Difference Between Atomic Weight and Mass Number

Atomic Weight vs Mass Number

Atoms are characterized by their atomic numbers and mass numbers. In the periodic table, atoms are arranged according to their atomic number. Mass number of an element is more related with the mass of it. However, it is not giving the exact mass of the atom. Atomic weight is another way of expressing the weight of atoms, but this is different from the atomic mass. It is important to identify the meaning of these terminologies separately, because in measurements they can make large differences if used interchangeably.

Atomic Weight

Atoms are mainly composed of protons, neutrons, and electrons. Atomic mass is simply the mass of an atom. Most of the atoms in the periodic table have two or more isotopes. Isotopes differ from each other by having a different number of neutrons, even though they have same amount of proton and electron. Since their neutron amount is different, each isotope has a different atomic mass. Atomic weight is the average weight calculated considering all the masses of isotopes. Each isotope presents in the environment, in different percentages. When calculating the atomic weight, both isotope mass and their relative abundances are taken into consideration.

Moreover, the masses of atoms are extremely small, so we cannot express them in normal mass units like grams or kilograms. The weights given in the periodic table are calculated like above, and are given as relative atomic mass.

IUPAC defines the atomic weight as follows:

“An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12C.”

The mass of the most abundant isotope contributes more towards the atomic weight. For example, Cl-35 natural abundance is 75.76%, and Cl-37 abundance is 24.24%. The atomic weight of Chlorine is 35.453 (amu), which is closer to the mass of the Cl-35 isotope.

Mass Number

Mass number is the total number of neutrons and protons in a nucleus of an atom. The collection of neutrons and protons is also known as nucleons. Therefore, mass number can also be defined as the number of nucleons in a nucleus of an atom. Normally, this is denoted in the left upper corner of the element (as superscript) as an integer value. Different isotopes have different mass numbers, because their neutrons vary. So the mass number of an element gives the mass of the element in integers. The difference between the mass number and the atomic number of an element gives the number of neutrons it has.

What is the difference between Atomic Weight and Mass Number?

- Atomic weight is the average mass calculated considering all the isotopes. But mass number gives the mass of the specific isotope.

- Most of the time, the mass number is considerably different from the atomic weight.

For example, bromine has two isotopes. The mass number of one isotope is 79, whereas the mass number of the other isotope is 81. Atomic weight of bromine is 79.904, which is different from both the isotope masses.