& average atomic mass atoms with the same number of protons (they are the same element) but...

ISOTOPES& Average Atomic Mass

ISOTOPESAtoms with the same number of protons (they are the same element) but different number of neutrons.

ISOTOPES For some elements, there can be more than one possible mass number.

Why? The neutrons are present to stabilize the protons in the nucleus so that they don’t repel (bounce off each other) too much and keep the atom stable.

Different numbers of neutrons can be effective for making a separation. Example: Imagine an atom with 3 protons:

AVERAGE ATOMIC MASS If all isotopes have an atomic mass that’s a whole number, why are all the atomic masses on the periodic table decimals? There are many isotopes for each element, and the atomic mass

given on the periodic table is a weighted average of all their isotope masses.

AVERAGE ATOMIC MASSDependent upon both mass and the relative abundance of each of the elements isotopes.

Once you have the isotopic abundances, you can find the average atomic mass of the elements using the following equation:

EXAMPLENaturally occurring copper exists with the following abundances:

69.17% is Cu-63 w/ atomic mass 62.93 amu30.83% is Cu-65 w/ atomic mass 64.93

(.6917)x(62.93) + (.3083)x(64.93)= 63.55 amu

PROBLEM3 Isotopes of Ar occur in nature

0.337% as Ar-36, 35.97 amu0.063% Ar-38, 37.96 amu99.6% Ar-40, 39.96 amu

Calculate the Average Atomic Mass

ANSWER CHECK (.00337)x(35.97) + (.00063)x(37.96) + (.996)x(39.96)= 39.95amu