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ATOMIC MASS

Atoms on the degree of precision and smallness so it was difficult to estimate their atomic masses, however it has been possible to assign their mass with great accuracy, for example, it has found that a hydrogen atom mass reach 1.64×10-24 g.

it has also been possible to obtain relative masses of atoms by setting the mass of elements with another element, provided that the relative number of atoms in the compounds is known, so atomic mass used to express an ele- ment mass for another element atom mass and it is agreed to use in determining the relative masses of all the elements of the periodic table.

In 1961 in Geneva held a Conference of the International Union of Pure and Applied Chemistry (IUPAC) which was agreed on the definition of the standard unit of atomic mass and Atomic Mass Unit named after (AMU) as equivalent to one of twelve part of carbon isotope atom mass 12 and that it's atomic mass considered equal to 12 units and therefore:-

Atomic mass unit (amu) = (mass of an atom of carbon isotope 12)/12

i.e.; 1 (amu)= (1/12) mass of an atom of carbon isotope 12

As the mass of carbon 12 isotope atom=

As the mass of carbon 12 isotope atom=


 

Thus, the atomic mass we use today and found in the periodic table are not ac- tual masses, but relative masses show the relationship in terms of atomic mass between different atoms. For example, the atomic mass of hydrogen isotope 1 is (1/12) the atomic mass of the isotope carbon-12 which is about 1 amu, either nucleus oxygen 16 have mass equal to(16/12) or (4/3) from the mass of carbon 12 isotope, so when atomic mass estimates in grams it called the gram-atomic mass.

 

 

Atomic mass unit

Atomic mass unit (AMU or amu) An atomic mass unit (symbolized AMU or amu) is defined as pre- cisely 1/12 the mass of an atom of carbon-12. The carbon-12 (C-12) atom has six protons and six neut- rons in its nucleus.

 

 

The gram-atomic mass for oxygen = 16 g and for silver= 107.9 g and each mass of these masses has Avogadro's number of atoms which is equal to 6.023×10*23 atoms, for example:

1  g of hydrogen contains 6.023×10*23 atoms of hydrogen

2  39 g of potassium contain 6.023×10*23 atoms of potassium

3  207 g of lead contain 6.023×10*23 atoms of lead

The absolute atomic mass is the mass of one atom of the element. i.e.;

The absolute atomic mass of an element=gram-atomic mass of the element/ Avogadro's number

 

  

Example

Calculate the absolute atomic mass of the oxygen given that it's atomic mass is equal to 16.

Solution:

The absolute atomic mass of an element=gram-atomic mass of the element/ Avogadro's number

Calculate the absolute atomic mass of the oxygen given that it's atomic mass is equal to 16.


 

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