Question

What is the reason behind the fractional mass number of chlorine, though the mass number is defined as the sum of the number of protons and number of neutrons?

• A. It exists in three isotopes .
• B. It has no isotopic form.
• C. It exists in four isotopes .
• D. It exists in two isotopes .

Answer 1

The correct option is D It exists in two isotopes .

The reason behind the fractional atomic mass of chloine is the different isotopes of an chlorine present in nature. Chlorine has more two isotopes having different masses. Since the atomic mass of an element is the average relative mass of all the natural isotopes of that element.
Chlorine in found to exist as two isotopes i.e. Chlorine 35 and chlorine 37 in ratio of 75% and 25% respectively.
This means that isotope of mass 35 u will contribute 75% whereas isotope of mass 37 u will contribute 25% to average atomic mass of Chlorine.
$Averageatomicmass=35\times \frac{75}{100}+37\times \frac{25}{100}$
$=\frac{2625}{100}+\frac{925}{100}$
$=26.25+9.25$
$=37.5u$
Thus, average atomic mass of Chlorine is 35.5 u that is in fraction.