Atomic Weight Molecular Weight

Atomic Weight Molecular Weight

Atom and Atomic Weight
The  smallest particle of an element that takes part in chemical reaction is called atom. An atom is so minute that it cannot be detected even with the most powerful microscope, let alone placed on a balance pan and weighed. So there is no question of determining the absolute weight of an atom. So chemists decided to determine the relative masses of atoms (i.e., how many times one atom of an element is heavier than another). Hydrogen atom was first selected as standard. i.e.
        Atomic weight of an element =\( \frac{{Weight\,\,of\,\,1\,\,atom\,of\,\,the\,\,element}} {{Weight\,\,of\,\,1\,\,atom\,\,of\,\,hydrogen}} \)
        When we state that the atomic weight of chlorine is 35.5, we mean that an atom of chlorine is 35.5 times heavier than an atom of  hydrogen. It was later felt that the standard for reference for atomic weight may be oxygen, the advantage being that the atomic weights of most other elements became close to whole numbers.
Atomic weight of an element = \( \frac{{Weight\,\,of\,\,1\,\,atom\,\,of\,\,the\,\,element}} {{\frac{1} {{16}} \times weight\,\,of\,\,1\,\,atom\,\,of\,\,oxygen}} \)
The modern reference standard for atomic weights is carbon isotope of mass number 12.
Atomic weight of an element = \( \frac{{Weight\,\,of\,\,1\,\,atom\,\,of\,the\,\,element}} {{\frac{1} {{12}} \times weight\,\,of\,\,1\,\,atom\,\,of\,carbon - 12}} \)
On this basis, atomic weight of oxygen 16 was changed to 15.9994.
Nowadays atomic weight is called relative atomic mass and denoted by amu 
(atomic mass unit). The standard for atomic mass is C^₁₂.
For example: On the experimental result the atomic weight of 1 atom of Aluminium is “27”
We know that weight of 1atom of carbon-12 is 12 
Therefore  Atomic weight of an element Aluminium
         = \( \frac{{Weight\,\,of\,\,1\,\,atom\,\,of\,the\,\,element}} {{\frac{1} {{12}} \times weight\,\,of\,\,1\,\,atom\,\,of\,carbon - 12}} \) =  \( \frac{{27}} {{\frac{1} {{12}} \times 12}} \) = 27
(i)    Atomic weight is not a weight but a number.
(ii)   Atomic weight is not absolute but relative to the weight of the standard reference element  (C¹²).
(iii)  Gram atomic weight is atomic weight expressed in grams, but it has a special significance with reference to a mole.
IMP NOTE: 
What is 1 amu ( atomic mass unit )
    1 amu or atomic mass unit is \( \frac{1} {{12}}^{th} \) of the weight of 1 atom of carbon-12 i.e., 
    1 amu = \( \frac{1} {{12}} \times weight\,\,of\,\,1\,\,atom\,\,of\,carbon - 12 \)
Therefore  Atomic weight of an element Aluminium in amu
         = \( \frac{{Weight\,\,of\,\,1\,\,atom\,\,of\,the\,\,element}} {{\frac{1} {{12}} \times weight\,\,of\,\,1\,\,atom\,\,of\,carbon - 12}} \) =  \( \frac{{27}} {{\frac{1} {{12}} \times 12}} \) = 27 amu