuclei released, the bigger the chain reaction will be

Nuclear Fission of Uranium-235

Figure 18

Figure 18

Figure 18

Figure 19

Figure 19

Figure 19

Figure 19

Figure 19

Figure 19

Figure 19

Figure 19

Figure 19

Animation

Critical Mass

There has to be enough of the fissionable material in order for a chain reaction to occur The minimum mass of a fissionable isotope in

which a nuclear chain reaction can occur is called the isotope’s critical mass

This is how they make nuclear bombs

Nuclear Fusion

Just like energy is obtained when nuclei break apart, energy can be obtained when very light nuclei are combined to form a heavier nuclei This type of nuclear process is called fusion

Its how stars make energy Requires LARGE amounts of energy