The position of an individual electron can be described by the four quantum numbers: n, l, m l , and m s . The Pauli exclusion principle states that electrons cannot possess the same set of quantum numbers. That means it describes whether the part of the electron lies mostly on the x,y, or z axis of the three dimensional grid. The values of the magnetic quantum number are the negative and positive values of l.
Since the reduction in mass upon the formation of an atom are small, the mass number is usually the same as the atomic weight rounded to the nearest integer. (For example, the atomic weight of chlorine-37 is 36.966, which is rounded to 37.) If there are several isotopes of an element in nature, then of course the experimentally observed atomic weight (the natural atomic weight) will be the weighted average of the isotope weights.
Another possibility is a carbon with three bonds and a single, unpaired (free radical) electron: in this case, the carbon has a formal charge of zero. (One last possibility is a highly reactive species called a ‘carbene’, in which a carbon has two bonds and one lone pair of electrons, giving it a formal charge of zero.