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RadioisotopesThe nuclei of some atoms are unstable and undergo spontaneous changes called radioactive decay.One such change is called beta decay.During beta decay a neutron changes into a proton and an electron transforming the atom to an element with an atomic number which is one higher while the atomic mass barely changes.
+
Tritium atoms, H-3, undergo spontaneous beta decay. Shown below is a tritium nucleus.
1 proton2 neutrons
+ 2 proton1 neutron1 electron
+
vvvvv
The highly energetic electron is ejected from the nucleus as radiation.It travels at a speed of 1.3 x 105 km/s. The equation is:
1H3
2He3
+ 1-e0
Two other forms of radiation from radioactive decay are:alpha particle emission andgamma rays.An alpha particle contains 2 protons and 2 neutrons while gamma rays do not result in the release of particles.The rate of release of radiation is expressed as a half-life.A half-life is the length of time required for half of the original material to decay.
Tritium has a half-life of 12.26 years.12.26 a (annum is latin for years)If 10 g of tritium were left for 12.26 a there would be 5 g left.After 24.52 a there would be 2.5 g left.Here is a table showing the quantity of tritium remaining after different time periods.
Time (a)Mass of
Tritium (g)
0.00 10.0012.26 5.0024.52 2.5036.78 1.2549.04 0.6361.30 0.3173.56 0.16
Mass of Tritium vs Time
10.00
5.00
2.50
1.25
0.630.31 0.16 0.080.00
2.00
4.00
6.00
8.00
10.00
12.00
0.00 20.00 40.00 60.00 80.00 100.00
Time (a)
Mas
s (g
)
Here is an example of Alpha decay.Alpha decay involves the emission of a helium-4 nucleus. Write an equation which shows how uranium-235 undergoes alpha decay.
92
235U 2
4He +
90
231Th
Different radioactive isotopes decay at different rates.If 100 g of a radioactive material decays for 10 years and 50 g remains this substance is said to have a half life of 10 years.
5 y 5 y
After 10 y only 50 g remain
If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left?
200 g -------> 100 g -------> 50 g
If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left?
200 g ------->
200 g
If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left?
200 g -------> 100 g
100 g left after 5 years
If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left?
200 g -------> 100 g -------> 50 g
50 g left after 10 years
If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left?
200 g -------> 100 g -------> 50 g ----> 25g
25 g left after 15 years
Show the decay sequence for 512 g of a substance with a half-life of 25 da.
512 g
Show the decay sequence for 512 g of a substance with a half-life of 25 da.
256 g
512 g ---> 256 g25 da
Show the decay sequence for 512 g of a substance with a half-life of 25 da.
128 g
512 g ---> 256 g ---> 128 g25 da 25 da
Total - 50 da
Show the decay sequence for 512 g of a substance with a half-life of 25 da.
64 g
512 g ---> 256 g ---> 128 g ---> 64 g25 da 25 da 25 da
Total - 75 da
Show the decay sequence for 512 g of a substance with a half-life of 25 da.
32 g
512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g25 da 25 da 25 da 25 da
Total - 100 da
Show the decay sequence for 512 g of a substance with a half-life of 25 da.
16 g
512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g25 da 25 da 25 da 25 da
Total - 125 da 16 g25 da
Show the decay sequence for 512 g of a substance with a half-life of 25 da.
512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g25 da 25 da 25 da 25 da
Total - 150 da 16 g25 da
8 g
8 g
25 da
Show the decay sequence for 512 g of a substance with a half-life of 25 da.
512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g25 da 25 da 25 da 25 da
Total - 175 da 16 g25 da
4 g
8 g
25 da
4 g
25 da
Show the decay sequence for 512 g of a substance with a half-life of 25 da.
512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g25 da 25 da 25 da 25 da
Total - 200 da 16 g25 da
2 g
8 g
25 da
4 g
25 da
2 g25 da
Show the decay sequence for 512 g of a substance with a half-life of 25 da.
512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g25 da 25 da 25 da 25 da
Total - 225 da 16 g25 da
1 g
8 g
25 da
4 g
25 da
2 g25 da
1 g25 da
If U-235 has a half-life of 7.1 x 108 y. How many years would it take 32 g to decay to 2 g?32 g --> 16 g --> 8 g --> 4 g --> 2 g4 half lifes2.84 x 109 y.
Cs-136 has a half-life of 13 da. If 1024 g was left to decay for 65 da how much of the original material would be left?65/13 = 5 hl1024 g -> 512 g -> 256 g -> 128 g -> 64 g -> 32 g
or 1024 g x (1/2)5 = 32 g
To find the quantity of material remaining use this formula
Massremaining =
OriginalMass
x 12
# of Half-lives
Pb-212 has a half-life of 10.6 h. If 12.5 g of Pb-212 is left for 84.8 h how much of the original material is left?
Massremaining =
OriginalMass
x 12
# of Half-lives
12.5 g x (0.5)84.8/10.6
= 0.0488 g
