1.2.3 Relative Atomic Mass and Atomic Mass Unit

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Relative Atomic Mass

As we know that the mass of an atom is too small to be determined practically. However, certain instruments enable us to determine the ratio of the atomic masses of various elements to that of carbon-12 atoms. This ratio is known as the relative atomic mass of the element. The relative atomic mass of an element is the average mass of the atoms of that element as compared to 1/12 (one-twelfth) the mass of an atom of carbon-12 isotope (an element having a different mass number but the same atomic number). Based on the carbon-12 standard, the mass of an atom of carbon is 12 units and l/12th of it comes to be 1 unit. When we compare the atomic masses of other elements with the atomic mass of carbon-12 atoms, they are expressed as relative atomic masses of those elements. In other words, relative atomic mass can be defined as:

"The relative atomic mass of an element is defined as the weight in grams of the number of atoms of the element contained in 12.00 g of carbon-12."

Atomic mass unit

The unit for relative atomic masses is called the atomic mass unit, with the symbol "amu". One atomic mass unit is 1/12th the mass of one atom of carbon-12.

"amu" in Grams

When this atomic mass unit is expressed in grams, it is:

1 amu = 1.66 x 10^(-24) grams

For Example mass of protons, neutrons, and electrons in "amu" are converted into grams in the following.

Mass of Proton

In amu = 1.0073 amu

In grams = 1.672x10^(-24) g

Mass of Neutron

In amu = 1.0087 amu

In grams = 1.674x10^(-24) g

Mass of Electron

In amu = 5.486 x 10^(-4) amu 

In grams = 9.106x10^(-28) g

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Hello, this is Misbah ud Din a student of STEM. This is my blog chemisTREEpk which provides a vast knowledge about Chemistry(and sometimes other sciences) fully free.

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