Chemistry Formulas

Atomic Mass Formula

What is Atomic Mass?

This article deals with atomic mass formula. Atomic mass refers to the sum of the masses of an atom’s protons, neutrons, and electrons. Furthermore, atomic mass can also refer to the average mass in a group consisting of atoms. Also, electrons have much lesser weight than both protons and neutrons. Consequently, electrons don’t really matter in the calculation. Therefore, atomic mass effectively is the sum of masses of neutrons and protons.

Atomic Mass Formula

Atomic Mass Formula

An element’s atomic mass happens to be equal to the weighted average of the isotopes belonging to that element. Furthermore, isotopes refer to atoms having the same atomic number but the number of neutrons differs. Moreover, the number of protons is the determinant of an atom’s identity. Also, isotopes consist of atomic numbers that are identical. So, atoms belong to the same element. The number of neutrons is not equal which means that isotopes will not be the same. Most noteworthy, the measurement of atomic mass takes place in atomic mass units (amu). Here, one amu will be roughly equivalent to a single proton or neutron’s mass.

An important point to understand is that the isotopes of an element in nature don’t occur in equal percentages. Therefore, one must take a weighted average so as to achieve the element’s atomic mass. Furthermore, isotopes that occur in higher percentages will have a greater effect on the atomic mass. On the other hand, isotopes that occur in lower percentages shall have an effect that is smaller.

One can derive the atomic mass by multiplying it with the isotope’s mass by its relative abundance. Afterward, one must add the individual masses together.

Atomic Mass on the Periodic Table

The first step in finding the atomic mass of an element is to learn how to make use of a periodic table. Furthermore, this number usually is available below the symbol of an element. Now one must look at the decimal number which happens to be a weighted average of the atomic masses of an element’s natural isotopes.

For example, If one is required to tell the atomic mass of carbon, one would first understand its symbol, C. Now, one must certainly search for C on the periodic table. Furthermore, one number is carbon’s atomic number or element number. Most noteworthy, the atomic number will increase as one goes across the table. This is certainly not the value you want. The atomic mass or atomic weight happens to be the decimal number, The number of important figures varies in accordance with the table, but the value is around 12.01.

This value on the periodic table is available in atomic mass units or amu. However, for chemistry calculations, one would most probably write the atomic mass in terms of grams per mole or g/mol. The atomic mass of carbon turns out to be 12.01 grams per mole of carbon atoms.

Sum of a Single Atom’s Protons and Neutrons

In order to calculate the atomic mass of a single atom of an element, one must add up the mass of protons and neutrons.

For example, consider a scenario where one must find the atomic mass of a carbon’s isotope that consists of seven neutrons. Moreover, one can understand from the periodic table that 6 is the atomic number of carbon, which is its number of protons. The atomic mass of the atom certainly happens to be the mass of the protons in addition to the mass of the neutrons, 6 + 7, or 13.

Weighted Average of an Element’s Atoms

The atomic mass of an element is a weighted average of each and every isotope of an element on the basis of their natural abundance. It becomes simple to calculate the atomic mass of an element following this methodology.

Solved Example For You

Question- Find out the atomic mass of chlorine if the information provided is- isotope atomic mass of 34.96885 with percentile abundance of 75.78 and isotope atomic mass of 36.96590 with percentile abundance of 24.22.

Answer- In order to find out the atomic mass of chlorine, each isotope’s atomic mass must be multiplied with the relative percentile abundance and then individual masses must be added together.

34.96885 x 0.7578 = 26.50

36.96590 x 0.2422 = 8.95

26.50 + 8.95 = 35.45

Hence, chlorine atomic mass is 35.45 amu.

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