Difference Between Molar Mass and Atomic Mass

Edited by Diffzy | Updated on: April 30, 2023


Difference Between Molar Mass and Atomic Mass

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Atomic mass is the number of protons and neutrons in an atom. In contrast, the molecular mass is the mass of a single mole of an element.

  • Atomic mass is "unitless," whereas molar mass is measured in grams per mole.
  • Mass spectrometry is used to calculate molar mass while measuring atomic mass.
  • The weight of a single atom is its atomic mass. The mass of particles in one mole is known as the molar mass (atoms and molecules).
  • It is expressed in a.m.u, sometimes known as daltons (Da) or you.
  • The measurement is in grams per mole.
  • The atomic mass of the nucleus's protons and neutrons is known as the atomic mass.
  • The mass of Avogadro's constant is called molar mass (6,022 x 1023 particles).
  • Atomic weight is another name for atomic mass, but mass is the standard term.
  • It's common to refer to a molecule's weight as its molecular mass.
  • It is used on an atomic (micro) scale.
  • It is used on a large scale.

The masses of isotopes are included in the average atomic mass. However, isotopes' relative atomic masses can differ. Molar masses also include isotopic masses because they are big-scale atomic masses.

Atomic Mass vs. Molecular Mass

  • Atomic mass and a mass number have different meanings in chemistry. The first is an element's average weight, and the second is the total amount of nucleons in an atom's nucleus.
  • Atomic weight is another name for atomic mass. Atomic mass is the weighted average mass of an element's atoms, calculated from the frequency of the element's various atom types in nature.
  • The total number of protons and neutrons in the nucleus of an atom is counted to determine the mass number.

Difference Between Atomic Mass and Molar Mass in Tabular Form

Parameter of Comparison Atomic Mass Molar Mass
Definition It refers to the mass of a single atom. It is the particle mass of one mole.
Particles Protons, electrons, and neutrons Avogadro's particle count
Units AMU g/mol
Know that Atomic density Molar weight
Utilised as Atomic scale Macroscopic level
Isotopic values Included Included
Examples Na = 22.99 AMU Na = 22.99 g/mol

What is Atomic Mass?

Unless the number of protons and neutrons in an atom's nucleus is known, its mass may be determined regardless of size. The spectrometric methods used for this are successful. The mass of an atom, also known as atomic mass or weight, is determined by comparing the mass number (number of protons and neutrons) of a sample atom to the standard 12C (1/12th of the mass).

Since atoms have relatively minuscule masses, they cannot be measured in grams or kilograms. Because of this, chemists substitute the relative atomic mass scale for atomic mass. Atomic mass or atomic weight refers to the relative mass of an atom.

Errors in the masses of the atomic particles

  • Common isotopes have nuclear binding energies. A graph showing the proportion of mass number to atomic mass is similar.
  • The following chart shows how far from 1 the ratio of atomic masses to mass numbers has deviated: For hydrogen-1, the deviation starts positively before becoming smaller until it reaches a local minimum for helium-4. Lithium, beryllium, And boron isotopes are less intimately bonded than helium isotopes, as indicated by their growing mass-to-mass ratios.
  • The ratio of mass (measured in daltons) to mass number is defined as one at carbon, and it then falls until it reaches a minimum at iron-56 (with only marginally higher values for iron-58 and nickel-62) before rising to positive values in the heavier isotopes as the atomic number increases. This makes sense given that nuclear fission in materials heavier than zirconium produces energy, whereas it consumes energy in elements lighter than niobium. On the other hand, the fusion of elements heavier than calcium requires energy, but the fusion of two atoms of an element lighter than scandium (apart from helium) produces energy. Energy would be required to fuse two 4He atoms to create beryllium-8, and the beryllium would quickly disintegrate. Both tritium (3H) and 3He can fuse with 4He; these events took place during the nucleosynthesis of the Big Bang. Three 4He atoms are fused in the triple-alpha reaction to create elements with more than seven nucleons. Instead of lithium, beryllium, or boron, carbon-nine is used.

Atomic mass unit or AMU

The mass of an atom of carbon-12 is equivalent to one-twelfth of an atomic mass unit.

The periodic table's atomic numbers and atomic masses are not the same. It is because most elements are mixtures of isotopes in nature. The atomic masses of atoms are used to calculate the average mass of radioactive isotopes. Due to this, atomic masses are typically expressed as decimals. C, for instance, has an atomic mass of 12.011. It could be calculated as displayed below.

Estimating the typical atomic mass

  • Accurately calculating the number of isotopes and their relative abundances is the first step in determining the average atomic mass. These details are acquired using spectrometric methods (mass spectrometer).
  • For instance, chlorine has two isotopes (Cl35 = 75%, Cl 37 = 25%), hydrogen has three isotopes (1H1 99.972%, 1H2 0.0156%, and 1H3 10-18%), etc.
  • Multiplying the masses of an element's isotopes by the quantity of each one will yield the element's atomic mass.

What is Molecular Mass?

The mass of a sample of a chemical compound is divided by the quantity of the substance present in the sample, which is the number of moles measured in moles, to determine the compound's molar mass in chemistry. The molar mass of a substance is a bulk characteristic, not a molecular characteristic. The molar mass is an average of many instances of the compound, whose masses regularly shift due to the existence of isotopes. The molar mass is usually estimated using the standard atomic weights and is consequently a function of the relative abundance of the component element isotopes on Earth. It is possible to convert between a material's mass and quantity for bulk amounts using the molar mass.

  • Particularly for molecular compounds, the phrases molecular mass and formula mass are sometimes used synonymously with molar mass; nevertheless, the most trustworthy sources describe it differently.
  • The weight of a single particle or molecule is its molecular mass, whereas the average weight of several particles or molecules is its molar mass.
  • The term "molar mass" is frequently used to refer to the formula weight of non-molecular substances such as ionic salts.
  • An essential characteristic of a substance unaffected by sample size is its molar mass. The consistent unit for molar mass in the International System of Units (SI) is kilograms per mole. Molar masses are often expressed in grams per mole due to historical considerations.
  • The mole was created so that the average mass of a molecule, measured in daltons, is equal to the molar mass of a chemical, measured in g/mol. It was precisely equivalent to the 2019 redefinition of the mole; presently, it is only roughly equal, but the difference is minimal for all intents and purposes. Therefore, water has a molar mass of roughly 18,0153 g/mol and an average mass of about 18,0153 daltons.
  • For elements like carbon and metals that don't have separate molecules, the molar mass is computed by dividing by the number of moles of atoms. For instance, iron has a molar mass of about 55.845 g/mol.

The "quantity of material" has been recognized by SI as a separate unit of measurement since 1971. The mole was previously defined as the amount of a substance possessing the same number of particles as atoms in 12 grams of carbon-12. The molar mass of carbon-12 at that time was, by definition, precisely 12 g/mol. Since 2019, a mole of any substance is defined as the amount of that substance having 6.022140761023 particles, a known number of particles. As a result, the mass of a substance's molecules in grams equals the compound's molar mass in grams per mole.

Here are a few examples of determining the molar masses of various substances.

Molecular mass calculation

NaCl has a molar mass of 22.9897 g/mol for 1 Na atom, 35.453 g/mol for 1 Cl atom.

Atomic mass = (individual atom molar masses)

NaCl has a molar mass of 22.9897 g/mol plus 35.453 g/mol.

NaCl, 58.4427 g/mol. O2 has a molar mass of 15.999 g/mol per oxygen atom.

O2's molecular weight is 15.999 g/mol plus 15.999 g/mol. O2 = 31.998 g/mol

The molar mass of CO2 is falling.

  • 12.011 g/mol for 1 C atom.
  • 15.999 g/mol for 1 O atom.

Atomic mass = (individual atom molar masses)

  • CO2 molar mass equals 12.011 g/mol plus 15.999 g/mol plus 15.999 g/mol.
  • 44.009 g/mol of CO2

Difference Between Molar Mass and Atomic Mass in Points

  • The first definition of molar mass is the mass of a mole or one mole. Additionally, It distinguishes itself from other substances and individual components and molecules. The molar mass and molecular weight are equivalent. The primary distinction is that molecular weight solely refers to the relative weight of molecules relative to elements. The molar mass and molecular weight are represented in terms of grams per mole.
  • For example, let's calculate the molar mass or molecular weight of the substance water. The symbol for water is H2O. One oxygen atom and two hydrogen atoms make up its structure. Each mole of oxygen contains 16 grams of mass. Per mole, a hydrogen atom weighs one gram. Thus, doubling two hydrogen atoms would result in two grams of hydrogen per mole and sixteen grams of oxygen per mole for 18 grams per mole. Water, therefore, has a molecular weight or molar mass of 18 grams.
  • On the other hand, atomic mass refers to the mass of an atom while it is at rest. Proton and neutron counts are both included in the definition of atomic mass. The singular weight of an isotope, not its average weight, is referred to as the atomic mass of an atom. Using mass spectrometry, atomic mass can be determined. Calculating molar mass starts with atomic weight. Atomic masses are "unitless," meaning there are no units of measurement for them because they are relative masses.
  • The mass of a single atom, expressed in atomic mass units, is referred to as its atomic mass or weight (a.m.u). An element's mass is expressed as a fraction of carbon atom mass of 1 a.m.u. On the other hand, the mass of one mole of particles (atoms or molecules) is measured in grams per mole (g/mol).

Since atomic mass is so minuscule, it cannot be determined separately. For this reason, physicists must use relative terminology while defining the mass of atoms. For instance, one hydrogen atom's (protium) mass could not be determined precisely by the mass of an atom (a.m.u)

The relationship between them is 1/12 if we assume that C-12 is 12 AMU with 6 protons and six neutrons and that a hydrogen atom (with one proton) is 12 AMU lighter than the C-12 atom. The mass of one mole of carbon atoms, or 1.66 x 10-24 g, is precisely equal to the value of one a.m.u.(g/mol) Molar mass units

A macroscopic (large scale) unit of mass for atoms or molecules is called a molar mass. The number of grams of material in one mole of a substance (g/mol) is the unit for measuring molar mass, which is the weight of the Avogadro number of particles of a substance.

NA = 6.022 x 1023 Every mass is calculated at about 12C.

The size of an atom is on the order of picometers. Any method now in use cannot directly calculate its mass. It must be connected to a known mass that can be used as a benchmark. Since compounds also contain isotopes, the standard must be the one with the most incredible relative isotopic abundance for the results to be as accurate as possible. The element carbon, whose isotopic abundance for carbon-12 is close to 98.89%, satisfies this requirement (12C).


Despite having different numbers and units, molar and atomic mass have the same values. Like molar masses, molecular weights are different quantities with the same values.


  • Chemistry in Quantitative Language: Fundamentals of General Chemistry, by Christopher O. Oriakhi (MIT, Nike, hp, RSC, etc.)
  • Atomic masses (angelo.edu)


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"Difference Between Molar Mass and Atomic Mass." Diffzy.com, 2024. Thu. 16 May. 2024. <https://www.diffzy.com/article/difference-between-molar-mass-and-atomic-mass-1039>.

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