Nuclear Chemistry

Chapter 25What so special?

Radioactivity

• Discovered accidentally using Uranium salts

• Without sunlight, Uranium could fog a photographic plate

• The Curies showed the fogging was due to rays emitted by the Uranium atoms

• Penetrating rays and particles emitted by radioactive source = radiation

Nuclear reactions

• Differ from chemical reactions• Chemical = stable electron configs

• Electrons just relocate within cmpd; nuclei stay the same

• Nuclear= nuclei of unstable isotopes (radioisotopes) gain stability by undergoing changes• Always produces large amounts of energy

• Not affected by changes in temp, pressure or catalysts• Cannot be sped up, slowed down or stopped

Radioactive decay• Radioisotopes have unstable nuclei

• Stability depends on the ratio of protons to neutrons and the overall size of the nucleus

• Too many or too few neutrons causes instability

• An unstable nucleus releases energy by emitting radiation during process called radioactive decay• Unstable isotopes of one element becomes

stable isotopes of a different element• Decay is spontaneous and requires no

energy

Types of Radiation (3)• Alpha radiation (stopped by paper)• Release alpha particle

• He nuclei (2p+ & 2n0 & 0e-)

• U-238 Th-234 + He2+ (α particle)• Beta radiation (stopped by wood)• Neutron breaks into p+ & e-

• p+ stays in nucleus, e- released

• n0 H+ (proton) + e- (β particle)

• C-14 (radioactive) N-14 (stable) + e- (β)

Figure 25.2

Gamma Radiation (3rd)

• High energy photon• No mass• No electrical charge• Often emitted with alpha particle

• Thorium-230 Radon-226 + α + γ ray

• Does not alter the atomic number or mass number

• Extremely penetrating and dangerous• Can be almost completely stopped by several

m of concrete or cm of lead

Figure 25.3

Nuclear stability• Low atomic # (<20) 1:1 (n0 : p+)• Up to 1.5:1 for heavy elements • The n:p determines the type of decay

• Too many n0 • n0 turns into p+ and emits β

− beta emission− ↓ n0, ↑p+, ↑ e-

• Too few n0 • p+ becomes a n0 by nucleus engulphing an e-

− Called electron capture− ↑ n0, ↓ p+, ↓ e-

• OR a p+ changes to a n0 (Positron emission) − ↑n0, ↓ p+

− Positron = particle with mass of e- but + charge

Figure 25.4 red line = Band of Stability

Generalizations

• All elements > 83 are radioactive• Have too many n0 AND p+ to be stable• Most undergo α emission

• Inc n0 : p+ • Mass # - 4, at # - 2

• Mass is not conserved• Very small amount of mass is converted into

energy and released during radioactive decay

• (hence photographic plate fogging)

Half-Life (t1/2)

• Time required for ½ of nuclei of a radioisotope sample to decay

• So, after each half-life, half the existing radioactive atoms have decayed into atoms of a new element

• Some are billions of years long, others fractions of a second

Figure 25.5

Table 25.3½ lives and radiation of some naturally occurring radioisotopes

Isotope Half-life radiation

C-14 5.73E3 years β

K-40 1.25E9 years β, γ

Rn-222 3.8 days α

Ra-226 1.6E3 years α, γ

Th-234 24.1 days β, γ

U-235 7.0E8 years α, γ

U- 238 4.46E9 years α

Transmutation reactions

• The conversion of an atom of one element into the atom of another element• Can occur by radioactive decay• Or when particles bombard the

nucleus of an atom • Particles can be p+, n0, or α particles

− Remember, α particle = He nucleus

Transmutations where?

• Occur naturally • N-14 C-14 in upper atmosphere• U-238 (x 14) Pb-206

• In laboratories • First done in 1919 by Rutherford

• N-14 + α F-18 (quickly O-17 + p+)• Lead to discovery of p+

• Chadwick found neutron in 1932• Be-9 + α C-12 + n0

• Nuclear reactors• Transuranium elements

Transuranium Elements

• At # > 92 (aka U)• All undergo transmutation• None occur in nature• All are radioactive• All synthesized in nuclear reactors and

nuclear accelerators• Accelerators accelerate bombarding

particles to very high speeds• Reactors produce beams of low-energy

bombarding particles• Hadron Accelerator

Examples- fyino, you do not need to know these

• U-238 + n0 (very slow moving) U-239*• *U-239 is radioactive

• U-239 Np-239* + β• *Np-239 is also radioactive and thus

unstable

• Np-239 Pu-239 + β

• Both Np and Pu were synthesized in 1940 in Berkley, Ca

Fission of Atomic Nuclei

• U-235 and Pu-239 are the only nuclei that can undergo fission• The splitting of a nucleus into smaller

fragments as a result of bombardment by slow moving neutrons

• Chain rxn when neutrons given off during fission of one nucleus strike another fissionable atom

Fun Facts about Fission

• Releases HUGE amounts of energy… 1 kg U-235 energy equal to that of 20,000 tons of dyn-o-mite!• In uncontrolled nuclear chain reaction (like

an atomic BOMB) the energy is released in fractions of a second!

• Can be controlled in nuclear reactors to make use of the energy in small, slowly released amounts

Nuclear Fusion fun facts

• Fusion = nuclei combine to produce a nucleus of greater mass

• Energy released by the sun (Earth’s major source of energy) results from nuclear fusion

• fusion releases MORE energy from little nuclei than fission from big nuclei

• Catch= fusion only occurs at ridiculously high temps• > 40,000,000°C

Fusion to be used under control on earth?

• Attempts made to combine H-2 + H-3 He-4 + n0 + ENERGY

• Problem = temp• So far, only way to get temp up is to use

a fission bomb • like the one used to trigger the controlled

fusion reaction that is called a H bomb − Therefore, not a useful idea…

Detecting Radiation• Geiger counters

• detects α,β, & γ with audible clicks

• Scintillation counters • Uses phosphor-coated surface

• Film badges• Made of several layers of

photographic film

Using radiation

• Important in many scientific procedures• Used in agriculture as “tracers” to test effect

of pesticides, herbicides, and fertilizers

• To diagnose medical problems• I-131 used to identify thyroid disorders

• To treat some diseases• Pharmaceuticals sometimes used as radiation

therapy• EX: More I-131 than for test = absorbed and emits β

& γ rays