It is possible to tell isotopes of an element apart by looking at their mass numbers. Because each isotope of an element has a different number of neutrons, each mass number is slightly different.
An element's mass number is equal to the number of protons and neutrons in the element. It is possible to determine an element's mass number by rounding its atomic weight to the nearest whole number. Silicon, for example, has an atomic weight of 28.085 atomic mass units. The mass number would be 28, or 28.085 rounded to the nearest whole number. An element's atomic number corresponds to the number of protons in the element. Because silicon has an atomic number of 14, it has 14 protons. Subtracting 14 protons from the mass number of 28 yields a result of 14 neutrons.
Not all atoms of silicon have 14 neutrons, however. These atoms are called isotopes. Si-29 has 14 protons and 15 neutrons, while Si-30 has 14 protons and 16 neutrons. The number in each isotope refers to the isotope's mass number. Although there are more than a dozen silicon isotopes, only Si-28, Si-29 and Si-30 occur in nature. Scientists believe Si-28 may improve heat conductivity in semiconductors. Si-29 is used in nuclear magnetic resonance spectroscopy, while Si-30 is used to produce a radioisotope of silicon, Si-31.