1. Chapter 10 – Prentice Hall Physical Science
Nuclear Chemistry
Chapter 10 – Prentice Hall Physical Science

2. Nuclear Chemistry - Prentice Hall Physical Science Chapter 10
Review
All the chemistry we’ve discussed so far has involved electrons.
Questions:
If element X has a molar mass of 3 g/mol and element Y has a molar mass of 5 g/mol, what must be the molar mass of X2Y?
If you tossed 128 coins in the air, about how many would you expect to land heads-up?
What do the mass number and atomic number represent?
Which subatomic particles are found in the nucleus?

3. Radioactivity
Antoine Henri Becquerel (1896) experimented with uranium salts and discovered radioactivity
Radioactivity (or nuclear decay): an unstable nucleus emits charged particles and energy
Radioisotope: radioactive isotope - any atom that has an unstable nucleus. Examples:
Uranium-238 (used in Becquerel’s experiment)
Carbon-14 (used often in radioactive dating)

4. Isotope symbology
Isotopes are named using the element name followed by the mass number (see examples, slide #3)
The symbol for isotopes includes the element symbol, the mass number and the atomic number as follows:
Mass # on top
Uranium-238
Carbon-14
Polonium-210
Atomic # on bottom

5. 3 Types of Nuclear Radiation
Nuclear radiation: charged particles and energy that are emitted from the nuclei of radioisotopes
Radiation Type
Symbol
Charge
Mass (amu)
Common Source
Alpha particle a, 2+ 4
Radium-226
Beta particle b, 1-
Carbon-14
Gamma ray g
Cobalt-60

6. Alpha Decay Alpha particle, a
2 protons and 2 neutrons
Positively charged
Same as He nucleus
Least penetrating type of nuclear radiation
Travel only centimeters in air
Can be stopped by a sheet of paper or clothing

7. Beta Decay Beta particle, b More penetrating than a particles
1 electron
Negatively charged
Produced by a neutron that decomposes into a proton and an electron
More penetrating than a particles
Pass through paper
Stopped by a thin sheet of metal

8. Gamma Decay
Gamma ray, g
Penetrating ray of energy
Like X-rays and light, only very short wavelength
Most penetrating form of the three types discussed
Often accompanies alpha or beta decay
Several centimeters of lead or several meters of concrete required to stop it

9. Writing and Balancing Nuclear Reactions
Similar to chemical equations, but isotope symbols are used.
In a balanced nuclear equation:
Mass # on the left = sum of mass #s on the right
Atomic # on the left = sum of atomic #s on the right
You will need to use your PERIODIC TABLES!
Reactants → Products

10. Example: Math Skills p. 295
Write a balanced nuclear equation for the alpha decay of polonium-210.
Step 1: Define reactants and products. Use letters to represent the unknown values.
Step 2: Write and solve equations to find unknown atomic and mass #s.
Step 3: Look up the element symbol on the periodic table using the atomic #.
Step 4: Write the balanced nuclear equation and double-check your solution. +
Atomic # 82 = Pb (Lead)

11. Effects of Nuclear Radiation
Background radiation: naturally occurring in the environment
Sources:
Radioisotopes in air, water, rocks & living things
Cosmic radiation
Generally at safe levels
Nuclear radiation can ionize atoms. At levels significantly above background, this can damage DNA and proteins
Which type of nuclear radiation is the least harmful? Which the most?

12. Detecting Nuclear Radiation
Geiger counters
Use gas-filled tubes to measure ionizing radiation
Gas produces an electric current when exposed to ionizing radiation
Film badges
Photographic film wrapped in paper
Film is exposed with exposure to radiation like photographic film is “exposed” with exposure to visible light

13. Rate of Nuclear Decay
Nuclear decay rate describes how fast nuclear changes take place
Unlike chemical reactions, nuclear decay rate does NOT vary with external conditions – it is constant for a given radioisotope
Half-life: the time required for half of a radioisotope sample to decay

14. Rates of Nuclear Decay (cont’d)

15. Rates of Nuclear Decay (cont’d)
Different radioisotopes have different half-lives
To determine how many half-lives have elapsed for a sample, divide the total time of decay by the half-life
Known decay rates are used in radioactive dating
Radioisotope
Half-life
Radon-222
3.82 days
Iodine-131
8.07 days
Carbon-14
5730 years
Thorium-230
75,200 years
Uranium-238
4.47x109 years

16. Radiocarbon dating
Carbon-14 exists naturally in the atmosphere at a fairly constant ratio to C-12
CO2 absorbed while living (including some C-14)
Age of fossil determines by comparing C-14/C-12 ratio in fossil to atmospheric ratio
As C-14 decays, it’s replaced by C-14 absorbed from atmosphere
Tree dies – no more CO2 absorbed to replace decaying C-14

17. Radiocarbon dating (cont’d)
Used for objects less that 50,000 years old
For older objects, must use different isotope with longer half-life
What isotopes would work well to date a rock formation that is thought to be close to a trillion years old?

18. Artificial Transmutation
Transmutation: conversion of atoms of one element into atoms of another
Alchemists have attempted this for hundreds of years (but not through nuclear chemistry)
First artificial transmutation: Ernest Rutherford (1919) turned nitrogen into oxygen-17

19. Artificial Transmutation (cont’d)
Transmutation achieved by bombarding atomic nuclei with high-energy particles
Protons, neutrons or alpha particles
Example: Ernest Rutherford’s transmutation used which particle?
Transuranium elements
Many produced by artificial transmutation of a lighter element
All are radioactive

20. Nuclear Forces The strong nuclear force attracts protons and neutrons.
Strong nuclear forces:
Electric forces:
The strong nuclear force attracts protons and neutrons.
Stronger than electric forces over short distances
Decreases with distance (like gravity)
Electric repulsions push protons apart.
When a nucleus is large enough, the electric forces can overcome the strong nuclear forces.
Nuclei are unstable at this point.
Any atom with 83 or more protons is unstable – and, therefore, radioactive.
Proton from a small nucleus
Small nucleus
Proton from a large nucleus
Large nucleus

21. Fission Fission: splitting of nucleus into two smaller parts
Lise Meitner, Fritz Strassman and Otto Hahn’s experiments (1939) first demonstrated nuclear fission.
A small amount of the original mass is converted into a lot of energy
Neutron (( ))
(very unstable)
Energy

22. Fission (cont’d)
About how much energy was released from 6.2 kg of Plutonium-239 in the second atomic bomb explosion? (Note: Only about 1 kg underwent fission – the rest was scattered.)
This quantity = 2.5 x 1010 kWh, or enough energy to power my house for over 3.6 million years!

23. Fission and Chain Reactions
Fission can result in a chain reaction.
Neutrons released from the first reaction can trigger another reaction, and so on – similar to a rumor spreading.
Energy +
Energy
Neutron
Energy +
Energy +

24. Chain Reactions (cont’d)
For a chain reaction to happen, each split nucleus must produce at least one neutron with enough energy to split another nucleus
This only happens when a specific mass of fissionable material is available – called the critical mass.
Controlled chain reactions are used to generate electricity in nuclear power plants.
Uncontrolled chain reactions are used in nuclear weapons

25. Nuclear Fusion Fusion: nuclei of two atoms combine
The sun and other stars are powered by fusion of H into He
Requires extremely HIGH temperatures
What state is matter in at such high temperatures? PLASMA
Fusion +
ENERGY (17.6 MeV)