Atomic Weights
Atomic weights of elements were first determined with reference to hydrogen (Dalton), then with reference to oxygen-16, and at present with reference to carbon-12 which is arbitrarily assigned 12 units of weight.
Thus atomic weight of an element may be defined as a number which indicates how many times \(\frac 1{12}\) heavier is an average atom of that element as compared with of the mass of an atom of carbon 12 (C12).
At. wt. of an element = \(\frac{\text{Wt. of an average atom} }{\frac 1{12} \times \text{Mass of an atom of }C^{12}}\)
Thus the atomic weights of elements are average relative weights.
Units of atomic weight: The modern unit of atomic weight is \(\frac 1{12}\) of the mass of the atom of C-12. Thus atomic weight of an element is a simple ratio and has no units but expressed in amu.
1 amu = \(\frac 1{12}\) x mass of C-12 atom
The real mass of one atom of C-12 has been determined as 1.9924 x 10-23 g.
1 amu = \(\frac{1.9924 \times 10^{-23}}{12} g = 1.66 \times 10^{-24} g\)
1.66 x 10-24 g is referred to as the gram equivalent of a.m.u.
The lightest known atom is that of hydrogen and the heaviest naturally occurring atom is of uranium-238.
Atomic weights of many elements are not whole numbers due to the presence of stable isotopes. For example, natural chlorine is a mixture of two isotopes, viz Cl-35, (75% abundance) and Cl-37 (25% abundance). Thus atomic weight of chlorine is
\(\frac{75 \times 35 + 25 \times 37}{100} = 35.5\) amu
Thus the actual atomic weight of an element depends upon (i) the mass numbers of isotopes and (ii) relative abundances of these isotopes.
Average wt. of one atom of an element = At. wt. x 1.66 x 10-24 g
Thus, average wt. of 1 atom of hydrogen = 1.008 x 1.66 x 10-24 g = 1.673 x 10-24 g
Gram-atomic weight: When atomic weights of elements are expressed in grams, they are called gram atomic weights. Thus the weight of an element (in grams) which is equal to its atomic weight is known as the gram atomic weight (GAW) of that element. For example, atomic weight of sulphur is 32 and its gram atomic weight is 32 grams.
Thus
1 GAW of Na = 23 g of Na
1 GAW of Ca = 40 g of Ca
Gram atom or Mole atom: The atomic weight of an element in grams is one gram-atom or mole atom of that element. For example, one gram atom of sodium represents 23 gram sodium.
No. of gm atoms of an element = \(\frac{\text{Wt. of element in g}}{\text{GAW of that element}}\)
Remember 1 GAW of every element contains 6.023 x 1023 atoms of that element.
