Explanation:
aam= 90%•20amu+10%•22amu/100
aam= 2020/100
aam= 20.2
Taking into account the definition of atomic mass, isotopes and atomic mass of an element, the average mass of the element is 20.2.
First of all, you have to know that atoms are made up of protons and neutrons in a central nucleus, as well as an electronic cloud (the extra-nuclear zone), a place where there is a greater probability of finding electrons orbiting the nucleus of the atom.
Then, it is neccesary to know that the atomic mass (A) is obtained by adding the number of protons and neutrons in a given nucleus.
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.
This is, isotopes are atoms whose atomic nuclei have the same number of protons but different numbers of neutrons.
Because the isotopes of an element have different atomic masses, the relative atomic mass or atomic weight is an average of the atomic masses of the different isotopes in a sample taking into account the relative abundance of each of them.
In this case, 90% of the isotopes have a mass number of 20 amu, while 10% have a mass number of 22 amu. Then, the average mass of the element can be calculated as:
20 amu×0.90 + 22 amu×0.10= 20.2
Finally, the average mass of the element is 20.2.
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