Nuclear Chemistry Pre Ap2008

Nuclear Chemistry • In nuclear chemistry, an atom is referred to as a nuclide and is identified by the number of protons and neutrons in the nucleus. • Nuclear reaction – a reaction that affects the nucleus of an atom. • Transmutation – a change in the identity of a nucleus as a result of a change in the number of protons.

Radioactive Decay • Radioactive decay – spontaneous disintegration of a nucleus into a slightly lighter and more stable nucleus, accompanied by emission of particles, electromagnetic radiation, or both. • Energy & matter released in nuclear radiation • Radiation is damaging. • During radioactive decay, radiation exits the nucleus & interacts with matter nearby.

Nuclear Radiation • Nuclear radiation can transfer its energy to the electrons of atoms or molecules and cause ionization. • The roentgen is a unit used to measure nuclear radiation; it is equal to the amount of radiation that produces 2 x 109 ion pairs when it passes through 1 cm3 of dry air. • One rem is the quantity of ionizing radiation that does as much damage to human tissue as is done by one roentgen of high-voltage X-rays. • Everyone is exposed to environmental background radiation.  Average exposure for people living in the U.S. is 0.1 rem per year.

Sources of Radiation

Doserem 5-20 20-100

Effects Possible late effects; possible chromosomal damage. Temporary reduction in white blood cells.

100-200

Mild radiation sickness within a few hours: vomiting, diarrhea, fatigue; reduction in resistance to infection.

200-300

Serious radiation sickness effects as in 100-200 rem and hemorrhage; exposure is a Lethal Dose to 10-35% of the population after 30 days (LD 10-35/30).

300-400

Serious radiation sickness; also marrow and intestine destruction; LD 50-70/30.

400-103 Acute illness, early death; LD 60-95/30. 103-5000 Acute illness, early death in days; LD 100/10.

Types of Nuclear Radiation • 5 types of nuclear radiation:     

alpha particles (α) beta particles (β) gamma rays (γ) electron capture of an inner orbital electron, positron emission to form neutrons

• Nuclear Decay — when an unstable nucleus emits alpha or beta particles, the number of protons & neutrons may change.  This process can be written in a nuclear equation/nuclear reaction.  Transmutation occurs. (Nucleus changes identity.)

Alpha Emission • Alpha Particle – two protons and two neutrons bound together emitted from the nucleus during radioactive decay.  The reaction produces a helium nuclei.

• The atomic number decreases by two, the mass number decreases by four. • Example: 210 Po → 206Pb + 4He 84

82

2

Beta Emission • Beta Particle – an electron emitted from the nucleus as a beta particle • The atomic number increases by one, the mass number stays the same – a neutron is changed into a proton. • Example: 14 C → 14N + 0β 6

7

-1

Gamma Emission • Gamma Particle – high energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground energy state. • Gamma emission usually occurs immediately following other types of decay. • Of the three types of nuclear radiation, gamma rays have the greatest penetrating power.

Nuclear Radiation

Don’t Forget • Remember:  Mass #’s add up to be the same on each side of the arrow.  Atomic #’s add up to be the same on each side of the arrow.

Balance the following Nuclear Reactions 253 99

Es + 4He → 1n + ? 2

0

1 0

n + 256Md 101

Ca + 0β

43 43 19

K → 4320Ca + ?

20

206 210 84

Po →

Pb + ?

206 82

Pb →

Bi + ?

210 83

Pb + 4He

82 210

210 82

-1

83

2

Bi + 0β -1

Half Life • Half-Life – the time it takes for half a radioactive element to undergo radioactive decay. • The half-life of radioactive elements can be used to date rocks, fossils, etc. • Each radioactive isotope has a characteristic halflife. • Equation: Initial mass (½ )(# of ½ lives) = final mass

Calculating ½ Life • The ½ life of an element is 21.5 min. How many milligrams of this element remain after 86 min. if you start with 80 mg? Initial mass (½ )(# of ½ lives) = final mass

• 86 min/21.5 min = 4 half lives • Final mass = 80 mg ( ½ )(4) • Amount of element remaining = 80.0 mg x ½ x ½ x ½ x ½ = 5 mg

Nuclear Fission • Nuclear Fission – an atom breaks up into smaller atoms.  energy is released. E=mc2 or E= m ( 9 x 1016 )

• Fission is the primary system powering nuclear reactors, nuclear missiles, and nuclear powered submarines and air craft carriers.

Nuclear Fusion

• Nuclear fusion – atoms join together to make 1 atom. (opposite of fission)  Energy is released after the reaction but lots of energy is needed to start the reaction. • Very high temperatures and pressures are used to combine light atoms, such as hydrogen, to make heavier atoms, like helium.  Ex. The Sun: Overall 4 hydrogen atoms make a helium atom

• Creating and maintaining a fusion reaction is more complex and expensive than performing fission.

Nuclear Power Plants • Nuclear power plants use heat from nuclear reactors to produce electrical energy. • They have five main components:     

Shielding Fuel Control rods Moderator Coolant

• Shielding – radiation-absorbing material that is used to decrease exposure to radiation from nuclear reactors. • Current Problems with nuclear power: environmental requirements, safety of operation, construction costs, and storage and disposal of spent fuel and radioactive wastes.

How long did the Soviet government wait until formally announcing that the accident at Chernobyl had, in fact, happened?

2 days!