CHAPTER 28 Nuclear Chemistry Radioactive Decay Radioactive Decay

CHAPTER 28

Nuclear Chemistry

CHAPTER 28

Nuclear Chemistry

Radioactive Decay

Radioactive Decay

A. Nuclear StabilityA. Nuclear Stability

Nuclide = atom of an isotope

A. Nuclear StabilityA. Nuclear Stability

Nuclear stability – stable nuclei always have at least as many neutrons as protons.

A. Nuclear StabiityA. Nuclear Stabiity

For an odd/even or even/odd nucleus, if the mass number is different by more than 1 amu from the rounded atomic mass, the nuclide is unstable.

Ex: N177

A. Nuclear StabilityA. Nuclear Stability

For an even/even nucleus, if the mass number is different by more than 3 amu from the rounded atomic mass, the nuclide is unstable.

Ex: O208

A. Nuclear StabilityA. Nuclear Stability

For odd/odd nuclei, only four stable isotopes are found in nature:

H21 Li6

3 B105 N14

7

He42

B. Nuclear DecayB. Nuclear Decay

Alpha particle () helium nucleus paper2+

Beta particle (-) electron e0

-11-

leadPositron (+)

positron e01

1+

Gamma () high-energy photon 0

concrete

B. Nuclear DecayB. Nuclear Decay

Alpha Emission

He Th U 42

23490

23892

parentnuclide

daughternuclide

alphaparticle

Top and bottom numbers must balance!!

B. Nuclear DecayB. Nuclear Decay

Beta Emission

e Xe I 0-1

13154

13153

electronPositron Emission

e Ar K 01

3818

3819

positron

B. Nuclear DecayB. Nuclear DecayElectron Capture

Pd e Ag 10646

0-1

10647

electronGamma Emission

Usually follows other types of decay.

Transmutation Atom of one element changes into an

atom of another element.

B. Nuclear DecayB. Nuclear Decay

Why nuclides decay… need stable ratio of neutrons to protons

He Th U 42

23490

23892

e Xe I 0-1

13154

13153

e Ar K 01

3818

3819

Pd e Ag 10646

0-1

10647

DECAY SERIES TRANSPARENCY

C. Half-lifeC. Half-life

Half-life (t½) Time required for half the atoms of a

radioactive nuclide to decay. Shorter half-life = less stable.

D. Radiocarbon DatingD. Radiocarbon Dating

Carbon-14 is in all living things through the carbon cycle.

Amount of carbon-14 stays constant until organism dies, then it begins to decay.

N C 147

0-1

146

D. Radiocarbon DatingD. Radiocarbon Dating

Amount of carbon-14 can be expressed as either a percentage or as a decimal number.

Example: amount of carbon-14 in a dead tree could be expressed as 38% or 0.38 of the original amount.

N C 147

0-1

146

D. Radiocarbon DatingD. Radiocarbon Dating

Half-life of carbon-14 : 5730 years

E. FissionE. Fission

Occurs when isotopes are bombarded with neutrons and split the nucleus into smaller fragments, accompanied by the release of neutrons and a large amount of energy. (Each atom can capture 1 neutron.)

E. FissionE. Fission

Chain reaction – occurs when atomic nuclei that have split release energetic neutrons that split more nuclei.

E. FissionE. Fission

Two steps in controlling fission: Neutron moderation – water or carbon

slows down the neutrons Neutron absorption – decreases the

number of slow neutrons through the use of control rods made of neutron-absorbing materials (usually cadmium)

F. FusionF. Fusion

Occurs when two light nuclei combine to produce a nucleus of heavier mass, accompanied by the release of a large amount of energy.

F. FusionF. Fusion

Occurs in all starsHigh temperatures are necessary to

initiate fusion (no cold fusion yet)Possible future energy sourceHydrogen bomb is a fusion reaction

(fusion of two deuterium nuclei).

G. Methods of DetectionG. Methods of Detection

Geiger Counters (primarily beta)Scintillation counter – coated screen

detects radiation particles. Film badge – several layers of

photographic film encased in a holder. Detects beta and gamma.

H. Radioisotopes in Medicine

H. Radioisotopes in Medicine

X-rays: Useful in imaging soft-tissue organs.

Tracers: Iodine-131 is used to check for thyroid problems

Radiation treatment: Some cobalt isotopes are used as radiation sources to treat cancer.