What is relative atomic mass with example?

Relative atomic mass is the average mass of an atom of an element compared to 1/12th of the mass of an atom of carbon-12. It is a dimensionless quantity and is usually expressed in atomic mass units (u).

For example, the relative atomic mass of carbon is 12.011 u. This means that on average, an atom of carbon is 12.011 times heavier than 1/12th of the mass of an atom of carbon-12.

Relative Atomic Mass (A)

The relative atomic mass of an element is the ratio of the average mass of its atoms to one-twelfth of the mass of a carbon-12 atom. It is a dimensionless quantity.

Example:

Consider the element chlorine (Cl). Chlorine has two naturally occurring isotopes:

- Chlorine-35 (Cl-35): Mass of one atom = 34.968855 amu
- Chlorine-37 (Cl-37): Mass of one atom = 36.965903 amu

The natural abundance of these isotopes is:

- Cl-35: 75.77%
- Cl-37: 24.23%

Calculating the Relative Atomic Mass:

To calculate the relative atomic mass of chlorine, we multiply the mass of each isotope by its abundance and then sum the products:

A(Cl) = (Mass of Cl-35 - Abundance of Cl-35) + (Mass of Cl-37 - Abundance of Cl-37)
A(Cl) = (34.968855 amu - 0.7577) + (36.965903 amu - 0.2423)
A(Cl) = 35.453 amu

Therefore, the relative atomic mass of chlorine is 35.453 amu.