Review

What type of decay will happen if the nucleus contains too many neutrons?

• Beta Decay

Alpha Decay

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Part II

•Nuclear Stability •Half-Life

Nuclear Stability

• Depends on the neutron to proton ratio.

Band of Stability

Number of Protons (Z)

Num

ber o

f Neu

tron

s, (N

)

What happens to an unstable nucleus?

• They will undergo decay

• The type of decay depends on the reason for the instability

Radioactive Half-Life (t1/2 ):

• The time for half of the radioactive nuclei in a given sample to undergo decay.

Common Radioactive Isotopes

Isotope Half-Life Radiation Emitted

Carbon-14 5,730 years , b g

Radon-222 3.8 days a

Uranium-235 7.0 x 108 years , a g

Uranium-238 4.46 x 109 years a

Radioactive Half-Life

• After one half life there is 1/2 of original sample left.

• After two half-lives, there will be 1/2 of the 1/2 = 1/4 the original sample.

Graph of Amount of Remaining Nuclei vs Time

A=Aoe-lt

A

Example

You have 100 g of radioactive C-14. The half-life of C-14 is 5730 years.

• How many grams are left after one half-life? Answer:50 g

• How many grams are left after two half-lives?

Formula

Amount Remaining = (Initial Amount) x (1/2)n

Where: n = to the number of half lives

You can calculate n by taking the elapsed time and dividing it by the duration of a half life of the substance.

Problem

A sample of 3x107 Radon atoms are trappedin a basement that is sealed. The half-life ofRadon is 3.83 days. How many radon atoms are left after 31 days?

answer:1.2x105 atoms