Question

Strontium consists of four isotopes with masses of $ 84 $ (abundance $ 0.50\% $ ), $ 86 $ (abundance of $ 9.9\% $ ), $ 87 $ (abundance of $ 7.0\% $ ), and $ 88 $ (abundance of $ 82.6\% $ ). What is the atomic mass of strontium?

Answer 1

Hint: In order to find the atomic mass of strontium, we would need the abundances and atomic masses of all its isotopes and then we will find an average atomic mass of it. As we can see that strontium with atomic mass $ 88 $ is the most abundant isotope of strontium and hence, the average atomic mass of strontium will be close to $ 88 $ .

Complete answer:
We know that the average atomic mass of any element is the weighted average mass of its isotopic atoms in the naturally occurring sample of the element.
The average atomic mass of any element is defined as the sum of the masses of its isotopes of and each mass multiplied by the fractional natural abundance value.
The formula for finding average atomic mass is the summation of the product of the atomic mass of each isotope and its abundance divided by $ 100. $
 $ A{M_{Avg}} = \sum {X_i}{M_i} $
Where, $ A{M_{Avg}} $ is the average atomic mass of a chemical element, $ {X_i} $ is the fraction abundance of isotope i and $ {M_i} $ is the atomic mass of isotope i.
Fraction abundance is calculated by dividing atomic mass of isotope by 100.
 $ AM = \left( {\dfrac{{0.5}}{{100}} \times 84} \right) + \left( {\dfrac{{9.9}}{{100}} \times 86} \right) + \left( {\dfrac{7}{{100}} \times 87} \right) + \left( {\dfrac{{82.6}}{{100}}} \right) \times 88 $
On calculating this, we get
 $ AM = 87.71 $
Therefore, the atomic mass of Strontium is found to be $ 87.71a.m.u. $
Where, a.m.u. is the S.I. unit of atomic mass.

Note:
We should not get confused between the terms atomic weight and average atomic mass. This is because the average atomic mass and atomic weight are the same. The average mass is defined as the unified atomic mass units (u) and 1u is equal to one- twelfth of the mass of the neutral carbon $ \; - 12 $ atom.