How To Find The Atomic Mass Of An Atom

two.2.5: Average Atomic Mass

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The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance.

Learning Objectives

Summate the average atomic mass of an element given its isotopes and their natural abundance

Key Points

An element can accept differing numbers of neutrons in its nucleus, simply information technology always has the same number of protons. The versions of an element with different neutrons accept different masses and are called isotopes.
The average diminutive mass for an element is calculated by summing the masses of the element's isotopes, each multiplied by its natural abundance on Earth.
When doing any mass calculations involving elements or compounds, ever use average atomic mass, which can be constitute on the periodic table.

Key Terms

mass number: The total number of protons and neutrons in an diminutive nucleus.
natural abundance: The affluence of a particular isotope naturally found on the planet.
average diminutive mass: The mass calculated past summing the masses of an element's isotopes, each multiplied by its natural affluence on Globe.

The diminutive number of an element defines the element'due south identity and signifies the number of protons in the nucleus of ane atom. For example, the element hydrogen (the lightest chemical element) will e'er have one proton in its nucleus. The element helium volition e'er have ii protons in its nucleus.

Isotopes

Atoms of the same chemical element tin, however, have differing numbers of neutrons in their nucleus. For instance, stable helium atoms exist that contain either one or two neutrons, but both atoms take two protons. These different types of helium atoms take different masses (3 or four atomic mass units ), and they are called isotopes. For whatsoever given isotope, the sum of the numbers of protons and neutrons in the nucleus is chosen the mass number. This is because each proton and each neutron weigh one atomic mass unit (amu). By adding together the number of protons and neutrons and multiplying by one amu, you tin summate the mass of the atom. All elements exist as a drove of isotopes. The word 'isotope' comes from the Greek 'isos' (significant 'same') and 'topes' (meaning 'place') because the elements tin can occupy the aforementioned place on the periodic table while existence different in subatomic construction.

Figure: **Lithium Atom**: Stylized lithium-7 atom: 3 protons (cerise), four neutrons (black), and three electrons (blue). (Lithium also has some other, rarer isotope with only 2 neutrons.)

Calculating Average Atomic Mass

The boilerplate diminutive mass of an chemical element is the sum of the masses of its isotopes, each multiplied by its natural abundance (the decimal associated with percent of atoms of that element that are of a given isotope).

Average atomic mass = f_aneThousand_one + f_twoM₂ +… + f_nM_{due north}where f is the fraction representing the natural abundance of the isotope and M is the mass number (weight) of the isotope.

The average atomic mass of an element can exist found on the periodic table, typically under the elemental symbol. When data are available regarding the natural affluence of various isotopes of an element, it is elementary to calculate the boilerplate atomic mass.

For helium, there is approximately one isotope of Helium-3 for every million isotopes of Helium-4; therefore, the boilerplate diminutive mass is very close to 4 amu (4.002602 amu).
Chlorine consists of ii major isotopes, one with eighteen neutrons (75.77 percent of natural chlorine atoms) and i with xx neutrons (24.23 percent of natural chlorine atoms). The atomic number of chlorine is 17 (information technology has 17 protons in its nucleus).

To calculate the average mass, first convert the percentages into fractions (divide them past 100). Then, calculate the mass numbers. The chlorine isotope with 18 neutrons has an affluence of 0.7577 and a mass number of 35 amu. To calculate the average atomic mass, multiply the fraction past the mass number for each isotope, so add them together.

Boilerplate atomic mass of chlorine = (0.7577 ⋅⋅ 35 amu) + (0.2423 ⋅⋅ 37 amu) = 35.48 amu

Another case is to calculate the atomic mass of boron (B), which has ii isotopes: B-10 with 19.9% natural abundance, and B-11 with 80.1% abundance. Therefore,

Average diminutive mass of boron = (0.199⋅⋅10 amu) + (0.801⋅⋅11 amu) = 10.80 amu

Whenever nosotros practice mass calculations involving elements or compounds (combinations of elements), nosotros always use average diminutive masses.

Source: https://bio.libretexts.org/Bookshelves/Microbiology/Book%3A_Microbiology_(Boundless)/2%3A_Chemistry/2.2%3A_Chemical_Bonds/2.2.5%3A_Average_Atomic_Mass

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