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Average atomic mass is not a direct measure of an atom’s mass. Instead, this is the average mass per atom from a typical sample of the element. If you can measure the masses of billions of individual atoms, you can find this value by calculating their average. We have a more practical method, which is based on the information of different isotopes of the chemical element.
Steps
Calculate average atomic mass
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Understanding isotopes and atomic masses. In nature, most elements exist in many forms or isotopes. The only difference between two isotopes of the same element is the number of neutrons in the atom, which affects the atomic mass. [1] X Research Source The average atomic mass calculation takes into account the effect of this difference, and tells you the average mass of each atom in a sample of that many atoms.
- For example, the element silver (Ag) has two isotopes in nature: Ag-107 and Ag-109 (or 107 Ag and 109 Ag). [2] X Research Source Isotopes are named after the “mass number,” or the sum of the number of protons and neutrons in an atom. [3] X Research Source Means that Ag-109 has two more neutrons than Ag-107, so its atom is slightly heavier.
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Find the mass of each isotope. You need two pieces of information for each isotope, which can be looked up in reference books or online, such as webelements.com. The first is the atomic mass or atomic mass of each isotope. Isotopes with more neutrons have more mass.
- For example, the silver isotope Ag-107 has an atomic mass of 106.90509 amu (the unit of atomic mass). The isotope Ag-109 is slightly heavier with a mass of 108.90470 .
- The pair of decimals at the end may be slightly different in the documents. Do not write any numbers in parentheses after the mass.
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Write down the percentage of natural existence of each isotope. This ratio indicates the prevalence of that isotope, expressed as a percentage of the total number of atoms of that element. You can find this information in the same document that says mass atomization. The natural prevalence of all isotopes should be 100% (although slightly different due to rounding errors).
- Ag-107 isotope has a proportion of 51.86%. The isotope Ag-109 is less common at 48.14%. That is, a typical silver sample has 51.86% Ag-107 and 48.14% Ag-109.
- Ignore any isotope that does not have this survival rate. These isotopes do not exist naturally on earth.
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Convert isotope percentage to decimal. Scaling this by 100 yields the same value as a decimal.
- In the aforementioned silver sample, the ratios of the isotopes are 51.86 / 100 = 0.5186 and 48.14 / 100 = 0.4814 .
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Find the average atomic mass. The average atomic mass of an element with n isotopes equals (atomic mass isotope 1 * isotope ratio 1 ) + (atomic mass isotope 2 * isotope ratio 2 ) + … + (atomic mass) isotope mass n * isotope ratio n This is an example of a “mass average”, meaning that the greater the isotope survival rate, the greater the effect that isotope has on the result. Use this formula for silver as follows:
- Average atomic mass Ag = (atomic mass Ag-107 * ratio Ag-107 ) + (atomic mass Ag-109 * ratio Ag-109 )
=(106,90509 * 0.5186) + (108.90470 * 0.4814)
= 55,4410 + 52.4267
= 107.8677 amu. - Look for that element on the periodic table to check the results. The average atomic mass is always written below the element’s chemical symbol. [4] X Research Sources
Use results
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Convert mass to atomic number. The average atomic mass shows the relationship between the mass and the number of atoms in a typical sample of that element. This is very useful in chemistry labs because it is nearly impossible to count the number of atoms accurately, but the mass is easy to determine. For example, you might weigh a sample of silver and know that for every 107.8677 amu there is a silver atom.
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Convert to mp volume . Atomic mass units are very small, so chemists often use grams for mass. Luckily we have definitions of these concepts so the conversion should be easy. Just multiply the average atomic mass by 1 g/mp (mass constant mp) to get the result in g/mp. For example, 107.8677 grams of silver contains one mp of silver atoms.
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Find the average molecular mass. Since a molecule is a collection of atoms, you can add up the masses of all the atoms to find the molecular mass. If you use the average atomic mass (instead of the mass of a particular isotope) the result will be the average molecular mass of a sample of the substance in nature. Here is an example:
- A water molecule with the chemical formula H 2 O contains two hydrogen atoms (H) and one oxygen atom (O).
- Hydrogen has an average atomic mass of 1.00794 amu. Oxygen has an average atomic mass of 15.9994 amu.
- So the average molecular mass of H 2 O is equal to (1.00794)(2) + 15.9994 = 18.01528 amu, equivalent to 18,01528 g/mp.
Advice
- The concept of relative atomic mass is sometimes used synonymously with average atomic mass. There is a slight difference because the relative atomic mass has no units; it’s a measurement of the relative mass of a carbon-12 atom. As long as you use atomic mass units in your average atomic mass calculation, the two values are the same.
- The number in parentheses after the atomic mass tells us the error. [5] X Research Source For example, an atomic mass of 1.0173 (4) means that the normal atom of that element has a mass range of 1.0173 ± 0.0004. You do not need to take this number unless asked for it.
- On the periodic table, the average atomic mass of the latter element will be greater than that of its predecessor, with few exceptions. Here’s a quick way to check your results.
- 1 atomic mass unit is 1/12 the mass of a carbon-12 atom.
- The isotope survival rates are calculated according to a typical pattern that occurs naturally on earth. Substances that are not found in nature such as meteorites or a substance made in the laboratory can have different isotope ratios, so the average atomic mass is also different.
Warning
- Atomic mass is always written in atomic mass units (amu or u), sometimes called daltons (Da). Never write another unit of mass (such as kilogram) after this number without changing it.
Things you need
- Pencil
- Paper
- Laptop
- Data on the prevalence of isotopes in nature.
- Data on atomic mass units of isotopes.
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This article is co-authored by a team of editors and trained researchers who confirm the accuracy and completeness of the article.
The wikiHow Content Management team carefully monitors the work of editors to ensure that every article is up to a high standard of quality.
This article has been viewed 52,271 times.
Average atomic mass is not a direct measure of an atom’s mass. Instead, this is the average mass per atom from a typical sample of the element. If you can measure the masses of billions of individual atoms, you can find this value by calculating their average. We have a more practical method, which is based on the information of different isotopes of the chemical element.
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