Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The...

Nuclear Chemistry

The Nucleus

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

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

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

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

http://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass

Most nuclei are stable.

It is the ratio of neutrons to protons that determines the stability of a given nucleus.

Radioactivity

• It is not uncommon for some nuclei to be unstable, or radioactive.

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

• Radioisotopes = isotopes that are unstable and thus radioactive

• There are several ways radionuclides can decay into a different nuclide.

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).

• Transmutation = the reaction by which the atomic nucleus of one element is changed into the nucleus of a different element

pHET simulations of alpha decay of Polonium-211 to form Lead-207 and of

Beta decay of Hydrogen-3 to Helium-3

• http://phet.colorado.edu/en/simulation/alpha-decay

• http://phet.colorado.edu/en/simulation/beta-decay

Types of Radioactive Decay Alpha Decay

= Loss of an -particle (a helium nucleus)

He42

U238

92 Th234

90 He42+

CorrectionAtomic # decreases by 2# of protons decreases by 2# of neutrons decreases by 2Mass # decreases by 4

Types of Radioactive Decay Beta Decay

= Loss of a -particle (a high energy electron)

0

−1 e0

−1or

I131

53 Xe131

54 + e0

−1

Atomic # increases by 1# of protons increases by 1# of neutrons decreases by 1Mass # remains the same

Types of Radioactive Decay Positron Emission

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

e01

C11

6 B11

5 + e01

Atomic # decreases by 1# of protons decreases by 1# of neutrons increases by 1Mass # remains the same

Types of Radioactive Decay Gamma Emission

= Loss of a -ray (a photon of high-energy light that has no mass or charge & that almost always accompanies the loss of a nuclear particle; often not shown when writing nuclear equations)

00

Artificial Transmutation= done by bombarding the nucleus with high-energy particles (such

as a neutron or alpha particle), causing transmutation

4020Ca + _____ -----> 40

19K + 11H

9642Mo + 2

1H -----> 10n + _____

**Natural transmutation has a single nucleus undergoing change, while artificial transmutation will have two reactants (fast moving particle & target nuclei.**

Nuclear Fission• Nuclear fission is the type of reaction carried out in

nuclear reactors.• = splitting of large nuclei into middle weight nuclei and

neutrons

Nuclear Fission

• Bombardment of the radioactive nuclide with a neutron starts the process.

• Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

• http://phet.colorado.edu/en/simulation/nuclear-fission

• This process continues in what we call a nuclear chain reaction.

• = the combining of light nuclei into a heavier nucleus

• 21H + 2

1H 42He + energy

• Two small, positively-charged nuclei smash together at high temperatures and pressures to form one larger nucleus.

Nuclear Fusion

Energy changes in Nuclear Reactions E =mc2

• Einstein E =mc2

• mass defect For nuclear reactions

• E = energy in Joules (J = kg•m2/s2)

• m = mass in kg• C = speed of light • (2.9979 x 108 m/s)

Half-Life

= the time it takes for half of the atoms in a given sample of an element to decay

- Each isotope has its own half-life; the more unstable, the shorter the half-life.

- Table T Equations:

fraction remaining = (1/2)(t/T)

# of half-lives remaining = t/T

Key: t = total time elapsed T = half-life

PhET simulation of decay and half-life

• http://phet.colorado.edu/en/simulation/radioactive-dating-game

Sample Half-Life Question 1AMost chromium atoms are stable, but Cr-51 is an unstable isotope with a half-life of 28 days. (a) What fraction of a sample of Cr-51 will remain after 168 days?

Step 1: Determine how many half-lives elapse during 168 days.

Step 2: Calculate the fraction remaining.

Sample Half-Life Question 1B

(b) If a sample of Cr-51 has an original mass of 52.0g, what mass will remain after 168 days?

Step 1: Calculate the mass remaining:

mass remaining = fraction remaining X original mass

(Note: Mass remaining can also be calculated by dividing the current mass by 2 at the end of each

half-life.)

Sample Half-Life Question 2

How much was present originally in a sample of Cr-51 if 0.75gremains after 168 days?

Step 1: Determine how many half-lives elapsed during 168 days.

Step 2: Multiply the remaining amount by a factor of 2 for each half-life.

Some practical uses of Radioisotopes (dating, chemical tracers, industrial applications, medical

applications, nuclear power plants)

Medical Uses• 60Co (cobalt-60) used in

cancer treatments and used to kill bacteria in food products

• 226Ra (Radium-226) used in Cancer treatment

• 131I diagnosis and treatment of thyroid disorders

• 11C Positron emission tomography (PET scans)

Other Uses• 14C archaeological dating (of

once living things) and radiolabelled organic compounds

• 238U archaeological dating (U-238 to Pb-206 ratio)

• 241Am (Americium-241) smoke detectors

• 235U nuclear reactors and weapons