Chapter 39
The Atomic Nucleus and Radioactivity
Conceptual Physics
Hewitt, 1999
Bloom High School
39.1 The Atomic Nucleus
• Nucleons- particles in the
nucleus of an atom
– Neutrons (n0) & Protons (p+)
– Almost equal masses
– Neutrons are “glue” in a nucleus
• Electrical forces- like repels like
– p+ repel other p+ in the nucleus
– Acts over a distance
– Inverse-square law
Neutrons & Nuclear Strong Forces
• Nuclear Strong force
– Acts between nucleons
– Only acts in close proximity
• Neutron- nucleon that is
unstable when alone
– More neutrons are needed
for more protons
39.2 Radioactive Decay
• Alpha (a) particlehelium nucleus
– 2p+ with 2n0 ejected
from nucleus
– Positively charged
particle
• Gamma (g) rayelectromagnetic
radiation
Beta emissions
• b- particle- electron (e-)
– With extra n0’s, 1n0 is
transformed into 1p+ +
1e• Conservation of charges
– (-) charged particle
• b+ particle- positron
(e+)
– With extra p+’s, 1p+ is
transformed into 1n0 +
1e+
• Conservation of charges
– (+) charged particle
39.3 Radiation Penetrating Power
• g ray- penetrate the most
– No charge or mass to slow
them down
– Need a very high density
substance to block them
• b particle- penetrates
slightly
– Loses energy with a small
number of collisions
– Thin sheets of metal can
block them
• a particle- penetrates the
least
– Relatively slow and heavy
– Paper and skin can stop them
39.4 Radioactive Isotopes
• Atomic number- equal to the number of p+
– Carbon, 6p+
• Atomic mass number- equal to the number of
nucleons
– Carbon-12 (99% of all carbon)
– Hydrogen-1
• Deuterium-2 (1n0 & 1p+) stable
• Tritium- 3 (2n0 & 1p+) radioactive
Ions vs. Isotopes
• Ion- charged particle
– Gain or loss of an e- through chemical reaction
– Neutral atom has equal numbers of p+ and e-
• Isotope- gain or loss of n0 through nuclear
reaction
– Number of p+ must be constant
– Isotope number (upper number) is (n0 + p+)
•
235U
(U-235) is 92p+ + 143n0
39.5 Radioactive Half-Life
• Half-Life- amount of time for half of the substance
to change or decay
– If half-life is 1000 years, 50% remains at 1000 years
– At 2000 years, ½ of ½ remains (25% or ½2)
– At 3000 years, ½ of ½ of ½ remains (12.5% or ½3)
39.6 Natural Transmutation of Elements
• Transmutation- changing one element into another
• Radioactive Decay- can change to another element
– a-decay- loss of 2p+
• Atomic number decreases by 2
• Atomic mass decreases by 4
• 238U  234Th + 4He
– b--decay- gain of 1p +
• Atomic number increases by 1
• Atomic mass unchanged
• 234Th  234Pa + 0e-
– g-decay- no gain or loss of p +
• No change in atomic number or mass
• 60Co  60Co + 0g
Calculating a-decay Reactions
• a-decay- loss of 2p+
– Atomic number decreases by 2
– Atomic mass decreases by 4
– The total mass and atomic number before must equal
the total mass and atomic number after the reaction
•
234Pa
 __ + 4He
– Number of nucleons must be equal
• 234=x + 4
– Number of p+ must be equal
• 91=Z + 2
– Missing element must be 230Ac
Calculating b--decay Reactions
• b--decay- gain of 1p +
– Atomic number increases by 1
– Atomic mass unchanged
– The total mass and atomic number before must equal
the total mass and atomic number after the reaction
•
216Po
 __ + 0e-
– Number of nucleons must be equal
• 216=x + 0
– Number of p+ must be equal
• 84=Z + -1
– Missing element must be 216At
Calculating g-decay Reactions
• g-decay- no gain or loss of p +
– No change in atomic number or mass
– The total mass and atomic number before must equal
the total mass and atomic number after the reaction
•
60Co
 __ + 0g
– Number of nucleons must be equal
• 60=x + 0
– Number of p+ must be equal
• 27=Z + 0
– Missing element must be 60Co
39.7 Artificial Transmutation of Elements
• Elements can be bombarded to change into other
elements
– 14N + 4He  17O + 1H
• Transuranic element- elements after Uranium
– Half-life’s are relatively short, so they are not found in
nature
39.8 Carbon Dating
• C-12 very stable (99% of carbon is C-12)
• C-14 radioactive
– Found in living tissue and is constantly replaced
– Ratio of C-12 to C-14 fixed in living tissue
– In dead tissue, C-14 is not replaced and decreases
over time
– Not found in non-living tissue (metals, rocks, etc.)
39.9 Uranium Dating
• Uraniumdecays in a
predictable
pattern
– Lead is a decay
product, so is
found in all
uranium
samples
39.10 Radioactive Tracers
• Radioactive tracerradioactive isotope
used to follow a path
• Used in agriculture to
determine path of
fertilizer and water
• Used in medicine to
determine metabolic
pathway of medicine
or blood
39.11 Radiation and You
• Radiation
naturally occurs
everywhere
– Underground, it’s
the cause of
warmth
• Radiation more
strong at high
altitudes because
there is less
atmospheric
shielding from
cosmic radiation
Lab 97: Nuclear Marbles
• Groups of 4
– 10 marbles
– 3 metersticks
• Computed diameter
– P=2N(R+r)/L
– P=H/T
– Replace (R+r) with “d,” combine formulae and solve for d
• Procedure Step 1
– 200 trials
– DON’T AIM for the marbles, aim for 1cm, then 2cm, etc.
and go back and forth until you have 200 trials
Lab 98: Half-Life
• Groups of 4
– 50 pennies
• Procedure Step 1
– Keep the “heads” pennies out of the box!
• Data Table A
– Number of Pennies removed it the TOTAL removed,
not just from that trial
• Analysis 4: Percent error
– (Analysis#2-theory)/theory x 100 = %