Learning ObjectiveCalculate the median atomic mass of an aspect given that isotopes and also their organic abundance
Key PointsAn element can have differing numbers of neutron in that is nucleus, however it always has the same number of protons. The versions of an element with various neutrons have various masses and also are dubbed isotopes.The median atomic mass because that an aspect is calculate by summing the masses of the element’s isotopes, every multiplied by its natural abundance on Earth.When doing any kind of mass calculations involving elements or compounds, constantly use average atomic mass, which can be uncovered on the routine table.

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Termsnatural abundanceThe diversity of a specific isotope naturally discovered on the planet.average atom massThe fixed calculated by summing the masses of one element’s isotopes, every multiplied by its herbal abundance top top Earth.mass this number total number of protons and also neutrons in an atom nucleus.

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


Atoms the the same element can, however, have actually differing number of neutron in your nucleus. For example, steady helium atom exist the contain either one or two neutrons, however both atoms have actually two protons. These different types of helium atoms have different masses (3 or 4 atom mass units), and also they are referred to as isotopes. For any given isotope, the sum of the number of protons and also neutrons in the cell core is called the massive number. This is due to the fact that each proton and each neutron sweet one atomic mass unit (amu). By including together the variety of protons and neutrons and multiplying by 1 amu, you deserve to calculate the fixed of the atom. All facets exist as a arsenal of isotopes. The word ‘isotope’ originates from the Greek ‘isos’ (meaning ‘same’) and also ‘topes’ (meaning ‘place’) due to the fact that the elements can accounting the same ar on the routine table while being different in subatomic construction.

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

Calculating typical Atomic Mass

The median atomic fixed of an facet is the sum of the masses the its isotopes, each multiplied by its organic abundance (the decimal associated with percent of atom of that element that space of a offered isotope).

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

The average atomic mass of an facet can be uncovered on the regular table, commonly under the elemental symbol. As soon as data space available concerning the natural abundance of various isotopes of one element, that is simple to calculation the mean atomic mass.

For helium, over there is around one isotope the Helium-3 because that every million isotope of Helium-4; therefore, the average atomic mass is very close come 4 amu (4.002602 amu).Chlorine consists of two significant isotopes, one through 18 neutrons (75.77 percent of herbal chlorine atoms) and one v 20 neutron (24.23 percent of organic chlorine atoms). The atomic variety of chlorine is 17 (it has 17 proton in that nucleus).

To calculation the mean mass, very first convert the percentages into fractions (divide them by 100). Then, calculation the mass numbers. The chlorine isotope with 18 neutrons has an abundance of 0.7577 and also a mass variety of 35 amu. To calculate the average atomic mass, main point 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 calculation the atomic mass the boron (B), which has actually two isotopes: B-10 v 19.9% natural abundance, and also B-11 v 80.1% abundance. Therefore,

Average atomic mass of boron = (0.199\cdot

10 amu) + (0.801\cdot

11 amu) = 10.80 amu

Whenever we carry out mass calculations involving elements or link (combinations the elements), we always use mean atomic masses.

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