Difference Between Atomic Mass and Molecular Weight

Atomic Mass vs Molecular Weight

Atoms can join in various combinations, to form molecules and other compounds. Molecular structures give the exact ratios of atoms; thus, we can write molecular formulas for compounds. These are important in determining the molecular weight.

Atomic Mass

Atoms are mainly composed of protons, neutrons and electrons. Atomic mass is simply the mass of an atom. In other words, it is the collection of masses of all the neutrons, protons and electrons in a single atom, specifically, when the atom is not moving (rest mass). Mass at rest is taken because, according to the fundamentals of physics, it has been shown that when atoms are moving at very high velocity the masses increase. However, the mass of electrons is considerably very small compared to the masses of protons and neutrons. So we can say that the electrons’ contribution to an atomic mass is less. 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 the same proton and electron amount. Since their neutron amount is different, each isotope has a different atomic mass.

Moreover, the masses of atoms are extremely small, so we cannot express them in normal mass units like grams or kilograms. For our purposes, we are using another unit call atomic mass unit (amu) to measure the atomic mass. 1 atomic mass unit is the one twelfth of the mass of a C-12 isotope. When a mass of an atom is divided by the mass of one twelfth of the mass of a C-12 isotope, its relative mass is obtained. However, in the general use when we say the relative atomic mass of an element, we mean their atomic weight (because it is calculated considering all the isotopes).

Molecular Weight

Molecular weight is the collection of weights of all the atoms in a molecule. The SI unit of the molecular weight is g mol-1. This gives the amount of the atoms/molecules/compounds present in one mole of the substance. In other words, it is the mass of Avogadro number of atoms/molecules or compounds. It is important to measure the weight of atoms and molecules in the practical scenario. But it is difficult to weigh them as individual particles, since their masses are extremely small according to the normal weighing parameters (grams or kilograms). Therefore, to fulfill this gap and measure the particles in a macroscopic level, molar mass concept is useful. The definition of molecular weight is directly related to the carbon-12 isotope. The mass of one mole of carbon 12 atoms is exactly 12 grams, which is its molar mass is exactly 12 grams per mole. Molecular weight of molecules containing the same atoms like O2 or N2 is calculated by multiplying the number of atoms by the atomic weight of the atoms. Molecular weight of the compounds like NaCl or CuSO4 is calculated by adding the atomic weights of each atom.


What is the difference between Atomic Mass and Molecular Weight?

• Atomic mass is the mass of a single atom, which is its collective mass of neutrons, protons and electrons. Molecular weight is the weights of the atoms in the molecule.

• Molecular weight gives the average mass of the molecule (mass of Avogadro number of molecules), whereas atomic mass gives the mass of a single atom (without considering the average mass of the other isotopes).