The measurement unit used for atomic mass is the unified atomic mass that has the symbol 'u.' This is a derived unit from the carbon-12 isotope, where 12 u is the atomic mass of carbon-12. On the periodic table, students can find the atomic mass written below the element's name. Mass is a physical property of matter and is expressed in SI units as kilogram. The atomic mass of elements is not actually the mass of their atoms in kilograms, rather it is a value giving the. The chemists resisted making the amu one-sixteenth the mass of an oxygen-16 atom; it would change their atomic weights by about 275 parts per million. Making the amu one-twelfth the mass of a carbon-12 nucleus, however, would lead to only a 42 parts per million change, which seemed within reason. The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element. The average atomic masses are the values we see on the periodic table. (2.1.5) 0.7577 (34.969) + 0.2423 (36.966) = 35.453. Most naturally-occurring elements consist of mixtures of isotopes, so you can't just add the masses of the protons and neutrons in a single atom. Look up the atomic mass in the Periodic Table.
How do I calculate the average atomic mass of element X?
isotope: abundance: mass:
221x 74.22 220.9
220x 12.78 220.0
218x 13.00 218.1
isotope: abundance: mass:
221x 74.22 220.9
220x 12.78 220.0
218x 13.00 218.1
1 Answer
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The atomic mass is the weighted average of the atomic masses of each isotope.
In a weighted average, we multiply each value by a number representing its relative importance.
In this problem, the percent abundance represents the relative importance of each isotope.
Here's how to do the calculation:
The average relative atomic mass of element X is 220.4.
Related questions
The atomic mass of an element is the average mass of the atoms of an element measured in atomic mass unit (amu, also known as daltons, D). The atomic mass is a weighted average of all of the isotopes of that element, in which the mass of each isotope is multiplied by the abundance of that particular isotope. (Atomic mass is also referred to as atomic weight, but the term 'mass' is more accurate.)
The Atomic Mass Of An Element Is Defined As
For instance, it can be determined experimentally that neon consists of three isotopes: neon-20 (with 10 protons and 10 neutrons in its nucleus) with a mass of 19.992 amu and an abundance of 90.48%, neon-21 (with 10 protons and 11 neutrons) with a mass of 20.994 amu and an abundance of 0.27%, and neon-22 (with 10 protons and 12 neutrons) with a mass of 21.991 amu and an abundance of 9.25%. The average atomic mass of neon is thus:
| 0.9048 | × | 19.992 amu | = | 18.09 amu |
| 0.0027 | × | 20.994 amu | = | 0.057 amu |
| 0.0925 | × | 21.991 amu | = | 2.03 amu |
| 20.18 amu |
The atomic mass is useful in chemistry when it is paired with the mole concept: the atomic mass of an element, measured in amu, is the same as the mass in grams of one mole of an element. Thus, since the atomic mass of iron is 55.847 amu, one mole of iron atoms would weigh 55.847 grams. The same concept can be extended to ionic compounds and molecules. One formula unit of sodium chloride (NaCl) would weigh 58.44 amu (22.98977 amu for Na + 35.453 amu for Cl), so a mole of sodium chloride would weigh 58.44 grams. One molecule of water (H2O) would weigh 18.02 amu (2×1.00797 amu for H + 15.9994 amu for O), and a mole of water molecules would weigh 18.02 grams.
The Atomic Mass Of An Element Is Equal To
The original periodic table of the elements published by Dimitri Mendeleev in 1869 arranged the elements that were known at the time in order of increasing atomic weight, since this was prior to the discovery of the nucleus and the interior structure of the atom. The modern periodic table is arranged in order of increasing atomic number instead.