Presentation on theme: "Average Atomic Mass. An element can exist in a number of forms, called isotopes. Isotopes are forms of the same atom that vary in mass as a result of."— Presentation transcript:
An element can exist in a number of forms, called isotopes. Isotopes are forms of the same atom that vary in mass as a result of a different number of neutrons.
There are two naturally occurring isotopes of copper. One isotope weighs in at 62.93 amu, the other has a mass of 64.94 amu. (atomic mass units) These two isotopes have different proportions in a natural sample of copper. The lighter isotope is more common with an abundance of 69.09% of all naturally occurring copper. What is the percent abundance of the other isotope? Isotopes of Copper
The remainder of the atoms, 30.91 %, have a mass of 64.94 amu.
The calculation of the average atomic mass is a WEIGHTED AVERAGE. To find the AVERAGE ATOMIC MASS of an atom, we take into account all of the isotopes that exist and the percentage of each type.
Average atomic mass = Σ (mass of isotope × relative abundance) Σ = sum or to add them all together
Since there are two isotopes for copper, we will be adding the contributions of 2 isotopes. (That’s where the Σ sign comes in) The relative abundance is simply the percentage of the isotope, but in decimal form. 69.09% corresponds to a relative abundance of 0.6909.
Average atomic mass of copper = (62.93 amu × 0.6909) [the mass and abundance of isotope #1] + (64.94 amu × 0.3091) [the mass and abundance of isotope #2] = 63.55
An AVERAGE atom of copper has a mass of 63.55 amu. Notice that in this problem, we could predict that the average is closer to the weight of the lighter isotope. Why? The lighter form of copper is more abundant.
Isotope nameIsotope mass (amu)Relative abundance Silicon – 2827.9892.21% Silicon – 2928.984.70% Silicon – 3029.973.09% Isotope nameIsotope mass (amu)Percent abundance Silver-107106.9050951.86 Silver-109108.90470remainder Isotope nameIsotope mass (amu)percentage Iron – 5453.945.90% Iron – 5655.9391.72% Iron – 5756.942.10% Iron – 5857.930.280%