Question

prepwork part i: atomic mass practice

1. examine the data for all naturally occurring isotopes of unknown element z below. use it to answer questions below.

isotope	mass (amu)	percent abundance in nature
2	64.93	30.83%

a. **make a prediction - what do you think the average atomic mass will be based on the natural abundances of both isotopes? ______________
b. **based on your prediction, what do you think the likely identity of element z is? ______________
c. represent the isotopes and their natural abundances on the graph below. complete the x - axis.
!graph(graph for isotopes)
d. **calculate the average atomic mass of the element.
e. **write the isotopic notation and atomic symbol for each isotope, using the element’s correct name.
flip over

Explanation:

Step1: Recall average atomic mass formula

The formula for average atomic mass is $\sum_{i} (m_i\times p_i)$, where $m_i$ is the mass of the isotope and $p_i$ is the percent - abundance (in decimal form) of the isotope.

Step2: Convert percent - abundances to decimals

Isotope 1: $69.17\%=0.6917$, Isotope 2: $30.83\% = 0.3083$

Step3: Calculate the contribution of each isotope

For Isotope 1: $m_1\times p_1=62.93\times0.6917 = 62.93\times0.6917=43.527681$
For Isotope 2: $m_2\times p_2=64.93\times0.3083 = 64.93\times0.3083 = 19.917919$

Step4: Calculate the average atomic mass

$43.527681 + 19.917919=63.4456\approx63.45$ amu

Answer:

a. A prediction could be that the average atomic mass is closer to 62.93 amu since the first isotope has a higher natural abundance.
b. Copper (Cu) has an average atomic mass of approximately 63.55 amu, so element Z is likely copper.
c. On the x - axis, label two points at 62.93 amu and 64.93 amu. Draw a bar above 62.93 amu that reaches to 69.17 on the percent - abundance axis and a bar above 64.93 amu that reaches to 30.83 on the percent - abundance axis.
d. 63.45 amu
e. For the first isotope of copper: $^{63}Cu$ (isotopic notation), Cu (atomic symbol). For the second isotope of copper: $^{65}Cu$ (isotopic notation), Cu (atomic symbol).