Learning ObjectiveCalculate the mean atomic fixed of an aspect given that is isotopes and their herbal abundance
Key PointsAn element can have differing number of neutron in that nucleus, but it always has the same variety of protons. The execution of an aspect with various neutrons have different masses and also are called isotopes.The average atomic mass because that an facet is calculation by summing the masses the the element’s isotopes, every multiplied through its organic abundance on Earth.When doing any mass calculations involving elements or compounds, always use mean atomic mass, which can be discovered on the regular table.

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Termsnatural abundanceThe variety of a certain isotope naturally uncovered on the planet.average atom massThe fixed calculated by summing the masses of an element’s isotopes, every multiplied by its natural abundance top top Earth.mass numberthe total variety of protons and neutrons in an atomic nucleus.

The atomic number of an aspect defines the element’s identity and signifies the variety of protons in the nucleus of one atom. For example, the aspect hydrogen (the lightest element) will always have one proton in that is nucleus. The element helium will constantly have 2 protons in that is nucleus.


Atoms that the same element can, however, have differing number of neutron in your nucleus. Because that example, secure helium atom exist that contain one of two people one or two neutrons, but both atoms have actually two protons. This different species of helium atoms have various masses (3 or 4 atom mass units), and they are dubbed isotopes. For any given isotope, the amount of the numbers of protons and neutrons in the cell core is called the mass number. This is since each proton and each neutron sweet one atom mass unit (amu). By including together the number of protons and also neutrons and multiplying through 1 amu, you deserve to calculate the mass of the atom. All elements exist as a collection of isotopes. The word ‘isotope’ comes from the Greek ‘isos’ (meaning ‘same’) and ‘topes’ (meaning ‘place’) since the facets can accounting the same place on the periodic table while being different in subatomic construction.

Lithium AtomStylized lithium-7 atom: 3 proton (red), 4 neutron (blue), and 3 electrons (black). (Lithium likewise has another, rarer isotope with just 2 neutrons. )

Calculating average Atomic Mass

The typical atomic mass of an element is the amount of the masses the its isotopes, each multiplied by its natural abundance (the decimal connected with percent of atoms of that aspect that room of a provided isotope).

Average atomic mass = f1M1 + f2M2 + … + fnMn wherein f is the portion representing the herbal abundance the the isotope and M is the massive number (weight) of the isotope.

The median atomic mass of an aspect can be discovered on the periodic table, generally under the elemental symbol. Once data are available about the herbal abundance of various isotopes of an element, it is straightforward to calculate the average atomic mass.

For helium, over there is approximately one isotope the Helium-3 because that every million isotopes of Helium-4; therefore, the mean atomic mass is really close to 4 amu (4.002602 amu).Chlorine consists of two significant isotopes, one with 18 neutrons (75.77 percent of natural chlorine atoms) and also one with 20 neutron (24.23 percent of natural chlorine atoms). The atomic variety of chlorine is 17 (it has 17 protons in that is nucleus).

To calculate the mean mass, an initial convert the percentages right into fractions (divide castle by 100). Then, calculate the mass numbers. The chlorine isotope v 18 neutron has an abundance of 0.7577 and a mass variety of 35 amu. To calculation the mean atomic mass, multiply the portion by the fixed number because that each isotope, then include them together.

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Average atom mass of chlorine = (0.7577 \cdot 35 amu) + (0.2423 \cdot 37 amu) = 35.48 amu

Another example is to calculate the atom mass the boron (B), which has actually two isotopes: B-10 with 19.9% organic abundance, and also B-11 through 80.1% abundance. Therefore,

Average atom mass of boron = (0.199\cdot

10 amu) + (0.801\cdot

11 amu) = 10.80 amu

Whenever we do mass calculations involving elements or compounds (combinations that elements), we always use typical atomic masses.

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