Nuclear Chemistry Review

Isotopes of atoms can be stable or unstable Isotopes of atoms can be stable or unstable. Stability of isotopes is based on the number of protons and neutrons in its nucleus. Some nuclei are unstable and spontaneously decay, emitting radiation. For small nuclides (atomic numbers 20 or less) equal numbers of protons and neutrons are stable. For larger numbers, more neutrons are needed to ensure stability.

Each radioactive isotope has a specific mode and rate of decay (half life). There are 24 nuclides listed in Table N. Usually one of these appears on a question somewhere. Remember that they may not tell you that you can look it up!

Half-Life Problems The key to any half life problem is to figure out the number of half-lives. Often the fraction remaining is given: ½ = 1 half life ¼ = 2 half lives 1/8 = 3 half lives 1/16 = 4 half lives 1/32 = 5 half lives

How much is left?

How old is it? How long did it take?

What is the Half-Life?

What is the half-life?

Which Isotope? (What is the half-life?)

Spontaneous decay can involve the release of alpha particles, beta particles, or gamma radiation from the nucleus of an unstable isotope. These emissions differ in mass, charge, and penetrating power. Be able to recognize and write the symbols for all of the particles and radiation emitted from radioactive nuclides – they’re all found on Table O.

Penetrating Power The larger the particle, the smaller the penetrating power. Alphas are the least dangerous; Gamma radiation is the most dangerous because it is massless, high energy, and moving at the speed of light.

Any change in the nucleus of an atom that converts it from one element to another in called transmutation. This can occur naturally or can be induced by the bombardment of the nucleus by high-energy particles. Artificial transmutation equations have two particles on the left side of the equation. One of them is either a neutron or an alpha particle. Natural transmutation has only one particle, the radioactive nuclide.

Nuclear transitions can be represented by equations that include symbols that represent atomic nuclei (with the mass number and atomic number), subatomic particles (with mass number and charge) and/or emissions such as gamma radiation.

Be able to distinguish nuclear decay equations from chemical equations. You won’t see mass numbers or atomic numbers in chemical equations.

Be able to find some component X in a transmutation equation. The mass numbers and the atomic numbers on both sides of the equation have to add up.

For alpha decay, the mass number decreases by 4 and the atomic number decreases by 2. For beta minus decay, the mass number doesn’t change and the atomic number increases by one. For positron decay (beta plus) the mass number is unchanged and the atomic number decreases.

Energy released in a nuclear reaction (fission/fusion) comes from the fraction amount of mass converted into energy. Nuclear changes convert matter into energy according to E=mc2. Fission: Two small atoms (usually hydrogen) make a larger one (usually helium) Fusion: A large atom is split into two smaller atoms by a neutron. Several neutrons are also produced.

Fission vs Fusion

Energy released during nuclear reactions is much greater than the energy released during reactions. The mass  energy question is frequently asked on the regents. The mass converted into energy is small, but the energy produced is enormous.

There are inherent risks associated with radioactivity and its uses, such as long-term storage and disposal of radioactive isotopes, and nuclear accidents. Often there are reading passages followed by questions on the free response section.

Radioactive isotopes have many beneficial uses. Radioactive isotopes are used in medicine and industrial chemistry, e.g., radioactive dating, tracing chemical and biological processes, industrial measurement, detection and treatment of diseases. This is another common reading assignment on the test. Medical radioisotopes must have short half-lives, for obvious reasons. Dating rocks requires isotopes with long half-lives.

Four Types of Decay Problems The key is to find the number of half lives. Figuring out the fraction remaining will usually help you do that: ½ = 1 half life ¼ = 2 half lives 1/8 = 3 half lives 1/16 = 4 half-lives 1/32 = 5 half-lives   Remember these fractions.

“How much is left?” Divide the initial mass in half as many times as you have half-lives.

“How long did it take?” They have to give you the fraction remaining. Once you figure out the number of half lives, multiply that number by the half life (often on Table N)

The Half-Life of an Unfamiliar Radionuclide They must give you the fraction remaining, so you can figure out the # half-lives. Take the time elapsed and divide it by that number.

“How much is did you start with?” The number of half lifes can be determined by taking the time elapsed and dividing it by the half-life. Then take the amount remaining and double it as many times as you have half-lives.