# Definition of Relative Atomic Mass | Atomic Mass Unit | Formula and Worked Examples

The size of an atom is too small that it is difficult to measure its mass by using standard methods. So since 1961, a system was devised in which Carbon-12 is used as the standard and the atomic mass of an atom of an element is measured by comparing it to carbon-12. The relative atomic mass of carbon-12 is 12. Previously, the relative atomic mass was referred to as atomic weight.

## Definition of relative atomic mass

The relative atomic mass of an element is the average mass of the atoms of that element relative to one-twelfth of the mass of an atom of carbon-12. The relative atomic mass is represented by the symbol Ar. The atomic mass unit (amu) is the unit of relative atomic mass.

It indicates how many times an element's average atom is weightier compared to one-twelfth of a carbon atom-12 from a given sample. Throughout the periodic table, this number increases gradually as protons and neutrons are added to the nuclei.

### Formula of relative atomic mass

It can be measured by using the following formula.

### Worked example 1:

Calculate the relative atomic mass of Sodium-23    .

Solution:

The mass of one atom of Sodium = 23
Relative atomic mass of sodium = 23 x 12/12 = 23 amu

### Relative isotopic mass

Many elements contain isotopes. An isotope has the same number of protons but the different number of neutrons. As the number of neutrons changes the atomic mass of isotopes also changes. So the relative isotopic mass is the average mass of isotopes of an element compared to 1/12 of the mass of carbon-12 atom.

### Formula of relative isotopic mass

It can be measured by using the following formula.

### Worked example 2:

In a sample, 25% chlorine-35 and 75% chlorine-37 are present. Calculate the relative atomic mass of chlorine atoms.

Solution:

### Worked example 3:

Calculate the relative atomic mass of X element by using the data given in the table.

 Realtive isotopic mass Abundance (%) 23.99 78.7 25.98 10.17

Solution:

### what is atomic mass unit?

The atomic mass unit is the unit of relative atomic mass. The atomic mass unit is defined as the mass equivalent to one-twelfth of the mass of one atom of carbon-12. In short, it is represented as amu. If the atomic mass unit is expressed in grams its value is 1.66 x 10-24 g.

### Examples of atomic mass unit

Some examples of atomic mass unit are:

• An atom of helium-4 has a relative atomic mass of 4.0026 amu.
• One atom of oxygen-16 has a relative atomic mass of 15.999 amu.
• An atom of sulfur-32 has a relative atomic mass of 31.972 amu.
• One atom of hydrogen-1 has a relative atomic mass of 1.008 amu.