Most elements occur naturally as a mix of different isotopes. An element\'s atom

Question

Most elements occur naturally as a mix of different isotopes. An element's atomic mass is the weighted average of the isotope masses. In other words, it is an average that takes into account the percentage of each isotope. For example, the two naturally occurring isotopes of boron are given here.

The atomic mass of boron is calculated as follows:

(10.0×0.199)+(11.0×0.801)=10.8 amu

Because the heavier isotope is more abundant, the atomic mass is closer to 11 amu than it is to 10 amu.

Part A

What is the atomic mass of a hypothetical element that consists of the following isotopes in the indicated natural abundances?

Express your answer to three significant figures and include the appropriate units.

Explanation / Answer

we know that an element is mix of it's isotopes(If isotopes exit for that element)

so atomic mass of that element = weighted average of the isotope masses

mass of isotope 1 = 85.9

relative abundance of isotope 1 = 0.119

mass of isotope 2 = 87.9

relative abundance of isotope 2 = 0.111

mass of isotope 3 = 91.9

relative abundance of isotope 3 = 0.770

atomic mass of element

= mass of isotope 1 * relative abundance of isotope 1 + mass of isotope 2 * relative abundance of isotope 2 + mass of isotope 3 * relative abundance of isotope 3

=85.9*0.119 + 87.9*0.111 + 91.9*0.770

=90.742 amu

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