AP Chemistry Chapter 23 Notes

Henri Becquerel ruined some photographic plates with x-rays from a uranium source and radioactive decay was discovered in 1896.

Henri Becquerel’s experiment – (1896) Tried to see if fluorescent minerals would give off X-rays. Set some out in the sun with covered photographic film. If minerals gave of X-rays when they fluoresced, the film should darken – and it did. Accidentally set some of these minerals in a dark drawer for a few days with undeveloped film, and was surprised to see the film strongly exposed. He knew they gave off X-rays when charged by the sun - but these results suggested the X-rays were coming from the mineral itself – Natural Radioactivity – No external energy source required!

Radioactivity One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie (1876-1934). She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces.

Marie and Pierre Curie’s experiments with pitchblende – Discovered Radioactive Naturally occurring elements, particularly Uranium, Radium, and Polonium. Curium was named after Marie posthumously

THE GREAT DISCOVERY   W.K. Roentgen’s experiment (1895) - Fluorescence –Certain substances will absorb photons of energy when exposed to a source (i.e. cathode rays, the sun), and then emit them over a period of time – thus they glow in dark when exposed to UV light Cathode rays –beams of electrons Cathode ray tube (CRT) –Vacuum tube that has electric current passed through it . Component of television sets –that’s why they call it “the tube”   X-rays –Name given by Roentgen to unusual stray energy observed to cause fluorescence across the room when CRT was used… X-ray because he did not know what the heck it was….and the name stuck

BETA PARTICLES Consists of – high speed electron (from disintegration of neutron) Tissue damage potential – much greater than Alpha Harmful if ingested? – not as much as Alpha Can be blocked? – by glass, will penetrate skin

GOLD FOIL EXPERIMENT Ernest Rutherford and the Gold Foil Experiment GOLD FOIL EXPERIMENT Ernest Rutherford and the Gold Foil Experiment   Disproved Thompson’s plum pudding model Proved the existence of a nucleus with a positive charge 

ALPHA PARTICLES Consists of – He nucleus Tissue damage potential – great – if internalized Harmful if ingested? – yup, very Can be blocked? – by layer of skin, or cardboard Note that atoms are NOT conserved in nuclear reactions, but mass numbers and atomic numbers are.

NUCLEAR RADIATION   Ernest Rutherford and the Lead block experiment (1899) -   Alpha rays () Beta rays ()– Gamma rays ()

Thompson 1906 Rutherford 1913 Bohr 1924 How did Rutherford’s gold foil experiment change the theory of the structure of the atom? Thompson 1906 Rutherford 1913 Bohr 1924

ARCHITECTURE OF THE ATOM Atomic Number – Number of protons Determine what type of element an atom is Mass Number – Sum of total number of protons and neutrons in an atom Can change for an element depending upon the number of neutrons present Isotopes – Elements with the same atomic number, but different mass numbers Due to the difference in number of neutrons Example: C-14 and C-12 H-1, H-2, and H-3 Radioisotope – Isotope that is unstable and undergoes decay, thus giving off radiation      

Subatomic Particles PARTICLE LOCATION CHARGE MASS Proton nucleus +  + 1 amu Neutron 1 amu  Electron Outside nucleus  - 0.00054 amu 

Protons (Atomic Number) Common Isotopes Symbol Name Protons (Atomic Number) Neutrons Mass Number Electrons 73Li  Lithium -7  3   4 7  146 C Carbon-14   6 8  14 6 6731Ga Gallium -67 31  36 67    Isotopes of Particular interest – C-14 used in radiocarbon dating I-131 used in thyroid cancer treatment U-235 used in nuclear power

ISOTOPES IN NATURE Atomic Mass -Weighted Average mass of all existing isotopes of an element  Can be calculated by: (percent isotope 1)(molar mass isotopes 1) + (percent isotopes 2)(molar mass isotope 2) +…..     Try this with your grades as an example….   Final grades will be determined by giving homework 10%, labs 30%, and tests 60%… Homework grade = 85% Lab grade = 80% Test grade = 60%    Final grade = (.10)(.85) + (.30)(.80) + (.60)(.60) = .69   

Nuclear Section B Introduction Approx. 90 known naturally occurring elements Approx. 350 known isotopes in our solar system Approx. 70 of these radioactive Radioactive – just means unstable – it naturally decays Approx. 1,600 Lab created isotopes There is a rather constant level of natural radiation in our environment – called background radiation

Change in Atomic Number TABLE OF CHANGES RESULTING FROM NUCLEAR DECAY Type Symbol Change in Atomic Number Change in Neutrons Change in Mass Number Alpha    -2 -2  -4  Beta    +1  -1 0  Gamma    0

Spontaneous Radioactive Stability Production of an  particle Production of a  particle Production of  rays Spontaneous Fission

1. production of a particle

2. production of b particle

3. production of g rays

4. Spontaneous Fission

DECAY SERIES Shows the nuclear decay steps that occur when a radioactive isotope decays to a final stable product

II. Nuclear Fission

Sub-Critical Critical Supercritical

then radioactive decomposition: ½ life = 1.3 sec

then radioactive decomposition: ½ life = 1.3 sec

then radioactive decomposition: ½ life 18.3 months

then radioactive decomposition: ½ life 18.3 months

Other Types of Nuclear Reactions K-capture: the capture of an electron from the first or K shell

Other Types of Nuclear Reactions Positron (0+1b): a positive electron 207

An electron and proton combine Formation of a Neutron An electron and proton combine to form a neutron. 0-1e + 11p --> 10n

Less mass more protons fewer protons

III. Nuclear Fusion Example #1 Requires 40,000,000 K to overcome electrostatic repulsion

Half life SM x (1/2)n = EM (1/2)n = EM / SM or EM / SM = (1/2)n n Log (1/2) = Log (EM / SM) n = Log (EM / SM) / Log (1/2) n = t / t1/2 life

ln (N/No) = ln (1/2)n ln (N/No) = - kt t1/2life k = ln (1/2) = 0.693 t1/2life = 0.693/k A = kN

thus, N/Not = - kN1 where N = amount [conc or counts] and k = rate constant  dN/dt = - kN

Half-life : time when

Half-life

energy released during degradation of a nucleus Binding Energy energy released during degradation of a nucleus

E = mc2 Energy = mass x speed of light2 1 gram of mass = 9 x 1013 joules = amount of energy needed to power your house for 1,000 years

E = mc2 or E = c2m where c = 3.00 x 108 m/sec

Nuclear Fission: Splitting of an atom into 2 or more “daughter particles” If daughter particles are unstable, then they will be radioactive

Fission Chain Reaction

Hydrogen bombs

Results of fission reactions

IONIZING RADIATION – HOW MUCH IS SAFE? Rem – Roentgen equivalent to man 1Rem = 1000 mRem Does not matter what type of radiation it is, it still has the same ionizing effect on living tissue 1 mRem of exposure to radiation increases risk of cancer death by 1 in 4 million Two things to consider: Radiation density Radiation dose

RADIATION DAMAGE: NOW AND LATER Radiation damage to your body can occur in several ways: Break apart essential molecules proteins (i.e. enzymes) nucleic acids (i.e. DNA) Mutations Kills cells Mutates sperm/ova Cancer Government recommends no greater than exposure to 500 mrem per year for general public Government recommends no greater than 5,000 mrem per year from the workplace

Factor Effect Dose Exposure time Area Exposed Tissue type Table of Factors Effecting Biological Damage from Radiation Factor Effect Dose Increase in dose produces proportional increase in risk Exposure time Spreading out over time decreases risk Area Exposed Larger area means greater risk Tissue type Rapidly dividing cells more susceptible

Radiation effects by dosage

EXPOSURE TO RADIATION Exposure to radiation can come from: Cosmic Rays Radioisotopes in rocks, soil, water, air Fallout from nuclear weapons testing Air travel Radioisotope release from nuclear power generation Government recommends no greater than exposure to ______________for general public Government recommends no greater than ___________ per year from the workplace

SOURCES OF EXPOSURE TO IONIZING RADIATION

RADON IN HOMES Radon gas comes from: Gas released from earth (from Uranium decay Radon gas exposure can lead to: lung cancer ___________% of lung cancer deaths are caused by radon exposure. ___________% of households in the U.S. have higher than recommended radon levels.