Aston Rule & Packing Fraction

# Aston Ryle and packing fraction: X 1919 Aston developed Mass spectrometer to Calculate accurate Atomic masses. Aston observed that Atomic masses are very close to the whole numbers which are actually the mass no. when expressed amy. For A < 20 and A> 180, Atomic masses slightly greater them Mass number A. For 20 LA < 180 ( intermediate nuclei) Atomic masses are slightly smaller than mass number A. This departure is called mass defect by Aston AM = MA- A Atomic-mass Packing fraction f= A AM f = MA-A MA = A I A

f 20 180 A C12 f=o by def. eg. Atomic Mass of H'= 1.0072 amy He 14 = 4.00 amu Li7 = 7.016 amu C12 = 12 amy (by def") N 14 = 14.003 amu 016 = 15.994 (exception) Mg = 23.985 amy f = ve Al 27 26.981 amu Packing fraction is measure of stability $ has rein with binding energy.