Ch. 4 Nuclei and radioactivity The atomic hypothesis Mass concentrated in tiny nucleus with electrons around it Atom 10 –10 m, nucleus is 10 –15 m Nucleus.

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Presentation transcript:

Ch. 4 Nuclei and radioactivity The atomic hypothesis Mass concentrated in tiny nucleus with electrons around it Atom 10 –10 m, nucleus is 10 –15 m Nucleus contains protons (positive) and neutrons (no charge) # of protons = atomic number protons + neutrons gives atomic weight

Periodic Table Elements arranged in a systematic way by their atomic numbers Elements in a group have similar chemical properties because of the way their outermost electrons are arranged Isotopes have different number of neutrons (same # protons) Interactive Periodic Table

Radiation When energy comes out of an atom, it’s usually called radiation. Caused by electrons: light, x-rays, low energy electrons Caused by nuclear processes: –  helium nucleus –  electron or positron –  high energy photon –p,nproton or neutron –Other energetic particles possible

Radiation Units Geiger counter video - watch dial curie (Ci) and becquerel (Bq) are certain numbers of decays per second rad and gray (Gy) measure how much energy is deposited in a kg of tissue. rem and sievert (Sv) measure how much damage is done. 1 Sv = 100 rem rem = rad * RBE It’s rems that harm you

Radiation Sickness Consider whole body doses < 100 remno short-term illness nausea, hair loss 300LD50 > 1000survival unlikely Alexander Litvinenko

Cancer and the linear hypothesis 2500 rem whole body gives 100% chance of excess cancer So 25 rem whole body gives 1% chance of excess cancer Give 100 people 25 rem each, expect one extra cancer (20 from other causes) Most people assume linear hypothesis at all dose levels, so 25 mrem gives 0.001% chance of excess cancer.

Radioactive half-life time T) = e –constant*T e –constant*T is called the decay factor T 1/2 is called the half-life At T = T 1/2, the decay factor = 1/2 At T = 2 T 1/2, the decay factor = 1/4 Half lives can range from tiny fractions of a second to billions of years Provides “clocks” for dating rocks, etc.

Fission and fusion energy Fission occurs when a nucleus splits Fragments have less mass than the original nucleus; E = mc 2 Fusion occurs when two lighter nuclei fuse into a single heavier nucleus. Again, mass goes missing and comes out as energy (lots of it) How can this happen in both cases?

Fusion generates lots of relatively clean power, but requires high temperature and probably magnetic confinement Fusion is the power source of stars Fusion bombs are the most powerful ever made