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Atomic mass is the total mass of protons, neutrons, and electrons in an atom or molecule. [1] X Research Source However, the mass of the electron is often ignored and not included in the formula because this value is very small. [2] X Research Source Atomic mass is often used to refer to the average atomic mass of all isotopes of an element, although such a usage is academically incorrect. The average atomic mass is actually the relative atomic mass, also known as the atomic weight , of an element. [3] X Research Source Atomic weight is determined based on the average mass of natural isotopes of the same element. Chemists need to distinguish these two types of atomic mass in their calculations, because taking the wrong atomic mass value can lead to errors when calculating the reaction efficiency.
Steps
Find the atomic mass on the periodic table
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Understand how mass atoms are represented on the periodic table. Atomic mass, i.e. the mass of an atom or molecule, can be expressed in SI standard mass units – grams, kilograms, etc. However, atomic masses are often so small that they are often expressed in terms of a single mass. common atomic mass unit (often abbreviated as “u” or “amu”). A standard atomic mass unit is equal to 1/12 of the standard atomic mass of the isotope carbon-12. [4] X Research Sources
- The atomic mass unit indicates the mass of one mp of an element or molecule in grams . Therefore, this is a very useful property in practical calculations because it allows us to easily convert between the mass and the number of mps of a given number of atoms or molecules in the same state.
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Locate the atomic mass on the periodic table. Most standard periodic tables list the relative atomic masses of the elements. Usually the atomic mass is written as a number below the element’s chemical symbol in the periodic table. This value is usually written as a decimal, not as an integer.
- It should be noted that the relative atomic mass given in the periodic table is the average mass of that element. Elements with multiple isotopes – isotopes are different variations of the same element and each has a different mass since they will have more or less one to a few neutrons in the nucleus of the element. atom. [5] X Source of study Therefore, the relative atomic mass given in the periodic table only represents the average mass of the atoms of an element, not the mass of an atom of that element. .
- Relative atomic mass, as stated in the periodic table, is used to calculate the mass mp of atoms and molecules. Technically, the atomic mass, when expressed in units of amu as shown in the periodic table, has no units. However, multiplying the atomic mass by 1g/mp yields a quantity that can be used to calculate the mass mp – the mass (in grams) of one mp of atoms – of an element.
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It should be understood that the values in the periodic table are the average atomic mass values of an element. As stated above, the relative atomic mass given in the periodic table is the average atomic mass of all isotopes. This mean is significant in practical calculations, such as calculating the mass mp of a substance made up of many atoms. However, when it comes to individual atoms, this average is often not enough.
- Since this is an average of many isotopes, the value given in the periodic table is not an exact atomic mass value for any one atom of that element.
- The atomic mass of individual atoms must be calculated by considering the exact number of protons and neutrons in each atom.
Calculate the atomic mass of a single atom
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Find the atomic number of the element or isotope. Atomic number is the number of protons in the atom of an element, this value never changes. [6] X Research Source For example, all hydrogen atoms, and only hydrogen, have a proton. Sodium has an atomic number of 11 because its nucleus has 11 protons, while oxygen has an atomic number of 8 because the nucleus of an oxygen atom contains 8 protons. You can find out the atomic numbers of elements by looking at the periodic table – in most periodic tables: the atomic number is written on the element’s chemical symbol. This number is always a positive integer.
- Consider the carbon atom. Carbon always has 6 protons, so the atomic number of carbon is 6. We can also look in the periodic table, in the box for carbon (C), we have the number “6” above the chemical symbol (C). , which denotes that carbon’s atomic number is six.
- It should be noted that the atomic number of an element does not directly affect the relative atomic mass given in the periodic table. Although it may seem that the atomic mass of a substance is twice the atomic number of that substance, especially for elements at the top of the periodic table, it is not possible to calculate the atomic mass by doubling the atomic number. atom.
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Find the number of neutrons in the nucleus of an atom. The number of neutrons in the nucleus can vary between atoms of the same element. Two isotopes of the same element are two atoms with the same number of protons and a different number of neutrons. Other than the constant number of protons in an element, the number of neutrons in an element’s atom can vary so much that the element’s average atomic mass needs to be written as a decimal.
- The number of neutrons can be determined based on the isotopic indication of the element. For example, carbon 14 is a naturally occurring isotope of carbon-12. An isotope is indicated by a number written above the element symbol: 14 C. The number of neutrons is calculated by subtracting the number of protons from the isotope indicator: 14 – 6 = 8 protons.
- Suppose we are considering a carbon atom with 6 neutrons ( 12 C). It is the most common isotope of carbon, accounting for about 99% of the carbon atoms present in nature. [7] X Research Source However, 1% of carbon atoms contain 7 neutrons ( 13 C). Other forms of carbon with more or less than 6 or 7 neutrons exist only in very small quantities.
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Add the total number of protons and neutrons. The total obtained is the atomic mass of that atom. You do not need to consider the number of electrons around the nucleus because the total mass of the electrons is very very small, so small that in practical calculations this value does not affect the calculation results obtained.
- The carbon atom has 6 protons + 6 neutrons = 12. The atomic mass of this carbon is 12. On the other hand, if it were the isotope carbon-13 , the atom would have 6 protons + 7 neutrons = 13 , in this case the atomic mass would be 13.
- The net atomic mass of carbon-13 is 13.003355 [8] X Research Source , this value is more accurate because it is determined experimentally.
- The atomic mass of an element is very close to the isotope indicator number. In simple calculations, we can treat the number of isotopic indicators as equal to the atomic mass. When determined experimentally, the atomic mass will be slightly larger than the isotope indicator number because at this time the very small mass of the electrons is also taken into account.
Calculate the relative atomic mass of an element
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Determine the isotope form present in the sample. Chemists often determine the relative ratios of isotopes in a sample by using a special instrument called a mass spectrometer. However, for student chemistry problems, this information is often given in the test or problem paper in the form of theoretically known values.
- For the purposes of this article, let’s assume we are dealing with isotopes of carbon-12 and carbon-13.
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Determine the relative abundance of each isotope present in the sample. Isotopes abundance is usually expressed as a percentage. Some isotopes are very common, while others are very rare in nature – sometimes so rare that they are difficult to identify. This information can be determined by mass spectrometry or can be found in the references.
- The carbon-12 abundance is 99% and the carbon-13 abundance is 1%. Other isotopes of carbon do exist, but in very small amounts, so for this example we can ignore their values.
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Multiply the atomic mass of each isotope by their ratio in the sample. Multiply each element’s atomic mass by the corresponding abundance (in decimal form). To convert from percentage to decimal, simply divide the percentage by 100. The sum of the decimals after this conversion is always 1.
- In the example under consideration, the sample contains carbon-12 and carbon-13. If carbon-12 makes up 99% of the sample, and carbon-13 makes up 1% of the sample, multiply 12 (atomic mass of carbon-12) by 0.99 and multiply 13 (atomic mass of carbon-13) by 0, 01.
- Reference books will likely have percentage information based on known isotopes of an element. Most chemistry textbooks have a table of this data at the end of the book. The mass spectrometer can also give results on the ratio of isotopes in the sample.
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Add results. Take the sum of the products obtained in the previous step. The result is the relative atomic mass of that element – that is, the average value of the atomic mass of the isotopes of that element. This value is used when discussing an element in general without referring to a particular isotope form.
- In our example, we have 12 x 0.99 = 11.88 with carbon-12 and 13 x 0.01 = 0.13 with carbon-13. So the relative atomic mass in this example is 11.88 + 0.13 = 12.01 .
Things you need
- Chemistry reference book
- Computer
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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 86,092 times.
Atomic mass is the total mass of protons, neutrons, and electrons in an atom or molecule. [1] X Research Source However, the mass of the electron is often ignored and not included in the formula because this value is very small. [2] X Research Source Atomic mass is often used to refer to the average atomic mass of all isotopes of an element, although such a usage is academically incorrect. The average atomic mass is actually the relative atomic mass, also known as the atomic weight , of an element. [3] X Research Source Atomic weight is determined based on the average mass of natural isotopes of the same element. Chemists need to distinguish these two types of atomic mass in their calculations, because taking the wrong atomic mass value can lead to errors when calculating the reaction efficiency.
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