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 (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 4000 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

• 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– 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 1: Percent error– (computed-measured)/measured x 100 = %