Isotopes+and+Atomic+Mass

ISOTOPES
Isotopes are atoms of a given element that differ in the number of neutrons in their nucleus. Because of the difference in the number of neutrons, each isotope has its own atomic mass.

Isotopes are written as their element with their mass number behind it. The mass number is the total number of protons and neutrons in the atom. For example, an isotope of carbon with 6 neutrons would be written as Carbon-12, since every carbon atom has 6 protons.

Below are three common isotopes of carbon:

Carbon-12 has 6 neutrons (12 - 6 protons = 6 neutrons) Carbon-13 has 7 neutrons (13 - 6 protons = 7 neutrons) Carbon-14 has 8 neutrons (14 - 6 protons = 8 neutrons)

ATOMIC MASS
One can determine the average atomic mass of an element by using the masses of its various isotopes and their relative abundances.

The atomic mass is expressed in amu's (atomic mass units).

To find the average atomic mass of an element, one must take into account all of the isotopes that exist and the percentage of each type.

Formula to calculate the atomic mass of an element: (% isotope 1) (mass of isotope 1) + (% isotope 2) (mass of isotope 2) + (% isotope 3) (mass of isotope 3).....

Example:

The three most common isotopes of oxygen are Oxygen-16, Oxygen-17, and Oxygen-18.

(.99762) (16 amu) + (.00038) (17 amu) + (.00205) (18 amu) = 16.00 amu

The atomic mass of oxygen is 16.00 amu.

http://en.wikipedia.org/wiki/Isotopes_of_oxygen