Question

Atomic mass of chlorine atom is:
(A) ${\text{35}}{\text{.5}}$
(B) ${\text{16}}$
(C) ${\text{32}}$
(D) ${\text{24}}$

Answer 1

In the above question, atomic mass of the chlorine atom is asked. We know that chlorine exists as a mixture of the isotopes chlorine-35 and chlorine-37 and both of them contribute to the calculation of the atomic mass.
Formula Used
${\text{A = }}\sum\limits_{{\text{i = 1}}}^{\text{n}} {{{\text{a}}_{\text{i}}}{{\text{p}}_{\text{i}}}}$
Where A = atomic mass of the element
${{\text{a}}_{\text{i}}}$ =atomic mass of ${{\text{i}}^{{\text{th}}}}$ isotope.
${{\text{p}}_{\text{i}}}$ = percentage of occurrence of ${{\text{i}}^{{\text{th}}}}$ isotope.
n= number of isotopes.

Complete step by step solution
Atomic mass of the element or group of elements can be determined by the mass of the atom, which involves the mass of electron, proton and neutron. Isotope of a particular element can be defined as the element having the same atomic number but different atomic mass. In other words, we can say that isotopes have the same number of protons but they differ in the number of neutrons.
Chlorine has two isotopes chlorine-35 and chlorine 37.
Chlorine-35 has 17 protons and 18 neutrons. And its atomic mass is 35. The percentage of its occurrence is $75\%$ .
Chlorine-37 has 17 protons and 20 neutrons. And its atomic mass is 37. The percentage of its occurrence is $25\%$
So, now we can find the relative atomic mass of Cl as:
${\text{A = }}\sum\limits_{{\text{i = 1}}}^{\text{n}} {{{\text{a}}_{\text{i}}}{{\text{p}}_{\text{i}}}}$
Substituting the value of n, we get:
${\text{A = }}\sum\limits_{{\text{i = 1}}}^{\text{2}} {{{\text{a}}_{\text{i}}}{{\text{p}}_{\text{i}}}}$
Thus, the average mass will be:
${\text{A = }}{{\text{a}}_{\text{1}}}{{\text{p}}_{\text{1}}}{\text{ + }}{{\text{a}}_{\text{2}}}{{\text{p}}_{\text{2}}}$
Substituting the atomic mass and percentage of occurrence of isotopes of chlorine, we get atomic mass of chlorine as:
${\text{A = 35}} \times \dfrac{{75}}{{100}}{\text{ + 37}} \times \dfrac{{25}}{{100}} = 26.25 + 9.25 = 35.5$
$\therefore$ The atomic mass of chlorine is $35.5$ .
So, the correct option is option A.

Note
In these types of questions, where we have to calculate the atomic mass of an element we have to take their isotopes mass and percentage of occurrence of that isotope into account. Most elements have isotopes but they are mostly radioactive so are neglected.