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Nuclear Chemistry

Chapter 10 – Prentice Hall Physical Science

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Review

All the chemistry we’ve discussed so far has involved electrons.

Questions:1. 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?

2. If you tossed 128 coins in the air, about how many would you expect to land heads-up?

3. What do the mass number and atomic number represent?

4. Which subatomic particles are found in the nucleus?

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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)

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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:

U23892

Uranium-238

C146

Carbon-14

Po21094

Polonium-210

Mass # on top

Atomic # on bottom

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3 Types of Nuclear Radiation

Nuclear radiation: charged particles and energy that are emitted from the nuclei of radioisotopes

Radiation Type

Symbol ChargeMass (amu)

Common Source

Alpha particle

, 2+ 4 Radium-226

Beta particle

, 1- Carbon-14

Gamma ray

0 0 Cobalt-60

He42

e01 1836

1

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Alpha Decay

Alpha particle, 2 protons and 2 neutronsPositively chargedSame as He nucleus

Least penetrating type of nuclear radiationTravel only centimeters in airCan be stopped by a sheet of paper or

clothing

He42

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Beta Decay

Beta particle, 1 electronNegatively chargedProduced by a neutron that decomposes into

a proton and an electron More penetrating than particles

Pass through paperStopped by a thin sheet of metal

e01

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Gamma Decay

Gamma ray, 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

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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!

Writing and Balancing Nuclear Reactions

ePaTh

HeThU

01

23491

23490

42

23490

23892

Reactants → Products

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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.

Po21084 He4

2 XAZ

isotope.product of symbol chemical X and ,# mass A ,# atomic Let Z

+

2064210

4210

A

A

82284

284

Z

Z

Atomic # 82 = Pb (Lead)

PbHePo 20682

42

21084

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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?

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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

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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

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Rates of Nuclear Decay (cont’d)

Nuclear Decay Rate

0

20

40

60

80

100

0 1 2 3 4 5

Time (# of Half-lives)

Rad

iois

oto

pe

Rem

ain

ing

(%

)

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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

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Radiocarbon dating

Carbon-14 exists naturally in the atmosphere at a fairly constant ratio to C-12

As C-14 decays, it’s replaced by C-14 absorbed

from atmosphere

Tree dies – no more CO2 absorbed to

replace decaying C-14

CO2 absorbed while living (including some C-14)

Age of fossil determines by

comparing C-14/C-12 ratio in fossil to

atmospheric ratio

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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?

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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

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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

HOHeN 11

178

42

147

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Nuclear 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.

Small nucleusProton from a small nucleus

Proton from a large nucleus

Strong nuclear forces:

Electric forces:

Large nucleus

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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 energyNeutron ((

((

(( ))

((

((

))

))

U23592 U236

92

(very unstable)

Energy

Kr9136

Ba14256

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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.)

J109s

m100.3kg1

s

m100.3

kg1

162

82

8

mcE

c

m This quantity = 2.5 x 1010 kWh, or enough energy to power my house for over 3.6 million years!

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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.

Neutron

U23592

Energy

Kr9136

Ba14256

U23592

U23592

U23592

Energy

Kr9136 Ba142

56

+ + +

Energy

Kr9136 Ba142

56

+ + +

Energy

Kr9136 Ba142

56

+ + +

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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

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Nuclear Fusion

Fusion: nuclei of two atoms combineThe sun and other stars are powered by

fusion of H into HeRequires extremely HIGH temperaturesWhat state is matter in at such high

temperatures? PLASMA

Fusion

+ + ENERGY (17.6 MeV)

H21

H31 He4

2 n10