Atomic Mass Calculator
A mass number counts nucleons and is always whole; an atomic weight averages over isotopes and almost never is.
The formula
protons = Z; neutrons = mass number − Z; electrons = Z − charge
Three numbers, three different questions
The proton count decides which element it is. The neutron count decides which isotope. The electron count decides the charge.
Change the first and you have transmuted an element, which needs a nuclear reaction. Change the third and you have made an ion, which happens whenever salt dissolves. They are not comparable operations.
Mass number and atomic weight are different quantities
A mass number counts nucleons and is always a whole number. Chlorine-35 has exactly 35.
An atomic weight averages over the isotopes as they occur naturally and is 35.45 for chlorine. One is a property of a single atom, the other of a sample, and they share a unit only by convention.
Anions are bigger, cations smaller
Adding an electron increases repulsion in the outer shell and the atom swells. Removing one lets the remaining electrons draw in towards the unchanged nuclear charge.
A sodium ion is markedly smaller than a sodium atom, and a chloride ion markedly larger than a chlorine atom — which is why ionic crystals pack the way they do.
Ideal behaviour is an approximation
The colligative and gas relations here assume dilute solutions and ideal gases, which real systems approach and do not reach.
At high concentration or high pressure the deviations become large, and the corrections are substance-specific. These figures are right where the assumptions hold and approximate where they do not.