To find Atomic mass, it is important to understand whether the given sample is an atom, a sample containing isotopes of the atom at a given ratio, or if it is a natural sample of the element.
What is Atomic Mass?
Atomic mass can be defined as the total mass of one atom of any given element. The unit of atomic mass is called the unified atomic mass unit (denoted by ‘u’). Most of the atomic mass of a substance is made up of protons and neutrons. Therefore, it is almost equal to its mass number.
Relative isotopic mass refers to the mass of an isotope of an element when compared to one-twelfth of the mass of the carbon 12 isotope (which is equal to 12). It is also called atomic weight.
How to Find Atomic Mass?
Find Atomic Mass for a Single Atom
Since the combined masses of protons and neutrons account for almost all of the mass of the given atom, the atomic mass of the single atom can be calculated by adding the total number of protons and the total number of neutrons of that particular isotope.
The number of protons in a given atom is always equal to its atomic number. For example, the atomic number of oxygen is 8, therefore the total number of protons in an oxygen atom is 8. The total number of neutrons is generally specified when describing which isotope the atom belongs to.
Example: Calculation of the atomic mass of an oxygen molecule with 9 neutrons.
Total number of protons in oxygen = Atomic number of oxygen = 8
Total number of neutrons in the isotope (given) = 9
Atomic mass of the given oxygen atom = 8+9 = 17
Find the Atomic Mass of a Natural Sample
The atomic mass of the given natural sample can be easily looked up on the periodic table. To find the element on the periodic table, the element symbol or its atomic number must be known. Once the required data is attained, it can be compared with the periodic table where the atomic mass of the natural sample in atomic mass units will be provided in decimal figures.
Find the Atomic Mass of a Given Ratio of Isotopes
When the sample is a mixture of isotopes of the given element in varying percentages, the following method can be used:
Step 1: Multiply the atomic mass of the isotope with its abundance percentage and divide the result by 100.
Step 2: Add the values gained from step 1 for each given isotope in the sample.
Example: Calculating the atomic mass of a given chlorine sample where two isotopes are mixed. The first isotope has an atomic mass of 34.96885 and has an abundance of 75.78%. The second isotope has an atomic mass of 36.96590 and has an abundance of 24.22%.
Step 1: (Atomic mass of each isotope) x (%Abundance /100)
34.96885*0.7578 = 26.50 (i)
36.96590*0.2422 = 8.95 (ii)
Step 2: Adding (i) and (ii), the atomic mass of the given sample is determined.
26.50 + 8.95 = 35.45
Thus, the atomic mass of the given sample of chlorine was found to be 35.45