Chapter 18
 Mass #
Symbol
 Element
 Parts
Name or symbol – Mass #
of a Reaction
Reactants  Products






Alpha emission or decay (a) –helium atom
238 U  4 He + 234 Th
92
2
90
Beta emission or decay (b)–
234 Th  234 Pa + 0 e
90
91
-1
0
-1e
Gamma emission or decay (g) 238 U  4 He + 234 Th + 0 g
92
2
90
0
4
2He
in the products
0
0g




Positron emission or decay 22 Na  0 e + 22 Ne
11
+1
10
+1e
Electron capture – beta particle in the
reactants
201 Hg + 0 e  + 201 Au
80
-1
79

Neutron emission or decay–
209 Po  1 n + 200 Po
84
0
84

Proton –

0
1
1H or
1
1p
1
0n

Mass # and the atomic # totals must be the
same for reactants and the products.
 3919K

35
17Cl
+ ___
-1e
+ ___
 20682Pb

 23894Pu
+ ___
0
4
2He +
235
92U

Alpha decay of Cu-68

Gamma emission of Thorium-235

Positron emission of P-18

Astatine-210 releasing 3 neutrons

Electron capture of Ti-45



Radioactive isotopes or nuclides all decay
because they are unstable, some just
breakdown much faster than others
Half-life – amount of time for half of the
original sample to decay
For two samples of the same isotope,
regardless of the sample size, after one halflife, only half of the original amount of
sample remains.









Isotopes
Carbon – 14
Sodium – 24
Bismuth – 212
Polonium – 215
Thorium – 230
Thorium – 234
Uranium – 235
Uranium – 238
Half-Live
5730 years
15 hours
60.5 seconds
0.0018 seconds
75400 years
24.1 days
7.0 x 108 years
4.46 x 109 years


Barium – 139 has a half-life of 86 minutes. If
you originally have a 10 gram sample of
Barium-139, how much will be left after 258
minutes?

How many days will it take 50 grams of
Radon – 222 (half-life of 3.82 days) to decay
to 3.125 grams?

If a sample of Cesium-135 decays from 10
grams to 2.5 grams over a period of 84 days,
what is the half-life of Cesium-135?


Fusion – combining two smaller nuclei into
one heavier, more stable nucleus.
3 He + 1 H  4 He + 0 e
2
1
2
1
Fission – splitting a large unstable nucleus
into two nuclei with smaller mass numbers.
209 Po  125 Te + 84 Ge
84
52
32