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Average Atomic Mass

Chemistry Notes

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Relative Atomic Mass Masses of atoms expressed in grams

are very small, for example: One atom of Oxygen-16 has a mass of 2.66x10-23g = 0.0000000000000000000000266g

The standard used to denote units of atomic mass is the carbon-12 nuclide.

It has been arbitrarily assigned a mass of exactly 12 atomic mass units

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Relative Atomic Mass

One atomic mass unit, or amu, is exactly 1/12 of a carbon-12 atom.

Atomic Mass is determined by comparing it with the mass of a carbon-12 atom.

Therefore, atomic mass is a measure of an atom’s mass relative to carbon-12.

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Average Atomic Mass Most elements occur naturally as a mixture

of isotopes. The percentage at which each isotope

occurs in nature is taken into account when calculating the element’s average atomic mass.

Average Atomic Mass- the weighted average of the atomic masses of the naturally occurring isotopes of that element.

Compare average atomic mass to mass number Mass number is the sum of the protons

and neutrons of ONE isotope of an element (whole number)

Average atomic mass is the weighted average of ALL of the isotopes of an element (usually not a whole number b/c its an average)

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How Do You Calculate a Weighted Average?

You have a box of 100 marbles. 25 of the marbles have a mass of

2.0g each. 75 of the marbles have a mass of

3.0g each.

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Weighted Average Calculation

Calculate the total mass of the mixture & divide by 100.

25 x 2.0g = 50g75 x 3.0g = 225g

Total mass = 275g 275g ÷100 = 2.75g (average mass of

marbles)

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Calculate the Average Atomic Mass of an Element (amu)

Isotope Mass Number

% Natural Abundance

Atomic Mass (amu)

Copper-63 63 69.17%(69.17 copper atoms out of 100 are this

isotope)

62.93

Copper-65 65 30.83%(30.83 out of 100 are this

isotope)

64.93

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Calculate the Average Atomic Mass of an Element (amu)

Copper-6369.17 x 62.93 amu = 4353 amu

Copper-6530.83 x 64.93 amu = 2002 amu

Total Mass = 6355 amu 6355 amu ÷ 100 = 63.55 amu What is Copper’s Atomic Mass on the

periodic table?

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Calculate the Average Atomic Mass of Oxygen

Isotope Mass Number

% Natural Abundance

Atomic Mass (amu)

Oxygen-16 16 99.76 15.995

Oxygen-17 17 0.038 16.999

Oxygen-18 18 0.200 17.999

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Answer

16.00 amu

Formula to calculate avg atomic mass Multiply the mass of each isotope by its percent

abundance and then add them all together; then divide the total by 100

(Mass of isotope 1 x % abundance) + (mass of isotope 2 x % abundance)+ 100

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Which isotope is most abundant?

The average atomic mass of an element is always closest in value to the most abundant isotope. For example, Carbon’s average atomic mass is

12.011, so the most abundant isotope of carbon is carbon-12.

The average atomic mass of sulfur is 32.06. The isotopes of sulfur are sulfur-32, sulfur-33, sulfur -34, and sulfur-36. What is the most abundant isotope of sulfur?

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Sulfur-32

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Assignment

Pg 117 practice problems #23-24 Pg 124 problem #81