Unit 6.3 Half- Life (radioactivity)

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Unit 6.3 Half- Life (radioactivity)

Half- Life: the amount of time it takes for half of the nuclei (nucleus) in a sample of radioactive atoms to decay.

The half-life is 

the time it takes for 

one-half of

 a given number

 of parent atoms to 

decay to

 daughter atoms.

Update:  New Option for how to solve

Example 1

ANSWER EXAMPLE 1

Example 2

Answer for Example 2

Example 3

fghj

Answer for Example 3

fghj

Example 4

dgh

Example 4

dgh

The following slides are the examples on the note sheet given on Day 1 of Half- Life

Example 1:

 If we start with 400 atoms of a radioactive substance, how many would we have after:

a.    One half-life: _______

b.    Two half-lives ________

c.    Three half-lives _______

d.     Four half- lives _________

Answers - Example 1: 

 If we start with 400 atoms of a radioactive substance, how many would we have after:

a.    One half-life:    200

b.    Two half-lives:    100

c.    Three half-lives:  50

d.     Four half- lives:   25

Example 2:

  There is a substance of 400 grams with a  

half-life of 10 years. How much is left after 20 years?  

Answer- Example 2:

  There is a substance of 400 grams with a  

half-life of 10 years. How much is left after 20 years?  

 10      20

200    100

g           g

Example 3:

  A 208g sample of Na-24 decays to 13 g of Na-24 within 60 hours. What is the half- life of this radioactivity? 

Answer:   Example 3:

  A 208g sample of Na-24 decays to 13 g of Na-24 within 60 hours. What is the half- life of this radioactivity? 

15hrs  30hr  45hr

104g   52g      26g

Example 3:

  A 208g sample of Na-24 decays to 13 g of Na-24 within 60 hours. What is the half- life of this radioactivity? 

Answer- Example 3:

  A 208g sample of Na-24 decays to 13 g of Na-24 within 60 hours. What is the half- life of this radioactivity? 

15hr                50%            104g

30 hr               25%             52g

45 hr                12.5%          26g

                        6.25%    

Example 4: 

 The chart below shows the decay 

rate for Cobolt- 14.