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INTRO TO NUCLEAR CHEMISTRY
A. Representation of an Atom
1. nucleons- description of atomic nuclei based on protons and neutrons particles
2. nuclide- used in nuclear chemistry as a reference to an atom, identified by the number of
protons and neutrons in its nucleus
3. Example:
Radium-228
OR
228
(atomic mass)
( name of element followed by atomic mass
Ra (symbol)
which is the # of protons + neutrons)
88
(atomic number)
B. Mass and Energy Relationships
1. mass defect- difference between the mass of an atom and the sum of the masses of the
subatomic particles composing that atom
2. Example: helium-4:
2 protons (2 x 1.007276 amu) = 2.014552 amu
2 neutrons (2 x 1.008665 amu) = 2.017330 amu
2 electrons (2 x 0.0005486 amu) = 0.001097 amu
total combined mass: 4.032979 amu
The actual mass of the atom is measured to be 4.00260 amu! That is 0.03038 amu less
then the sum of all of its particles.
3. The mass defect is caused by the conversion of mass (m) to energy (E) when the nucleus
was originally formed.
4. nuclear binding energy- the energy that was released when a nucleus is formed from
nucleons (E = mc2 ). It can also be thought of as the amount of energy required to break
apart the nucleus; therefore, the nuclear binding energy is also a measure of the stability of
a nucleus.
5. band of stability- a stable nuclei cluster shown as a neutron to proton ratio
a. most stable nuclei are 1:1 ratio
b. higher atomic masses show close to a 1.5:1 ratio; this is accounted for because proton
electrostatic repulsion is greater than the effect of nuclear forces at this point
C. What is Radioactivity?
1.
Radioactivity: is the act of emitting radiation spontaneously with the resulting emission
of radiation resulting in the formation of a new nuclei.
a. Does not need a source to travel through space and penetrate another material
2.
Atoms with unstable nuclei are radioactive
D. Transmutation:
1.
Elements with atomic numbers greater than 83 are radioisotopes
a. Those elements with atomic numbers less than 83 have isotopes and most have at least
1 radioisotope
2.
Radioisotopes try to stabilize
a. They try to transform into a new, stable element
b. This is transmutation
i. The change occurs due to changes in the nucleus and results in radioactive decay
HALF-LIFE REACTIONS
A. Half-life- the time required for half the atoms of a radioactive nuclide to decay (t 1/2) Each
radioactive nuclide has its own half-life. Example: Carbon-14 has a half-life of 5715-5730 years.
B. Example:
Phosphorus-32 has a half-life of 14.3 days. How many mg of phosphorus-32 remain after
57.2 days if you start with 4mg of the isotope?
Amount of Phosphorus-32
4 mg
2 mg
1 mg
0.5 mg
0.25 mg
Time Elapsed
0 days
14.3 days
28.6 days
42.9 days
57.2 days
Another way to work problems:
# of half-lives = time elapsed x ratio for half-life
# of half-lives = 57.2 days x 1 half-life = 4 half-lives
14.3 days
So now we know the answer will be:
4 mg x ½ x ½ x ½ x ½ = 0.25mg
C. Practice
1. The half-life of polonium-210 is 138.4 days. How many mg of polonium-210 remain after
415.2 days if you start with 2mg of the isotope?
2. The half-life of radon-222 is 3.824 days. After what time will ¼ of a given amount of radon222 remain?
D. Decay series- series of radioactive nuclides produced by successive decay until a stable form is
reached. Heaviest nuclide is parent, ones produced are daughter(s).