Nuclear Chemistry

The Nucleus

• Remember that the nucleus is comprised of the two nucleons, protons and neutrons.

• The number of protons is the atomic number.• The number of protons and neutrons together is

effectively the mass of the atom.

Isotopes

• Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms.

• There are three naturally occurring isotopes of uranium:– Uranium-234– Uranium-235– Uranium-238

Radioactivity

• It is not uncommon for some nuclides of an element to be unstable, or radioactive.

• We refer to these as radionuclides.• There are several ways radionuclides can

decay into a different nuclide.

Types ofRadioactive Decay

Alpha Decay:

Loss of an -particle (a helium nucleus)

He42

U23892 U234

90 He42+

Beta Decay:

Loss of a -particle (a high energy electron)

0

−1 e0−1or

I13153 Xe131

54 + e0−1

Positron Emission:

Loss of a positron (a particle that has the same mass as but opposite charge than an electron)

e01

C116 B11

5 + e01

Gamma Emission:

Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)

- results from a loss of mass when particles splits from the nucleus

00

Neutron-Proton Ratios

• Any element with more than one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus.

• A strong nuclear force helps keep the nucleus from flying apart.

Neutron-Proton Ratios

• Neutrons play a key role stabilizing the nucleus.

• Therefore, the ratio of neutrons to protons is an important factor.

Neutron-Proton Ratios

For smaller nuclei (atomic # 20) stable nuclei have a neutron-to-proton ratio close to 1:1.

Neutron-Proton Ratios

As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

Stable Nuclei

The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

Stable Nuclei

• There are no stable nuclei with an atomic number greater than 83.

• These nuclei tend to decay by alpha emission.

Radioactive Series

• Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation.

• They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

Half - Life

Half - Life

Definition: the amount of time that it takes for half of a radioisotope to naturally decay into another element

I – 131 has a half life of

- if 100 g of I- 131 were allowed to sit for 8.07 days 50 g would remain

- if allowed to sit for 16 days, 25 g would remainWhat would the I-131 turn into? (Write the

equation using table N.) 131I → 131Xe + -1e

- Half lives are not effected by temp or press

How can we make calculations using half life? (let’s use C-14 as an example)

# of half lives Time (min)

Fractionremaining

Massremaining

- To determine the time start at 0 (zero) and add one half life - To determine the mass remaining, begin with the original mass of the sample and divide by two every step down

How much radium – 226 will remain of a 10.0 g sample after 4800 years?

Transmutation

Def: a rxn in which the nuclei of an element is changed into the nuclei of another element– Natural radioactive decays • (Natural Transmutations), spontaneous

Positron

Artificial transmutations- caused by the bombardment of nuclei with high energy particles

- can bombard a nuclei with an alpha particle, neutrons, or protons

- must be supplied with sufficient energy to overcome force of repulsion from positive nucleus

4He + 27Al 30P + _____

Artificial transmutations

Identify the missing product:

14N + 4He X + 1H

Fission rxn that involves the splitting of heavy nuclei to produce

lighter nuclei produces tremendous amts of energy (much greater than

normal chem rxn) Conversion of 1.0 kg of matter produces

E = mc2

= 1.0 kg x (3.0 x 108)2 = 9.0 x 1016 J

compared to burning of 1.0 kg of methane, about a billion times

greater

Fission

• During fission a nucleus is bombarded with a neutron, which makes the nuclei unstable

1n + 235U 142Ba + 91Kr + 31n + energy

Fusion• the combining of 2 lighter nuclei to produce a larger nuclei

• occurs naturally on the sun, high temps needed to fuse together nuclei

• 1H + 1H 2H + +1e

• 1H + 2H 3He

• 3He + 3He 4He + 21H

Beneficial uses of radioisotopes

1. Uses in medicine a. I – 131 used in identifying thyroid problems

b. Co – 60 used in treating cancer

2. Uses in Geology/Anthropology

a. C – 14 to C – 12 ratio used to date geological organisms

b. Pb – 206 and U – 238 used to date geological formations