Unstable Nuclei & Radioactive Decay

RadioactivityRadioactivity

Nucleus of an element spontaneously Nucleus of an element spontaneously emits subatomic particles & emits subatomic particles & electromagnetic waves.electromagnetic waves.

Nucleus changes into a different Nucleus changes into a different element when it does this.element when it does this.

Original nucleus is called “unstable.”Original nucleus is called “unstable.” Process is called “Process is called “decaydecay” or ” or

““transmutationtransmutation.”.”

Rutherford & Rutherford & RadioactivityRadioactivity 1898 – Rutherford began 1898 – Rutherford began

experiments with radioactivity.experiments with radioactivity.

1899 – discovered alpha and beta 1899 – discovered alpha and beta “rays” from uranium.“rays” from uranium.

Types of RadiationTypes of Radiation

Radioactivity – typesRadioactivity – types

Characteristics of RadiationCharacteristics of RadiationSee Table OSee Table O

NameName SymbolSymbol Mass Mass (amu)(amu)

Relative Relative ChargeCharge

Alpha Alpha particleparticle

or or 44HeHe or or 44 44 +2+2

Beta Beta particleparticle

or or 00ee or or 00 or or --

00 -1-1

Gamma Gamma radiationradiation

or or 00 00 00

Positron Positron emissionemission

00 or or 00ee or or ++ 00 +1+1

22 22

-1-1 -1-1

00

+1+1 +1+1

2 Neutrons & 2 Protons.

Charge = +2Mass = 4

Beta Particle – fast moving electron.Beta Particle – fast moving electron.

Radioactive atom: Change occurs in nucleus.

ShieldingShielding

Can we predict exactly Can we predict exactly when an atom will decay?when an atom will decay? NO!NO!

For large #’s of atoms, we For large #’s of atoms, we CANCAN predict how many will decay on predict how many will decay on average in a given amount of average in a given amount of time.time.

Which elements are Which elements are radioactive?radioactive? All elements past Bismuth in the All elements past Bismuth in the

periodic table.periodic table. If the atomic number is If the atomic number is 83, it’s 83, it’s

radioactive!radioactive!

Other elements may have radioactive Other elements may have radioactive isotopes.isotopes.

Stability depends on neutron/proton Stability depends on neutron/proton ratio.ratio.

applet

What’s Going on in the What’s Going on in the Nucleus?Nucleus?

Electrostatic repulsionsElectrostatic repulsions between between protonsprotons. Want to fly apart.. Want to fly apart.

But But protons & neutronsprotons & neutrons all all attracted to each other by attracted to each other by nuclear nuclear strong forcestrong force..

So having neutrons helps hold a So having neutrons helps hold a nucleus together.nucleus together.

Strong ForceStrong Force

Works best if the nucleus isn’t too Works best if the nucleus isn’t too large.large.

As the nucleus gets larger, need As the nucleus gets larger, need to have more neutrons to help to have more neutrons to help counteract the electrostatic counteract the electrostatic repulsion between the protons.repulsion between the protons.

Eventually, the nucleus is too Eventually, the nucleus is too large to be stable.large to be stable.

Balancing ActBalancing Act

Balance exists between Balance exists between electrostatic repulsive force & electrostatic repulsive force & nuclear strong force.nuclear strong force.

Certain #’s of protons & neutrons Certain #’s of protons & neutrons make a stable nucleus. Other #’s make a stable nucleus. Other #’s of protons & neutrons are of protons & neutrons are unstable. So the atom decays.unstable. So the atom decays.

Beyond Element 83Beyond Element 83

No amount of neutrons can hold a No amount of neutrons can hold a nucleus together once it has more nucleus together once it has more than 83 protons.than 83 protons.

Elements 84 & above are Elements 84 & above are radioactive.radioactive.

Stability and the n/p ratioStability and the n/p ratio

For atoms below atomic number = For atoms below atomic number = 20, best neutron/proton ratio 20, best neutron/proton ratio 1. 1.

As atomic number As atomic number , atoms need , atoms need more neutrons to be stable.more neutrons to be stable.

So n/p ratio for stable atoms So n/p ratio for stable atoms increases to 1.5 for big atoms.increases to 1.5 for big atoms.

Stability Line

Type of radiation emitted depends on position relative to stable nuclei.

Blue: too many neutrons.Yellow: not enough neutrons for the protons.Red/Orange: too many protons and neutrons

Natural Radioactivity – Natural Radioactivity – Unstable Nuclei Emit RadiationUnstable Nuclei Emit Radiation

Spontaneous nuclear change to Spontaneous nuclear change to attain good n/p ratio (high stability, attain good n/p ratio (high stability, low energy state).low energy state).

Form a new kind of atom.Form a new kind of atom.

Each isotope or nuclide decays in a Each isotope or nuclide decays in a certain manner to get a better n/p certain manner to get a better n/p ratio. The ratio. The decay modedecay mode is named is named for the particle emitted. See Table for the particle emitted. See Table N.N.

Nuclear EquationsNuclear Equations

Describe the decay process.Describe the decay process.

reactant or starting side (left) reactant or starting side (left) product or ending side (right).product or ending side (right).

separates two sidesseparates two sides

Nuclear vs. ChemicalNuclear vs. Chemical

How is a nuclear change different How is a nuclear change different from a chemical change?from a chemical change?

Involve a change Involve a change in an atom’s in an atom’s nucleus.nucleus.

Radioactive atoms Radioactive atoms spontaneously spontaneously emit radiation and emit radiation and change into other change into other kinds of atoms.kinds of atoms.

Nuclear reactions Nuclear reactions involve 1,000,000 involve 1,000,000 X more energy X more energy than ordinary than ordinary chemical rxns.chemical rxns.

Involve changes in Involve changes in the outermost the outermost electrons.electrons.

1 or more 1 or more substances substances changed into new changed into new substances.substances.

Atoms are Atoms are rearranged, but rearranged, but their identities do their identities do not change.not change.

NUCLEAR CHEMICAL

Mass Mass Energy Energy

In nuclear reactions, mass is In nuclear reactions, mass is converted into energy.converted into energy.

– Mass DefectMass Defect is the difference is the difference between the mass of a nucleus and between the mass of a nucleus and the sum of the masses of its the sum of the masses of its constituent particles.constituent particles.

E = mcE = mc22

Binding EnergyBinding Energy

Energy released when a nucleus is Energy released when a nucleus is formedformed from its constituent particles. from its constituent particles.

It is a measure of the It is a measure of the stabilitystability of an of an atom formed:atom formed:

– The higher the binding energy the more The higher the binding energy the more stable the nucleus.stable the nucleus.

– The lightest and heaviest elements are the The lightest and heaviest elements are the most unstable (low BE)most unstable (low BE)

– Intermediate elements are the most stable Intermediate elements are the most stable (highest BE).(highest BE).

Ni-62Ni-62

Fe-56Fe-56

Binding EnergyBinding Energy