[newbe]

Dear friend,
I am getting confused in this question;what i learned till now is that number of proton+number of neutron=atomic mass. But while searching the net i got this equation
atomic mass = mass a x fract a + mass b x fract b. can any one tell me which is the correct (not approximate) method for finding atomic weight.

and my second question

is weight of proton+weight of neutron= atomic mass is this equation correct

advance thanks for your help

[gippgig]

Weight of protons + weight of neutrons - binding energy = atomic mass. The mass of the nucleus is always less than the sum of the mass of the protons & neutrons (except for protium).

[Wastrel]

newbe,

Your first sum gives the atomic mass of an element with only one isotope.  If the element has more than one isotope you work out the mass of each isotope using your first sum and then combine them using AbundanceA*MassA + AbundanceB*MassB + AbundanceC*MassC +. making sure that all your abundances (AbundanceA + AbundanceB + AbundanceC ...) total exactly 1.

gippgig's sum is more accurate and if you add the mass of the electrons it's 'right', but it's not that helpful as the most practical method we have for finding the binding energy is to find the real mass of an isotope experimentally and then subtract everything else.  Like pretty much everything in science, to approach the real answer you have to do an experiment.