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 lightCathode rays –beams of electronsCathode 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 EXPERIMENTGOLD FOIL EXPERIMENTErnest Rutherford and the Gold Foil Experiment Ernest Rutherford and the Gold Foil Experiment   

Disproved Thompson’s plum pudding modelDisproved Thompson’s plum pudding modelProved the existence of a nucleus with a positive charge Proved the existence of a nucleus with a positive charge 

•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.

ALPHA PARTICLES

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

How did Rutherford’s gold foil experiment change the theory of the structure of the atom?

Thompson1906

Rutherford1913

Bohr1924

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   

PARTICLE LOCATION CHARGE MASS

Protonnucleus

 +1 amu

Neutron nucleus 0 1 amu 

Electron Outside nucleus  - 0.00054 amu 

Subatomic Particles

Symbol Name Protons (Atomic Number)

Neutrons Mass Number

Electrons

73Li  Lithium -7  3   4 7  3 

146 C Carbon-14   6 8  14 6

6731Ga Gallium -67 31  36 67  31 

           

           

Common Isotopes

Isotopes of Particular interest –C-14 used in radiocarbon dating I-131 used in thyroid cancer treatmentU-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

Approx. 1,600 Lab created isotopes

Radioactive – just means unstable – it naturally decays

There is a rather constant level of natural radiation in our environment – called background radiation

Type Symbol Change in Atomic Number

Change in Neutrons

Change in Mass

NumberAlpha    -2 -2  -4 

Beta    +1  -1 0 

Gamma    0 0  0 

TABLE OF CHANGES RESULTING FROM NUCLEAR DECAY

Spontaneous Radioactive Stability

1. Production of an particle

2. Production of a particle

3. Production of rays

4. Spontaneous Fission

1. production of particle

? He U 42

23892

Th He U 23490

42

23892

2. production of particle

? eTh 01

23490

Pa eTh 23491

01

23490

3. production of rays

?U*238

92

UU 23892

23892

*

4. Spontaneous Fission

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

II. Nuclear Fission

Sub-CriticalSub-Critical

CriticalCritical

SupercriticalSupercritical

then radioactive decomposition:

?? n β Kr 10

01-

9236

½ life = 1.3 sec

then radioactive decomposition:

½ life = 1.3 sec

Rb n β Kr 9137

10

01-

9236

then radioactive decomposition:

?? βBa 01-

14156

½ life 18.3 months

then radioactive decomposition:

½ life 18.3 months

La βBa 14157

01-

14156

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+1): a positive electron

207 207

Formation of a Neutron

An electron and proton combine to form a neutron.

0-1e + 1

1p --> 10n

fewer protons

more protons

Lessmass

III. Nuclear FusionExample #1

n He H H 10

42

31

21

Requires 40,000,000 K to Requires 40,000,000 K to

overcome electrostatic repulsionovercome electrostatic repulsion

Half life Half life

SM x (1/2)SM x (1/2)nn = EM = EM

(1/2)(1/2)nn = EM / SM or EM / SM = (1/2) = EM / SM or EM / SM = (1/2)nn

n Log (1/2) = Log (EM / SM)n Log (1/2) = Log (EM / SM)

n = Log (EM / SM) / Log (1/2)n = Log (EM / SM) / Log (1/2)

n = t / tn = t / t1/2 life1/2 life

ln (N/Nln (N/Noo) = ln (1/2)) = ln (1/2)nn

ln (N/Nln (N/Noo) = - kt) = - kt

tt1/2life1/2life k = ln (1/2) = 0.693 k = ln (1/2) = 0.693

tt1/2life 1/2life = 0.693/k= 0.693/k

A = kNA = kN

thus, N/Not = - kN1

where N = amount [conc or counts]and k = rate constant

dN/dt = - kN

t

0

N

N

dtk N

dN

0

t0

NN |kt| N ln

0

ktN

Nln

0

0N ln-ktN ln

Half-life : time when

0N2

1N

21kt

N

N21

ln0

0

Half-life

k

2ln 2

1 t

k

1/2ln -2

1 t

Binding Energyenergy 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 = c2m

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

Table of Factors Effecting Biological Damage from Radiation

FactorFactor EffectEffect

DoseDose Increase in dose Increase in dose produces proportional produces proportional increase in riskincrease in risk

Exposure timeExposure time Spreading out over time Spreading out over time decreases riskdecreases risk

Area ExposedArea Exposed Larger area means Larger area means greater riskgreater risk

Tissue typeTissue type Rapidly dividing cells Rapidly dividing cells more susceptiblemore 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 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.

RADON IN HOMES