Question

uranium has three common isotopes that naturally occur in the earth: $^{234}u$, $^{235}u$, and $^{238}u$. the relative abundance of each atomic mass is $0.0054\\%$ for $^{234}u$, $0.7204\\%$ for $^{235}u$, and $99.2742\\%$ for $^{238}u$.
the average atomic mass is ____ (use significant figures to solve.)
92 amu
235 amu
236 amu
238 amu
14 choose the best answer.
why did mendeleev leave gaps in his periodic table?
to leave space for missing elements yet to be discovered
to account for extra atomic masses
wanted others to finish his work
ran out of space on the table

Explanation:

Step1: Calculate the contribution of each isotope

For \(^{234}U\): \(234\times0.0054\%=234\times\frac{0.0054}{100}= 1.2636\)
For \(^{235}U\): \(235\times0.7204\%=235\times\frac{0.7204}{100}=1.69394\)
For \(^{238}U\): \(238\times99.2742\%=238\times\frac{99.2742}{100}=236.2726\)

Step2: Sum up the contributions

\(1.2636 + 1.69394+236.2726=239.23014\approx238\) (using significant figures, since the most abundant isotope \(^{238}U\) has a large contribution and the relative abundance of \(^{238}U\) is dominant)

Mendeleev left gaps in his periodic table based on the pattern of properties. He predicted that there were elements yet to be discovered that would fit into those gaps. This was a forward - looking approach in organizing the elements.

Answer:

238 amu

For the second question: