The key difference between atomic mass and molecular weight is that the atomic mass is the mass of a single atom whereas the molecular weight is the sum of the weights of the atoms in the molecule.
Atoms are the building blocks of all matter. An atom has weight; it is atomic mass. 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.
What is Atomic Mass?
Atoms mainly contain 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). We take the mass at its rest because, according to the fundamentals of physics, when atoms are moving at very high velocity, the masses increase.
However, the mass of an electron is considerably very small compared to the masses of protons and neutrons. Therefore, 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; thus, 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 we divide the mass of an atom by the mass of one-twelfth of the mass of a C-12 isotope, we can get its relative mass. However, in the general use when we say the relative atomic mass of an element, we mean their atomic weight (because we calculate it considering all the isotopes).
What is Molecular Weight?
Molecular weight is the collection of weights of all the atoms in a molecule. The SI unit of this parameter is gmol-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 fulfil this gap and measure the particles in a macroscopic level, molar mass concept is very useful.
The definition of molecular weight directly relates 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. Moreover, we can calculate the molecular weight of molecules containing the same atoms like O2 or N2 by multiplying the number of atoms by the atomic weight of the atoms. However, 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 gives the mass of atoms while molecular weight gives the mass of molecules. However, the key difference between atomic mass and molecular weight is that the atomic mass is the mass of a single atom whereas the molecular weight is the sum of the weights of the atoms in the molecule. As another significant difference between atomic mass and molecular weight, the unit of measurement of atomic mass is amu while the unit for molecular weight is g/mol.
Moreover, there is a difference between atomic mass and molecular weight in the way we calculate these two quantities. We can determine the atomic mass easily via adding the masses of neutrons, protons and electrons of the atom. However, we determine the mass of a molecule or the molecular weight of a molecule via adding the average masses of atoms in the molecule.
Below illustration presents the difference between atomic mass and molecular weight in a tabular form.
Summary – Atomic Mass vs Molecular Weight
We use the terms atomic mass and molecular weight mainly in general chemistry calculations to determine the values regarding chemical reactions; such as the amount of reactants, products, etc. The key difference between atomic mass and molecular weight is that the atomic mass is the mass of a single atom whereas the molecular weight is the sum of the weights of the atoms in the molecule.
1.Helmenstine, Anne Marie, Ph.D. “How to Calculate Atomic Mass.” ThoughtCo, Dec. 5, 2018. Available here
1.”Periodic Table Of Elements Atomic Mass Black And White”By Dmarcus100 – Own work, (CC BY-SA 4.0) via Commons Wikimedia
2.”Boiling point vs molar mass graph”By Jpta – Own work, (CC BY-SA 3.0) via Commons Wikimedia