Answer :
Answer:
[tex]m_X=128.44 a.m.u[/tex]
Explanation:
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In this case, since the average atomic mass of an element, when given the atomic mass and the percent abundance of the naturally occurring isotopes is computed as shown below for this unknown element:
[tex]m_X=126*0.22+128*0.34+130*0.44[/tex]
Whereas the atomic mass of each isotope is multiplied by the percent abundancy; we obtain:
[tex]m_X=128.44 a.m.u[/tex]
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Considering the definition of atomic mass, isotopes and atomic mass of an element, the atomic mass of the element is 128.44 amu.
Definition of atomic mass
First of all, the atomic mass (A) is obtained by adding the number of protons and neutrons in a given nucleus of a chemical element.
Definition of isotope
The same chemical element can be made up of different atoms, that is, their atomic numbers are the same, but the number of neutrons is different. These atoms are called isotopes of the element.
Definition of atomic mass
On the other hand, the atomic mass of an element is the weighted average mass of its natural isotopes. In other words, the atomic masses of chemical elements are usually calculated as the weighted average of the masses of the different isotopes of each element, taking into account the relative abundance of each of them.
Atomic mass of the element in this case
In this case, you know:
- the first isotope has an atomic mass of 126 amu and a percent natural abundance of 22%.
- the second isotope has an atomic mass of 128 amu and a percent natural abundance of 34%.
- the third isotope has an atomic mass of 130 amu and a percent natural abundance of 44%.
Then, the average mass of lithium can be calculated as:
126 amu×0.22 + 128 amu×0.34 + 130 amu×0.44= 128.44 amu
Finally, the atomic mass of the element is 128.44 amu.
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