Answer to Question #95075 in General Chemistry for Jeremiah Oriade

Question

Chlorine has 2 commonly occurring isotopes. \ce{{75.78}\%}75.78% of \ce{Chlorine}Chlorine atoms have 1818 neutrons and 1717 protons, whereas the other \ce{{24.22}\%}24.22% has 2020 neutrons and 1717 protons. Using the isotopic composition provided, and the known masses of protons ( 1.007271.00727 amu) and neutrons ( 1.008661.00866 amu), calculate the expected mass of \ce{Chlorine}Chlorine . Round your answer to 3 decimal places.

Answer 1

Atomic mass is the average mass of an atom of that element, taking into account all the isotopes in their relative amounts.

Explanation:

Here is an example:

Chlorine exists in the form of two principle isotopes. One has 17 protons and 18 neutrons, and a mass of 34.969 u. It makes up 75.78% of all chlorine atoms. The other isotope has 17 protons and 20 neutrons, and accounts for the remaining 24.22% of atoms. The mass of these atoms is 36.966 u.

(u is the atomic mass unit equal to 1

12

 of the mass of a carbon 12 atom.)

Now, find the average mass of these atoms:

34.969

×

0.7578

=

26.50

u

36.966

×

0.2422

=

8.95

u

Add the two above values: 26.50

+

8.95

=

35.45

u

So, the average mass of a chlorine atom is 35.45 u. This is the atomic mass.

An atomic mass very close to a whole number usually means the element is composed of only one major isotope (with very small portions of lighter of heavier isotopes.) Chlorine has two major isotopes, and this leads to an atomic mass not close to a whole number.