Mass number is related to the atomic mass of any elements. The symbol used for mass number is ‘A’. It is derived mainly from the term atomic weight. Also, mass number of any element can be defined as the number of nucleons, which means the number of the electrons and the number of protons that is present in any element. Such number helps in determining the category of element of atom. In an atom, the sum of neutrons and protons is represented by the mass number. There is no unit of mass number and therefore, it is unit less. Mass number can be established by forming the atomic weight and it is basically referred to the total number of particles measurement in the nucleus of an atom. Moreover, the total number of neutrons in an atom is calculated by using the mass number. Therefore, it is easy to determine the mass number if the number of neutrons and protons are known. The atomic mass number is determined by computing the means of mass numbers for all the isotopes that are naturally occurring.

For example, the mass number of silver is 108 because the value being equivalent to the mass number of silver. This can be explained using the atomic number, atomic mass weight and number of protons. The atomic number of silver is 47 with the atomic mass weight of 107.87 and the number of protons in the silver at the value of 47.

There are a set of simple rules for determining the number of electrons, protons and neutrons in an atom.  Electrons are the negatively charged particle which is known to carry a negative on their heads, and its mass is, whereas a proton is a positively charged particle and it has a mass as well like Mass of proton is 1.0072766 a.m.u. or 1.6726 x 10^-27 kg.  Since, the protons the neutrons are called the baryons of the atomic elements, therefore the mass number is very much similar to the baryon number, as the nucleolus of the whole of the atom or whole of the ions.

It is also known that the mass number of every isotope element is found to be different. Also of the purpose of better understanding, it should be known that isotopes are the elements having same atomic number and different mass number. The atomic number is denoted by the letter ‘Z’. Atomic number can be defined as the number of protons which are present in an element. It should also be understood that the difference in between the atomic number and the mass number gives us the number of the neutrons which are present in an element.

The most common example of an isotope is the isotopes of carbon like carbon ahs 2 isotopes one is  carbon-12, which is also written as ¹²C, which means that carbon has 6 protons and 6 neutron. The full isotope symbol would also have the atomic number (Z) as a subscript to the left of the element symbol directly below the mass number: ¹²₆C.  This is technically redundant, as each element is defined by its atomic number, so it is often omitted. The term atomic mass is measured in a.m.u. and often denoted by the letter “u”.

 For example, if the carbon isotope is taken into consideration, ¹²C the isotopic mass is exactly 12, since the atomic mass unit is defined as 1/12 of the mass of ¹²C. For other isotopes, the isotopic mass is usually within 0.1 u of the mass number. For example, ³⁵Cl (17 protons and 18 neutrons) has a mass number of 35 and an isotopic mass of 34.96885. The difference between mass number of an atom and its isotopic mass is known as the mass excess. Mass excess should not be confused with mass defect which is the difference between the mass of an atom and its constituent particles (namely protons, neutrons and electrons).

There are two reasons for mass defect/excess:

1. Since, it is known that the neutron is slightly heavier than the proton. Therefore, consequently, it can be easily said that this increases the mass of nuclei with more neutrons than protons relatively to the atomic mass unit scale based on ¹²C with equal numbers of protons and neutrons.
2. The nuclear binding energy varies in between nuclei. If there is nucleus with greater binding energy it has a lower total energy, and therefore a lower mass according to Einstein's mass energy equivalence relation E = mc². For ³⁵Cl the isotopic mass is less than 35 so this must be the dominant factor. The energy mass relationship is a very significant relation which describes the nuclear binding energy and the mass relationship very smoothly.

It can be accordingly said that mass number gives an estimate of the isotopic mass measured in atomic mass units (u). An isotope of the element would have the different mass number but same atomic number and they differ basically in terms of number of neutrons. However, the total number of protons in all the atoms of the element is same and hence the atoms belonging to the same element with different mass number and same atomic number is referred as the isotopes. If the above discussed example is taken into consideration then, it can be said that  ¹²C the isotopic mass is exactly ‘12’, since the atomic mass unit is defined as 1/12 of the mass of ¹²C.While, for the  other isotopes, the  mass of the isotope  mostly is usually within 0.1 u of the mass number. Again, for an example, ³⁵Cl (17 protons and 18 neutrons) has a mass number of 35 and an isotopic mass of 34.96885.Therefor it can be said that the total difference between mass number of an atom and its isotopic mass is known as the mass excess. Mass excess and one should not get confused with mass defect which is the difference between the mass of an atom and its constituent particle named as the protons, neutrons and electrons) accordingly.