Question

Chlorine consists of $75\mathrm{\%}$ chlorine - 35 and $25\mathrm{\%}$ chlorine - 37 . Calculate the average atomic mass .
A. 35.9
B. 35.5
C. 35.2
D. 36.1

Answer 1

Hint: The average atomic mass of an element is the sum of the masses of its isotopes , each multiplied by its natural abundance on Earth and then the sum is divided by 100 . In the periodic table the average atomic mass of elements is written .

Complete step by step answer:
We know that an element can have different numbers of neutrons in its nucleus , but it always has the same number of protons . The versions of an element with different neutrons have different masses and are called isotopes . They have different subatomic construction but occupy the same place on the periodic table .
Now to calculate the average atomic mass of chlorine we will add the masses of the element's isotopes , each multiplied by its percentage abundance on earth and then the whole sum is divided by 100 .
So , we have $75\mathrm{\%}$ of chlorine - 35 and $25\mathrm{\%}$ of chlorine - 37 .
Therefore , Average atomic mass of chlorine = $\sum _{}^{}{\displaystyle \frac{p_i{A}_i}{100}}$
where , $p_i$ is the respective percentage and $A_i$ is the respective atomic mass of the element .
Average atomic mass = $($ ${\displaystyle \frac{(75\times 35)+(25\times 37)}{100}}$ = $35.5$
Hence , the average atomic mass of chlorine is 35.5.
Therefore,Option B is correct.

Note: The average atomic mass of an element can be found on the periodic table , under the elemental symbol . When data is available regarding the natural abundance of various isotopes of an element , it is simple to calculate the average atomic mass of that particular element .