Nuclear Chemistry

Brown, LeMay Ch 21AP Chemistry

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21.1: Radioactivity• Result of unstable nuclei

Nucleons: particles in the nucleus, n0 and p+

Radioisotopes: atoms that containing radioactive nuclei (or radionuclides)

Nuclear reactions or equations: express products of radioactive decay, fusion, or fission

Radioactive decay: process in which a radionuclide spontaneously decomposes

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Most common types of radioactive decayType Symbol Description Travels in air…

Ex:

alpha

or

energized He nucleusHe

4 2+2

A few cm; cannot penetrate

human skin

U23892

He42

Th234

90+

4

Alpha decay

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Ex:

Same as:

beta or

High energy electron

e0

-1

~300 cm; can penetrate skin, but

rarely

I13153

Xe131

54e

0-1

+

n 1 0

p 1

1e

0-1

+

A neutron converts to a proton and

electron

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Ex:

gamma

orphoton

0

0

Very far; can be stopped by ~5 cm of

Pb

Pu244

94+

0 0Pu

24494

*

• Represents energy emitted (i.e., radiation) when nucleons in an unstable radionuclide reorganize to become more stable

• Usually not written in a nuclear reaction.

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Ex:

Same as:

positron • Antimatter (positively charged) e; collides with e- and both are annihilated as gamma rays are created

e0

1

C116

B 11

5e

0 1

+

p 1 1

n 1

0e

0 1

+

A proton converts to a positron and

neutron

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Ex:

Same as:

Electron capture

• Capture of inner shell e- by nucleus

Rb8137

Kr 81 36

+

p 1 1

n 1

0

A proton and electron convert to a

neutron

e0

-1

e0

-1

e0

-1+

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21.2: Nuclear stabilityStrong nuclear force: pulls nucleons together to

form nuclei (actually acts on quarks)* Weak nuclear force: responsible for changes

in flavor of quarks

• Nuclei become unstable (radioactive) if the neutron-to-proton ratio “strays” too far from “normal range”

• * Nuclear shell model: when p and n fill nuclear shells, atoms are unusually stable:“Magic numbers” 2, 8, 20, 28, 50, 82, 126

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A radionuclide will decay until a stable ratio exists:

• If too many n, n will be converted to p by emission.

• If too few n, p will be converted to n by positron emission or electron capture.

• Nuclei with p ≥ 84 undergo emission 1

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21.4: Rates of DecayHalf-life (t½):• Time for ½ a radioactive (i.e., having an

unstable p/n ratio) material to decay (form 2 or more stable atoms)

1/21/2 t0.693

t2ln k

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21.6: Mass-energy relationshipsE = m c2

(mass in kg)

Mass → energy Mass lost during radioactive decay is released as

energy

Energy → mass Mass defect (m): mass difference between a nucleus

and its constituent nucleons; the nuclear bonding energy must be added to a nucleus to break it into its nucleons

When energy is added, the nucleons separate and gain mass

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21.7 & 21.8: Fission & FusionFission: splitting of a nucleus; some mass is

lost, which results in release of energy (ex: nuclear power plants, “atomic” bombs)

Ba +13956 Kr +94

36 3 n + energy10 U235

92n + 10

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Fusion: combination of 2 nuclei; some mass is lost, which results in release of energy (ex: stars, “H” bombs)

H + 31

H

21

He +42

n10

+ energy

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* Fundamental Particles and the Standard Model of the Atom

6 flavors of quarks:

Inc Mass

↓

Q = -1/3 Q = 2/3

Down (d) Up (u)

Strange (s)Charm (c):

discovered 1974 at 1.5 GeV

Bottom (b): discovered 1978 Top (t)

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* Ordinary matter is made of:

p+: u-u-d quark tripletn0: u-d-d quark triplete-: one of 6 leptons

decay: d quark in a n changes into u quark, making a p