Chapter 39 The Atomic Nucleus and Radioactivity Conceptual Physics Hewitt, 1999 Bloom High School
Dec 28, 2015
Chapter 39The Atomic Nucleus and Radioactivity
Conceptual PhysicsHewitt, 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) particle- helium nucleus– 2p+ with 2n0 ejected
from nucleus– Positively charged
particle
• Gamma (g) ray- electromagnetic 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, 12p+
• Atomic mass number- equal to the number of nucleons– Carbon-24 (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 is n0 + p+
• 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%)
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
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
• Uranium- decays in a predictable pattern– Lead is a decay
product, so is found in all uranium samples
39.10 Radioactive Tracers
• Radioactive tracer- radioactive 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
• Radiation more strong at high altitudes because there is less atmospheric shielding from cosmic radiation