The Nucleus

Chapter 18

Atomic Symbol Notation

N147Atomic

Number

Mass

Number

Some Common Nuclides

H11

H21

H31

Decay Particles

• Alpha Particle

• Beta Particle

• Gamma Ray

He42

e01

00

Stopped by heavy clothing, skin, does great amount of surface damage.

Converted to alpha particle by lead, does widespread damage.

Penetrates most substances. Is made of energy rather than matter.

More Decay Particles

n10

e01

• Neutron

• Positron

Nuclear Reactions:

• Alpha Particle Production

• Spontaneous Fission

• Positron Production

• Electron Capture

RaHeTh 22688

42

23090

ePaTh 01

23491

23490

NeeNa 2210

01

2211

0020179

01

20180 AueHg

Try Me!

• Write out the nuclear equations:• 11

6C produces a positron

• 21483Bi produces a beta particle

• 23793Np produces an alpha particle

Stability of element’s nucleus

The Band of Stability

• Small elements can support a 1:1 proton:neutron ratio.

• Larger elements need more neutrons to make it stable!

Thermostability of Nuclei

• Remember how Einstein said “E=mc2”?• It means that energy has mass.• Tiny differences in mass of a nucleus during a

nuclear reaction is due to the change in energy.• The change in mass is called the mass defect.• The quantity of energy required to decompose a

nucleus in a decay reaction is called the binding energy.

Rate =

Rate of Decay

• We use a negative sign because the number of original nuclides is decreasing.

• Half-life is given by the integrated first order rate law.

tN

2/1

)2ln(tk ln (A/Ao) = - kt

Try Me

• The rate constant for the decay of Technetium-99 is known to be 1.16 x 10-1h-1. What is the half-life of this nuclide?

Radioactivity Dating

• Carbon-14 is used for dating artifacts, though technically this process can be used on most radioisotopes.– A small percentage of all carbon present in the

universe is carbon-14. This percentage is constant in all living things.

eNC 01

147

146

– When an organism dies, the carbon-14 supply is not replaced, so whatever decays stays decayed

– Measurements of original quantities of nuclides and current quantities allow dendrochronologists and other archeologist types to date dead organic materials.

Try this out.

• The half-life of carbon-14 is 5730 years. An old piece of fire wood, found in a cave in Africa, has a carbon-14 decay rate of 3.1 counts per minute per gram. Assuming the decay rate of freshly cut wood is 13.6 counts/minute/gram, calculate the age of the old log.

Try Again

• A rock containing and was examined to determine its approximate age. Analysis showed the ratio of atoms to atoms to be 0.115. Asssuming no lead was originally present, that all of the lead formed remained in the rock, and intermediate forms of decay are negligible, calculate the age of the rock. The half life of uranium-238 is 4.5 x 109 years.

U23892 Pb206

82

U23892Pb206

82

Nuclear Fission

• A large nucleus, bombarded by a neutron, becomes unstable and splits into two smaller nuclei.

• This is an exothermic process, since stable

nuclei have less energy than unstable nuclei.

Chain Reactions

• A self-sustaining fission reaction that is caused by the production of one or more neutrons as products.

• Critical

• Subcritical

• Supercritical

• Critical Mass

Nuclear Fusion

• Two small nuclei combine to produce a larger, more stable nucleus and at least one neutron.

• This is an exothermic reaction, since stable nuclei have less energy than unstable.

Nuclear Transformations

• The process of changing one element’s nucleus into another.

• Particle Accelerators• Magnets• Circular or Linear

CERN: Particle Accelerator

• Game:Cern50.web.cern.ch/cern50/multimedia/LHCGame/StartGame.html

CERN movies

Detection Methods

• Geiger Counter• Tube of argon gas is ionized by

radioactive particles.• Amplifier changes energy from the ionization into a

noise.

• Scintillation Counter• Much more sensitive than a Geiger counter.• Light sensitive machinery records the

flashes when radioactive particles emit energy.

Nuclear Reactors

Effects of Radiation

• Somatic Damage: sickness, death, organism is harmed directly.

• Genetic Damage: no immediate damage is noticed. Offspring experience effects.

Your radiation exposure:

http://www.epa.gov/rpdweb00/understand/calculate.html

Factors Impacting Effects

1. Energy of Radiation (rads): more energy,more damage

2. Penetrating Ability of Radiation: alpha<beta<gamma

3. Properties of Radiation Source What kind of particle is

produced? How long does it take to decay? Does it get metabolized by body?

Medical Uses of Nuclides

• Radiotracers

• Iodine for Thyroid

• Thallium and Technetium-99m for Heart

• X rays