Isotopes are generally designated by the mass number, which is the number of protons and neutrons in a nucleus of that isotope. The binding energy of the nucleons in the nucleus causes the actual mass of the atom to be slightly different from the mass number, so the actual mass can only be determined experimentally.
Some isotopes occur naturally, and it is possible to calculate the percent abundance of two isotopes in nature if atomic masses and the element's average atomic mass are already known. Determine the atomic masses of the isotopes, as well as the element's average atomic mass.
The atomic weight of an element depends on the abundance of its isotopes. If you know the mass of the isotopes and the fractional abundance of the isotopes, you can calculate the element's atomic weight.
To calculate atomic mass, start by finding the atomic number of the element, which is the number above the element on the periodic table. Next, find the number of neutrons in the nucleus by subtracting the atomic number from the isotope number. Finally, add the atomic number and the number of neutrons to get the atomic mass.
If you're given the mass of each isotope of an element, and the average atomic mass, you can calculate the percent (%) abundance of each isotope. Let "x" be one of the abundances, and the other ...
Find average molecular mass. Since a molecule is just a collection of atoms, you can add the masses of the atoms together to find the mass of the molecule. If you use the average atomic masses (instead of the mass of a specific isotope), the answer is the average mass of the molecule as found in a naturally occurring sample. Here's an example:
To calculate the average mass, first convert the percentages into fractions (divide them by 100). Then, calculate the mass numbers. The chlorine isotope with 18 neutrons has an abundance of 0.7577 and a mass number of 35 amu. To calculate the average atomic mass, multiply the fraction by the mass number for each isotope, then add them together.
Isotope distributions can also be calculated using the Isotopes Calculator in the MS Interpreter tool in the NIST Mass Spectral Database. Program Design This program consists of several modules to calculate the molecular weights and isotopic distributions of the molecular formula input by the user.
elements have an average atomic mass, not isotopes. There maybe several isotopes of a certain element, each isotope having its own, whole-numbered, mass. When you do a weighted average based on the abundance of the isotope, you can get the average atomic mass of the element. To get the mass of an isotope, just add the protons and neutrons together.