Download
1 / 19
190 likes | 192 Views
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.
E N D
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 Nuclei The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability. 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
238 92 234 90 4 2 4 2 He U Th He + Types of Radioactive Decay Alpha Decay = Loss of an -particle (a helium nucleus) Atomic # increases by 2 # of protons decreases by 2 # of neutrons decreases by 2 Mass # decreases by 4
131 53 131 54 0 −1 0 −1 0 −1 e I Xe e + or Types of Radioactive Decay Beta Decay = Loss of a -particle (a high energy electron) Atomic # increases by 1 # of protons increases by 1 # of neutrons decreases by 1 Mass # remains the same
11 6 11 5 0 1 0 1 e C B e + Types of Radioactive Decay Positron Emission = Loss of a positron (a particle that has the same mass as but opposite charge than an electron) Atomic # decreases by 1 # of protons decreases by 1 # of neutrons increases by 1 Mass # remains the same
0 0 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)
Artificial Transmutation = done by bombarding the nucleus with high-energy particles (such as a neutron or alpha particle), causing transmutation 4020Ca + _____ -----> 4019K + 11H 9642Mo + 21H -----> 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. • This process continues in what we call a nuclear chain reaction.
Nuclear Fusion • = the combining of light nuclei into a heavier nucleus • 21H + 21H 42He + energy • Two small, positively-charged nuclei smash together at high temperatures and pressures to form one larger nucleus.
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
Sample Half-Life Question 1A Most 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 0.75g remains 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.
