unstable nuclei & radioactive decay radioactivity nucleus of an element spontaneously emits...
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Unstable Nuclei & Radioactive Decay
RadioactivityNucleus of an element spontaneously emits subatomic particles & electromagnetic waves.
Nucleus changes into a different element when it does this.
Original nucleus is called unstable.Process is called decay or transmutation.
Rutherford & Radioactivity1898 Rutherford began experiments with radioactivity.
1899 discovered alpha and beta rays from uranium.
Types of Radiation
Characteristics of RadiationSee Table O22-1-10+1+1
2 Neutrons & 2 Protons.
Charge = +2Mass = 4
Beta Particle fast moving electron.
Radioactive atom: Change occurs in nucleus.
Can we predict exactly when an atom will decay?NO!
For large #s of atoms, we CAN predict how many will decay on average in a given amount of time.
Which elements are radioactive?All elements past Bismuth in the periodic table.If the atomic number is 83, its radioactive!
Other elements may have radioactive isotopes.Stability depends on neutron/proton ratio.applet
Whats Going on in the Nucleus?Electrostatic repulsions between protons. Want to fly apart.But protons & neutrons all attracted to each other by nuclear strong force.
So having neutrons helps hold a nucleus together.
Strong ForceWorks best if the nucleus isnt too large.As the nucleus gets larger, need to have more neutrons to help counteract the electrostatic repulsion between the protons.Eventually, the nucleus is too large to be stable.
Balancing ActBalance exists between electrostatic repulsive force & nuclear strong force.
Certain #s of protons & neutrons make a stable nucleus. Other #s of protons & neutrons are unstable. So the atom decays.
Beyond Element 83No amount of neutrons can hold a nucleus together once it has more than 83 protons.
Elements 84 & above are radioactive.
Stability and the n/p ratioFor atoms below atomic number = 20, best neutron/proton ratio 1.
As atomic number , atoms need more neutrons to be stable.
So n/p ratio for stable atoms increases to 1.5 for big atoms.
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 Unstable Nuclei Emit RadiationSpontaneous nuclear change to attain good n/p ratio (high stability, low energy state).Form a new kind of atom.
Each isotope or nuclide decays in a certain manner to get a better n/p ratio. The decay mode is named for the particle emitted. See Table N.
Nuclear EquationsDescribe the decay process.
reactant or starting side (left) product or ending side (right).
separates two sides
Nuclear vs. ChemicalHow is a nuclear change different from a chemical change?
Involve a change in an atoms nucleus.
Radioactive atoms spontaneously emit radiation and change into other kinds of atoms.
Nuclear reactions involve 1,000,000 X more energy than ordinary chemical rxns.Involve changes in the outermost electrons.
1 or more substances changed into new substances.
Atoms are rearranged, but their identities do not change.
Mass EnergyIn nuclear reactions, mass is converted into energy.Mass Defect is the difference between the mass of a nucleus and the sum of the masses of its constituent particles.E = mc2
Binding EnergyEnergy released when a nucleus is formed from its constituent particles.It is a measure of the stability of an atom formed:The higher the binding energy the more stable the nucleus.The lightest and heaviest elements are the most unstable (low BE)Intermediate elements are the most stable (highest BE).