isotope

Meaning: Isotopes are atoms with the same atomic number Z but different neutron counts, so different mass numbers. Carbon-12, carbon-13 and carbon-14 are all carbon (6 protons) with 6, 7 and 8 neutrons. A neutral atom of each has the same electron configuration, so chemical behavior is nearly identical. Mass-dependent properties differ slightly, such as how far a particle is deflected in a mass spectrometer. Some isotopes have unstable nuclei and decay radioactively; others are stable. The atomic mass on the periodic table is the average of the isotope masses, weighted by natural abundance.

Equation: A = Z + N, where A is the mass number (number of nucleons, unitless), Z is the atomic number (protons, unitless) and N is the neutron number (unitless).

On the MCAT: You may be given a symbol such as ¹³C and asked for protons, neutrons and electrons. You may be given isotope abundances and asked for the average atomic mass, which falls close to the mass of the most abundant isotope. Mass spectrometry peaks show isotopes as separate masses. Radioactive isotopes appear in half-life, decay and tracer passages.

Common confusion: Students mix up isotopes and ions. Changing the neutron number makes a different isotope of the same element. Changing the electron number makes an ion and leaves the nucleus unchanged. Changing the proton number makes a different element.