Radioactivity

Atoms

Bond-fill outer valence shell

Decay-Breakdown into energy and particles

The ones that decay are usually unstable isotopes of an atom

Decay Types

1. Alpha decay-releases alpha particles (Helium atoms, large) and stopped by paper or clothing.

2. Beta decay-releases beta particles (electron, small) and stopped by aluminum.

3. Gamma decay-releases gamma rays (no charge or mass) and stopped by thick concrete or strong lead.

Half-Life• Amount of time it takes for half of a

radioactive substance to decay

• Used to tell how old various rock samples and fossils are

Isotope Half-Life Isotope Half-Life

U-238 4.5 BY Po-210 138 d

O-21 3.4 s N-13 10 m

H-3 12.3 y Ca-36 0.1 s

Half Life Example

If 10 mg of iodine 131 is given to a patient, how much is left after 24 days? The half-life of iodine-131 is 8 days.

Time 0 D 8 D 16 D 24 D 32 D

Mass 10 mg

Answer

Time 0 D 8 D 16 D 24 D 32 D

Mass 10 mg 5 mg 2.5 mg 1.25 mg

0.625 mg

After 24 Days you have 1.25 mg left and 3 half lives have past

Half Life Example

Iodine-131 is used to destroy thyroid tissue in the treatment of an overactive thyroid. The half-life of iodine-131 is 8 days. If a hospital receives a shipment of 200 g of iodine-131, how much I-131 would remain after 32 days?

Answer

Time 0 D 8 D 16 D 24 D 32 D

Mass 200 g 100 g 50 g 25 g 12.5 g

After 32 days only 12.5 g of the sample remains, which means 4 half lives have past.

Half-Life Example

C-14 half life = 5730 years

¾ of the original C-14 of a fossil decayed.

How many half lives have past?

How old is this fossil?

If you started with 250 grams of C-14, how much remain?

AnswerTime 0 Y 5730 Y 11460 Y 17190 Y 22920 Y

Half Lives Past

0 1 2 3 4

Original Mass Remaining %

100% 50% 25% 12.5% 6.25%

Original Mass Remaining (g)

250 g 125 g 62.5 g 31.25 g 15.625 g

¾ decay of sample means 2 half lives, 25% remains, 75% decayed and therefore in this example 62.5 g remain