SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 2

Atomic Structure and the Periodic Table of the Elements

DECAY SERIES, TRANSMUTATIONS, AND HALF-LIFE

The nuclei of uranium, radium, and other radioactive elements are continually disintegrating. It should be emphasized that spontaneous disintegration produces the gas known as radon. The time required for half of the atoms of a radioactive nuclide to decay is called its half-life.

TIP 

Know how to use half-life to determine the age of a substance.

See “Radioactive Dating” for more info.

For example, for radium, we know that, on the average, half of all the radium nuclei present will have disintegrated to radon in 1,590 years. In another 1,590 years, half of this remainder will decay, and so on. When a radium atom disintegrates, it loses an alpha particle, which eventually, upon gaining two electrons, becomes a neutral helium atom. The remainder of the atom becomes radon.

Such a conversion of an element to a new element (because of a change in the number of protons) is called a transmutation. This transmutation can be produced artificially by bombarding the nuclei of a substance with various particles from a particle accelerator, such as the cyclotron.

The following uranium-radium disintegration series shows how a radioactive atom may change when it loses each kind of particle. Note that an atomic number is shown by a subscript (92U), and the isotopic mass by a superscript (238U). The alpha particle is represented by the Greek symbol α, and the beta particle by β.

TIP 

Know how each type of emission affects the atomic mass and the atomic number.

The changes that occur in radioactive reactions and the subatomic particles involved are summarized in the following charts.

This is shown graphically in the radioactive decay series below.