02-05-2021

Isotopic Mass



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Isotopic mass synonyms, Isotopic mass pronunciation, Isotopic mass translation, English dictionary definition of Isotopic mass. The mass of an atom, usually expressed in atomic mass units. American Heritage® Dictionary of the English Language, Fifth Edition. Find out how isotopes can be detected using mass spectrometry. Learn about isotopes and how they relate to the average atomic mass of an element. Find out how isotopes can be detected using mass spectrometry. If you're seeing this message, it means we're.


For example, magnesium exists as a mixture of three isotopes, each with an atomic number of 12 and with mass numbers of 24, 25, and 26, respectively. These isotopes can be identified as 24 Mg, 25 Mg, and 26 Mg. These isotope symbols are read as “element, mass number” and can be symbolized consistent with this reading. Isotopic Abundances. Since 1931, the Commission regularly publishes the critical evaluation of isotopic abundances of elements. These values form basis for the standard atomic weights of elements. Below is the current table of Isotopic Compositions of the Elements. Isotopic Compositions of the Elements 2019. Mass distribution represented by a two-column matrix in which each row corresponds to an isotope. The first column lists the isotopic mass, and the second column lists the probability for that mass. Structure containing mass information for the peptide sequence or compound in the following fields.

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mass

[mas]
2. that characteristic of matter that gives it inertia. Symbol m.
atomic massatomic weight; see also atomic mass unit.
inner cell mass an internal cluster of cells at the embryonic pole of the blastocyst which develops into the body of the embryo.
lean body mass that part of the body including all its components except neutral storage lipid; in essence, the fat-free mass of the body.
relative molecular mass technically preferable term for molecular weight.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

a·tom·ic mass

(ă-tomik mas) Isotopic
Total number of protons, neutrons, and electrons in an atom.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012

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The periodic table only tells us a weighted average of the atomic masses of the different isotopes for an element. For example, if we take a weighted average for the isotopes of Carbon, we get an average atomic mass of 12.011 amu. Like Carbon, many elements exist in nature as a mixture of isotopes.

To find the average atomic mass of the element Carbon, we multiply the mass of each isotope by its percent abundance expressed as a decimal. The table below shows the exact mass of each isotope (isotopic mass) and the percent abundance (sometimes called fractional abundance) for the primary isotopes of Carbon.

IsotopeExact Weight
(Isotopic Mass) 		Percentage Abundance
Carbon-1212.0000 98.90 %
Carbon-1313.0033 1.10 %

To find the average atomic mass for Carbon:

Average Atomic Mass = (12.0000)(.9890) + (13.0033)(0.0110) = 12.011 amu

The answer, 12.011 amu, is the same value found for Carbon on the periodic table.

Exercise: Calculate the average atomic mass of Nitrogen (N) based on the information given.

Mass NumberExact Weight
(Isotopic Mass) 		Percentage Abundance
1414.003074 99.63%
1515.000108 0.37%

The exact mass of an isotope is something that is determined experimentally. To find the average atomic mass experimentally a mass spectrometer is used. This laboratory instrument works based on the differences in the mass of the different isotopes of an element. Because isotopes for an element have different masses, they can be separated, and the percentage of each isotope measured. From these measurements the average atomic mass can be calculated.

Isotopic Mass Equation

Review of Important Terms:

Isotopic Mass Of Carbon

  • Mass Number is the number of protons and neutrons in an isotope. It is a whole number. We use mass number in naming isotopes, like Carbon-12 or Oxygen-17.
  • Atomic Mass is the mass of the entire atom of an isotope. This includes electrons as well as a slight change in mass due to binding energy.
  • Average atomic mass is the weighted average of the isotopes for an element. This is what is listed below the element symbol on the Periodic Table.

Isotopic Mass Equation

Exercise: Use the simulation to compare the average atomic mass and determine why Hydrogen (1.0079) is close to a whole number but Chlorine (35.453) is not.

In the PhET simulation, click on 'Mixtures', and at the bottom of the page select 'Nature's Mix'. On the periodic table provided, select H or Cl.

Isotopic Mass Calculator