electrons have mass, it is so negligible in comparison to that of protons and neutrons that
it is given a mass number of 0.
Some Other Elementary Particles
On the SAT II, you will not need to apply your knowledge of any elementary particles
aside from the proton, the neutron, and the electron. However, the names of some other
particles may come up, and you will at least need to know what they are.
Quarks are the fundamental building blocks of the protons, neutrons, and mesons. They
generally have positive or negative charges in units of one-third to two-thirds of the
charge of the electron. Protons are neutrons composed of three quarks. Mesons are
composed of a quark–antiquark pair.
Radioactive Decay
Some configurations of protons and neutrons are more stable in a nucleus than others.
For instance, the carbon-12 atom is more stable than the carbon-14 atom. While carbon-
12 will remain stable, carbon-14 will spontaneously transform into a more stable isotope
of nitrogen, releasing particles and energy in the process. Because these transformations
take place at a very steady rate, archaeologists can date carbon-based artifacts by
measuring how many of the carbon-14 atoms have decayed into nitrogen. These
transformations are called radioactive decay, and isotopes and elements like carbon-14
that undergo such decay are called radioactive. There are three major kinds of
radioactive decay.
Alpha Decay
When an atom undergoes alpha decay, it sheds an alpha particle, , which consists
of two protons and two neutrons. Through alpha decay, an atom transforms into a
smaller atom with a lower atomic number. For instance, uranium-238 undergoes a very
slow process of alpha decay, transforming into thorium:
Notice that the combined mass number and atomic number of the two particles on the
right adds up to the mass number and atomic number of the uranium atom on the left.
Beta Decay
There are actually three different kinds of beta decay— decay, decay, and electron
capture—but SAT II Physics will only deal with decay, the most common form of beta
decay. In decay, one of the neutrons in the nucleus transforms into a proton, and an
electron and a neutrino, , are ejected. A neutrino is a neutrally charged particle with
very little mass. The ejected electron is called a beta particle,.
The decay of carbon-14 into nitrogen is an example of decay:
Note that the mass number of the carbon on the left is equal to the sum of the mass
numbers of the nitrogen and the electron on the right: 14 = 14 + 0. Similarly, the atomic