Cracking the SAT Physics Subject Test

RADIOACTIVITY

The stability of a nucleus depends on the ability of the nuclear force to balance the
repulsive Coulomb forces between the protons. Many nuclides are ultimately
unstable and will undergo spontaneous restructuring to become more stable. An
unstable nucleus that will spontaneously change into a lower-energy configuration
is said to be radioactive. Nuclei that are too large (A is too great) or ones in which
the neutron-to-proton ratio is unfavorable are radioactive, and there are several
different modes of radioactive decay. We’ll look at the most important ones: alpha
decay, beta decay (three forms), and gamma decay.

Alpha Decay

When a nucleus undergoes alpha decay, it emits an alpha particle, which

consists of two protons and two neutrons and is the same as the nucleus

of a helium-4 atom. An alpha particle can be represented as

or

Conservation of Matter

Matter cannot be created

nor destroyed, but it can

undergo other transformations

that do not break

this fundamental rule. The

quickest way to determine

whether something

undergoes alpha, beta, or

gamma decay is to find

the amount of nucleons

that have escaped and to

then work backward to

determine which kind of

decay occurred.

Very large nuclei can shed nucleons quickly by emitting one or more alpha

particles, for example, radon-222 ( ) is radioactive and undergoes alpha
decay.