Why are atomic weight of maximum elements are in fraction?
Answers
There are two reasons. First, most elements have more than one naturally occurring isotope, and the atomic weight is an average of all the isotopes (weighted according to their natural percentages), so that when you have a large number of atoms, the atomic weight will tell you the average mass of an atom of that element, which will permit you to calculate the number of atoms in a given mass.
However, even for elements that have only one naturally-occurring isotope (for example, sodium, with consists only of Na-23 (11 protons, 12 neutrons, 11 electrons)), we find that the atomic weight is not exactly 23, but 22.98977. What’s going on there?
This ties in to the most famous equation in physics, E=mc². When protons and neutrons join together to form an atomic nucleus, energy (“binding energy”) is released, and that is so much energy that it measurably reduces the mass of the nucleus compared to the separate protons and neutrons, by E/c². This decrease in mass is called the “mass defect”. Since the unit for atomic weights is not the mass of a single proton or a single neutron (which are a little different, anyway) but 1/12 the mass of a carbon-12 atom, atoms with a bigger mass defect than carbon weigh a little less than you get from adding up the number of protons + neutrons, and those that have a smaller mass defect weigh a little bit more. Either way, with both these effects (isotopes and mass defects) at work, it almost never comes out to an integer. (Not even for carbon, since it has little bits of C-13 and C-14 mixed in with the C-12).