CHAPTER 7 NUCLEAR CHANGES Chapter 7 Section 1: Radioactivity What is radioactivity? ◦ Radioactivity is the process by which an unstable nucleus emits.

CHAPTER 7CHAPTER 7

NUCLEAR CHANGESNUCLEAR CHANGES

Chapter 7 Section 1:Chapter 7 Section 1:RadioactivityRadioactivityWhat is radioactivity?

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

Nuclear RadiationNuclear RadiationRadioactive materials have

unstable nuclei

Nuclei change by releasing energy.

Elements can transform into isotopes of the same element or into entirely different elements

Nuclear DecayNuclear DecayAs elements “change” or

transform they undergo nuclear decay.◦Carbon dating uses rates of nuclear

decay

The released energy is called Nuclear Radiation◦Can cause damage to living tissue

When a radioactive atom decays, the nuclear radiation leaves the nucleus.

The radiation is now able to react with any surrounding matter

The reaction depends on the charge, mass, & energy of the nuclear radiation

Types of Nuclear RadiationTypes of Nuclear Radiation4 types of nuclear radiation:

◦Alpha Particles

◦Beta Particles

◦Gamma Rays

◦Neutron Emission

Alpha Particles Alpha Particles Positively charged & more massive than any other type of nuclear radiation

Discovered by Ernest Rutherford (also discovered the nucleus)

Consist of 2 protons & 2 neutrons

Alpha ParticlesAlpha ParticlesDo not travel far through materials

(can barely pass through paper)

The massive size of alpha particles limits their movement

Remove electrons from surrounding matter as they pass through it (_______________)

Ionization causes alpha particles to slow down

Beta Particles Beta Particles Fast-moving Electrons produced by neutron decay

Negatively charged particles

Travels farther through matter compared to alpha particles

How can electrons come from How can electrons come from a positively charged nucleus?a positively charged nucleus?

In the 1930’s, it was discovered that as a neutron (neutral charge) decays it forms a proton & electron

The electron has very little mass and gets ejected from the nucleus at a high speed as a Beta Particle

Gamma RaysGamma RaysVery High Energy Light

1898- Marie Curie isolated radioactive radium

1900- Paul Villiard discovered that radium emitted a new type of nuclear radiation…

Gamma Rays!Gamma Rays!More penetrating than Beta particles

Gamma rays are not made of matter & do not have an electric charge

Gamma rays are a form of electromagnetic energy

Have more energy than X-rays

Do not ionize surrounding matter

Can penetrate 60cm of aluminum & 7 cm of lead!

Not easily stopped by clothing or building materials◦Can be very harmful

Neutron EmissionNeutron EmissionNeutron Emission is the release of a high-energy neutron by some neutron-rich nuclei during radioactive decay.

Emitted from an unstable nucleusDo not ionize surrounding matterTravel farther through matter than or

Nuclear DecayNuclear DecayWhen an unstable nucleus emits alpha or beta particles, the # of protons or neutrons changes.

Example: Radium-226 changes to Radon-222 by emitting an alpha particle

A nucleus gives up 2 protons & A nucleus gives up 2 protons & 2 neutrons during 2 neutrons during alpha alpha decaydecayNuclear decay processes can be

written similar to a chemical reaction equation

The nucleus before the decay is like the reactant

The nucleus after the decay is like the product

A nucleus gains a proton & A nucleus gains a proton & loses a neutron during loses a neutron during beta decaybeta decayEquation is formed the same way except the symbol for a beta particle is used.

In all cases of beta decay, the mass number before & after the decay doesn’t change

When writing the equation…Write the original element on the

left side (reactants)

X is used for the unknown symbol A is used for the unknown massZ is used for the unknown atomic

#

Solve for the unknowns & rewrite the balanced equation

Radioactive decay ratesRadioactive decay rates

A substance’s half-life is the time in which half of the radioactive substance decays(A measure of how quickly a substance decays)

After each half-life passes half of the sample remains unchanged.

Practice ProblemPractice ProblemRadium has a half-life of 1599 years. How long would it take seven-eights of radium-226 sample to decay?

1- 7/8 = ______½ x ½ x ½ =______How many “half-lives”?______

Each half-life is 1599 years

It takes 3 half-lives to have 1/8 radium remaining.

3 x 1599 = 4797 years

More Practice! (pg. 228)

Carbon- 14 has a half life of 72 hours. How long would it take for 15/16 of the carbon sample to decay?

Uranium-238 decays very slowly, with a half-life of 4.47billion years. What percentage of the sample would remain after 13.4 billion years?

A sample of strontium-90 is found to have decayed to 1/ 8 of its original amount after 87.3 years. What is the half-life of strontium?

A sample of francium-212 will decay to 1/16 its original amount after 80 minutes. What is the half-life of francium-212?

What is the half-life of a 100.0 g sample of nitrogen-16 that decays to 12.5 grams in 21.6 seconds?

The half-life of hafnium-156 is 0.025 seconds. How long will it take a 560 g sample to decay to one-fourth      of its original mass?

Potassium-42 has a half-life of 12.4 hours. How much of an 848 g sample of potassium-42 will be left after      62.0 hours?

Half-life practiceHalf-life practiceGold-210 is a radioactive isotope that has a half-life of 12 hours. If a lab starts with a 13milligram sample of gold-210, how much will remain after 37 hours?

Chapter 7 Section 2Chapter 7 Section 2

Nuclear Fission & Fusion

Nuclear ForceNuclear ForceElements can have both stable &

unstable isotopes.

Example: Carbon-12 is stable Carbon-14 is unstable &

The stability of a nucleus depends on the nuclear forces that hold the nucleus together

Nuclei are held together by a Nuclei are held together by a particular forceparticular force

Strong Nuclear Force is the interaction that binds protons & neutrons together in a nucleus

Force is much stronger than the repulsion force between protons

Strong nuclear forces occur at very short distances (3 x 10-15 m)

In stable nuclei the attractive forces are stronger than repulsion forces

Too many neutrons or protons can cause a nucleus to become unstable & decay

Nuclei with too many neutrons or too few are unstable & will undergo decay

Nuclei with more than 83 protons are always unstable ◦Atomic numbers will be greater than _____

Nuclear FissionNuclear FissionThe process of producing lighter nuclei from heavier nuclei is called Fission

First observed by Otto Hahn & Fritz Strassman in 1939

Their experiment bombarded Uranium-235 with neutrons

The Product…The Product…2 lighter nuclei, neutrons, &

energy

Barium-137 & Kryton-84

The product includes 15 neutrons

Uranium can undergo many types of fission with different products

Energy is released during Energy is released during Nuclear FissionNuclear FissionDuring Fission the nucleus

breaks into smaller nuclei

The reaction releases large amounts of energy

Each dividing nucleus releases about 3.2 x10-11 J of energy

Hahn & Strassman determined the overall masses of the elements in their reaction had decreased after the reaction.

The missing mass had changed to Energy!

The Theory of RelativityThe Theory of Relativity

Presented by ___________ __________ in 1905

Describes the equivalence of mass & energy observed in nature

Equivalence means that matter can be converted into energy & energy into matter.

E = mcE = mc22

Energy = mass x Energy = mass x (speed of light)(speed of light)22

Neutrons released by fission can start a chain reaction!

When a nucleus splits into lighter nuclei, they need less neutrons so the neutrons are emitted.

The emitted neutron can then collide with surrounding nuclei and undergo fission

Nuclear Chain Reaction

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

Chain reaction is the principle behind the nuclear bomb!

Chain reactions can be controlled

The more neutrons that are produced per reaction the great the chances of creating a chain reaction

Specific materials can be used that will slow a fission chain reaction by absorbing neutrons

Nuclear Fusion (Hydrogen Bomb)Energy can be obtained when light

nuclei are combined into heavy nuclei

Fusion is the process in which light nuclei combine at extremely high temperatures forming heavier nuclei & releasing energy

Chapter 7 Section 3Chapter 7 Section 3

Dangers & Benefits of Nuclear Radiation

Dangers of Nuclear Dangers of Nuclear RadiationRadiation

Background Radiation is nuclear radiation that arises naturally from cosmic rays & from radioactive isotopes in the soil & air◦Example: Sun, soil, water & plants

Nuclear radiation can ionize atoms in living tissue.◦Example: Hemoglobin loses its ability

to carry oxygen when exposed to radiation

Skin helps keep low levels of radiation outside the body.

Nuclear radiation can cause burns in the skin as well as destroy bone marrow cells.

Radiation sickness results from exposure to high levels of nuclear radiation.

Observable effects from nuclear radiation exposure (low intensity) may not appear for days or years.◦Example: hair loss, sterility, death of bones, & cancer.

Nuclear Radiation can cause Nuclear Radiation can cause genetic mutationsgenetic mutations

Long-term effects of nuclear radiation appear when DNA molecules in the body are damaged.

Example:◦ Radioactive wastes from the Shiprock Uranium Mine contaminated water that was used for cattle & sheep.

◦ The radiation damaged the DNA in their reproductive cells causing offspring to have birth defects.

Beneficial Uses of Nuclear Beneficial Uses of Nuclear RadiationRadiation

Many uses in medicine & archeological dating

Smoke alarms

Cancer Treatment

Radioactive TracersRadioactive TracersUsed in agriculture, medicine, &

scientific research

Radioactive Tracers are added to substances so that its location can be detected later.

Used to locate tumors, measure the speed of a river, track drugs throughout a body

Nuclear PowerNuclear Power

Nuclear reactors are used to generate electricity (uses fission)

Nuclear fission has many benefits◦Produces not gaseous pollutants◦Much more energy than coal & oil reserves

Disadvantages of Nuclear Disadvantages of Nuclear FissionFissionSerious safety concerns with handling, treatment, and disposal of the Uranium

Safe operation of the nuclear reactor is also a concern◦Power plants can only be active for 40

years

Nuclear wastes must be safely stored

Half-life practiceHalf-life practiceA radioactive substance has a half-life of 10 years. What fraction of a sample of the substance would be left after 30 years?

Half-life practiceHalf-life practiceIron-109 is a radioactive isotope that has a half-life of 4 hours. What fraction of the Iron-109 will remain after 24 hours?

 1. The half-life of a radioactive

isotope is 1.0 hr. a)What fraction of the original

sample of radioactive nuclides is left after 3.0 h?

b) What fraction of the original sample of radioactive nuclides is left after 1.0 d?

2. A soil sample contains 40 mg of 90Sr. An atom of 90Sr masses 1.49295 x 10-25kg. Approximately, how much 90Sr will be in the sample 150 y from now?

Strontium's half-life = 50 years.

3. In 1898, Pierre and Marie Curie isolated about 10 mg of 226Ra from 8 tons of uranium ore. The half-life of Radium is 25 years. If this sample had been placed in a museum, how much of the radium would remain in the year 2100?

 A radioactive element has a

half-life of 20 days. How much of a 16mg sample would be undecayed after 80 days?