Transcript Isotopes Atomic mass Half-life calcs

Nuclear Reactions
6
3
Li
Protons and neutrons are
held together in nucleus by
the strong force.
The strong force is strong
enough to cancel out the
electrical repulsion between
protons in small nuclei.
Strong force
small nucleus
p/p repulsion
Net force
Strong force
P/p repulsion
stronger
Net force weaker
large nucleus
Isotopes: Atoms of the same element (same atomic
number=same number of protons), but different numbers of
neutrons ( different mass numbers).
As the number of neutrons in an isotope increases, the
stability of the nucleus decreases due to a decrease in the
strong force that holds the nucleus together.
1
1
H
hydrogen
2
3
1
1
H
deuterium
H
tritium
Normal hydrogen and deuterium are not radioactive, while
tritium is.
Atomic mass: the weighted average of all of the mass numbers
for an element’s isotopes.
Just in: a new element, Beakerium, has been
discovered in a local high school laboratory.
Isotope 1
Isotope 2
2/5 of every sample
mass number of 50 g
3/5 of every sample
mass number of 100 g
Q: What is the average mass of a sample of Beakerium?
Atomic mass = (2/5)(50 g) + (3/5)(100 g) = 80 g
Nuclear decay
The most common types of radiation given
off by unstable nuclei are:
Alpha particles: helium nuclei
4
Beta particles: electron emitted
FROM THE NUCLEUS
0
(are 10x more penetrating than a)
Gamma rays: high frequency
electromagnetic radiation
(are 1000x more penetrating than a)
2
He or a
e or 
-1

Example of alpha particle decay:
210
84
Po
4
2
He
+
206
Pb
82
*decay = giving off a particle
Notice that there are 84 protons on both sides of the
arrow. The Law of Conservation of Mass strikes again!!
Transmutation: the changing of one element into another.
• When Po decays to Pb by alpha particle emission, it has
transmutated.
Half-life: the amount of time required for half of the
unstable element to decay (transmutate) into another
element.
Time (s)
Red (g)
Green (g)
0
100
0
2
50
50
4
25
75
6
12.5
87.5
8
6.25
93.75
What is the half life of the red
element? 2 s
x
Time after
Activity
starting the
experiment (Disintegrations
per second)
(hours)
0
400
30
100
60
25
x
x
x
• To determine half-life, calculate
half of the original activity.
• 400 /2 = 200cps
• Use the graph to find the time
required to drop to this value.
• The half life is 15 hours.
x
x
x
A substance’s half-life is constant, regardless
of the size of your sample.
It takes 15 hours to go from:
x
400 cps  200 cps
x
200 cps  100 cps
x
100 cps  50 cps
x
The other
Isotopes say I’m a
little unstable
Hi, I’m
Lenny!
Radioactive M&Ms!!!
Mission 1: To determine the atomic mass of
Candium.
Mission 2: To determine the half-life of Candium.
Caution: Failure to follow the directions by
eating the M&Ms BEFORE you
have completed both missions
will cause your lab grade to selfdestruct!
Radioactive M&Ms!!!
Mission 1: To determine the atomic mass of
Candium.
Mission 2: To determine the half-life of Candium.
Caution: Failure to follow the directions by
eating the M&Ms BEFORE you
have completed both missions
will cause your lab grade to selfdestruct!
Ex: I-131 undergoes beta decay:
131
53
0
+
e
-1
I
131
54
Xe
Ex: Ba-137 undergoes electron capture:
137
56
Ba
+
137
0
e
-1
55
Cs
Ex: Np-237 undergoes two beta decays
237
93
Np
0
e
-1
+
0
e
-1
+
237
95
Am
Nuclear Fission
vs.
Nuclear Fusion
sUn
Spl tting  t nier
1 235
0 n  92 U
+
U

137
97
1
Te

Zr

2
32
40
0n
Te + Zr
+
+
Sm
shing  h
ger
2
2
4
H

H

1
1
2 He
H + H
He
Half-life calculations
Recall: for every half-life that passes, half of the
radioactive element transmutates into a different
element.
Ex: Element X has a half-life of 11 days. How many grams
are left after 33 days if you start out with 80 g?
1. Determine the number of half lives that have passed
#half-lives = total time/time for one half life
= 33 days/11 days = 3 half lives
2. Multiply the starting amount by ½ three times to
find out how much is left over.
Amt remaining = 80(0.5)(0.5)(0.5) =80(0.5)3 = 10 g
What if there isn’t an even number of half-lives
involved?
A radioactive element has a half-life of 5
minutes. How much of a 10 g sample is left
after 17 minutes?
# half-lives = total time/half-life
= 17 min/5 min = 3.4 half-lives
Amount remaining = 10 g(0.5)3.4 =
0.95 g