Atomic Mass And Molecular Mass: Definition, Formula and Examples

Edited By Shivani Poonia | Updated on Jul 02, 2025 07:38 PM IST

Atomic mass is generally called the atomic weight of any chemical element. it is the mass of a chemical element and is measured in the atomic mass unit. The mass shows the number of protons and neutrons in the nuclei of an atom. It was discovered by the scientist John Dalton at the start of the 19th century. Dalton said that each atom has a different atomic weight as their compositions are different. And this theory of Dalton builds the idea of atomic theory.

This Story also Contains

Atomic Mass And Molecular Mass And Dalton's Atomic Theory
Some Solved Examples
Summary

Atomic Mass And Molecular Mass: Definition, Formula and Examples

Molecular mass is the mass of molecules that are formed by the combination of elements. The whole mass of the molecule including all of its elements is called the molecular mass. This theory was evolved from Dalton's work but further established by another scientist named Dmitri Mendeleev along with another chemist. Mendeleev developed the periodic table which helps us in understanding the relationship between the elements and their compounds. The knowledge of molecular mass is important for calculating the stoichiometry of chemical reactions and also the properties of the substance.

Atomic Mass And Molecular Mass And Dalton's Atomic Theory

Atomic Mass :

One atomic mass unit is defined as a mass exactly equal to one-twelfth the mass of one carbon-12 atom.

And 1 amu = 1.66056 x 10^-24g

Mass of an atom of hydrogen = 1.6736 x 10^-24g

Thus, in terms of amu, the mass of hydrogen atom

=1.6736×10−24gm1.66056×10−24gm

= 1.0078 amu

= 1.0080 amu

Similarly, the mass of oxygen - 16 (¹⁶O) atom would be 15.995 amu.

Today, 'amu' has been replaced by 'u' which is known as unified mass.

When we use atomic masses of elements in calculations, we actually use average atomic masses of elements which are explained.

Average Atomic Mass :

Many naturally occurring elements exist as more than one isotope. When we take into account the existence of these isotopes and their relative abundance (percent occurrence), the average atomic mass of that element can be computed.

Average Atomic Mass =Σ( Mass of Isotopes )i×( \%abundance )i100

In the periodic table of elements, the atomic masses mentioned for different elements actually represented their average atomic masses.

Molecular Mass :

Molecular mass is the sum of the atomic masses of the elements present in a molecule. It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together.

Formula Mass :

It may be noted that in sodium chloride, One Na⁺ is surrounded by six CI^- and vice-versa. A formula such as NaCI is used to calculate the formula mass instead of molecular mass as in the solid-state sodium chloride does not exist as a single entity.

Thus. formula mass of sodium chloride = atomic mass of sodium + atomic mass of chlorine

= 23.0 u + 35.5 u = 58.5 u

Dalton's Atomic Theory

Matter consists of indivisible atoms.
Atom is indivisible and cannot be broken down.
All the atoms of a given element have identical properties, including identical mass. Atoms of different elements differ in mass.
Compounds are formed when atoms of different elements combine in a fixed ratio.
Chemical reactions involve the reorganization of atoms. These are neither created nor destroyed in a chemical reaction.

Some Solved Examples

Example.1 What is the standard for the present system of atomic masses?

1)H - 1

2) (correct)C - 12

3)He - 4

4)O - 16

Solution

The present system of atomic mass is based on Carbon - 12 as the standard and has been agreed upon in 1961.

Hence, the answer is an option (2).

Example.2 What is the mass equivalent to 1 amu?

1) (correct)112th of C-12 isotope

2)116th of O-16 isotope

3)H-1 isotope

4)14th of the He atom

Solution

1 amu = 112 thof the mass of C-12 atom.

Hence, the answer is an option (1).

Example. 3 What is the mass (in u) of one Nitrogen atom in u if its mass in g is 2.32597×10−23 g?

1)14

2)15

3)13.89

4) (correct)14.007

Solution

As we learned in

Relation between amu and Gram -

¹²C is assigned a mass of exactly 12 atomic mass units (amu).

1 amu = 1.66056×10^–24 g

mass in u=2.32597×10−231.66056×10−24=14.007u

Hence, the answer is (14.007u).

Example.4 What is the molecular mass of H₂SO₄?

(response should be like 67 or 70)

1)96u

2)100u

3)102u

4) (correct)98u

Solution

The molecular formula of H₂SO₄:
2H = 2u, S =32u, 4O = 64u

Thus, Molar mass of H₂SO₄ = 2 + 32 + 64 = 98u

Hence, the answer is (98u).

Example.5 5 moles of AB2 weigh 125×10−3 kg and 10 moles of A₂B₂ weigh 300×10−3 kg The molar mass of A(M_A) and molar mass of B(M_B) in kgmol−1 are :

1)MA=50×10−3 and MB=25×10−3

2)MA=25×10−3 and MB=50×10−3

3) (correct)MA=5×10−3 and MB=10×10−3

4)MA=10×10−3 and MB=5×10−3

Solution

Let the atomic mass of A = x & atomic mass of B = y

∴ For AB2 :

5(x+2y)=125g⇒x+2y=25g⋯(I)

& For A2B2 :

10(2x+2y)=300g

⇒x+y=15g⋯⋯(II)

Solving equations (I) & (II), simultaneously, we get:

x=5×10−3 kg & y=10×10−3 kg

Thus, MA=5×10−3 kg

MB=10×10−3 kg

Hence, the answer is the option (3).

Example.6 Given the reaction :

2A+6B+3C+3D→Ax−yBxC3+ByDz

What are the values of x, y & z?

1)3, 2, 3

2)2, 4, 3

3)4, 2, 1

4) (correct)4, 2, 3

Solution

The number of moles of atoms of reactant is equal to the number of moles of atoms of the product.

So, Balancing A

x−y=2−(1)

In B,

x+y=6−(2)

In D,

z=3

from (1) & (2),

x=4

y=2

Hence, the answer is the option (4).

Summary

Atomic mass is referred to as the average mass of the elements and basic framework of the periodic table as it helps to classify and in identification of the elements in the periodic table and it is also essential for calculating the stoichiometry of the reactant and the product which determine their quantity and by the help of atomic mass, we can calculate the formula mass of any compound to understand their property.

Frequently Asked Questions (FAQs)

1. What is atomic mass and how is it different from mass number?

Atomic mass is the average mass of an atom of an element, taking into account all its naturally occurring isotopes. It's measured in atomic mass units (amu). Mass number, on the other hand, is the total number of protons and neutrons in the nucleus of a specific isotope. The key difference is that atomic mass is an average, while mass number is a whole number for a specific isotope.

2. Why is the atomic mass of most elements not a whole number?

The atomic mass of most elements is not a whole number because it represents an average of the masses of all naturally occurring isotopes of that element, weighted by their relative abundance. Since different isotopes have different numbers of neutrons, their masses vary slightly, resulting in a weighted average that is often not a whole number.

3. How does the concept of relative atomic mass relate to the atomic mass unit (amu)?

Relative atomic mass is a dimensionless quantity that expresses the average atomic mass of an element relative to 1/12th the mass of a carbon-12 atom. The atomic mass unit (amu) is defined as exactly 1/12th the mass of a carbon-12 atom. This means that the numerical value of an element's relative atomic mass is the same as its mass in amu, making conversions between the two straightforward.

4. What is the relationship between atomic mass and isotopes?

Atomic mass is directly related to isotopes. It is calculated as the weighted average of the masses of all naturally occurring isotopes of an element. The mass of each isotope is multiplied by its relative abundance, and these products are summed to give the atomic mass. This relationship explains why atomic masses are often not whole numbers and can change if new isotopes are discovered or if isotopic abundances are refined.

5. How does molecular mass differ from atomic mass?

Molecular mass is the sum of the atomic masses of all atoms in a molecule, while atomic mass refers to the mass of a single atom of an element. For example, the molecular mass of water (H2O) is the sum of the atomic masses of two hydrogen atoms and one oxygen atom. Molecular mass applies to molecules, while atomic mass applies to individual atoms.

6. What is the significance of the mass-energy equivalence principle in understanding atomic mass?

The mass-energy equivalence principle, expressed by Einstein's famous equation E=mc², is fundamental to understanding atomic mass. It explains why the mass of an atom is slightly less than the sum of its parts. The difference, called the mass defect, represents the energy released when the nucleus forms. This principle underlies nuclear processes and explains why certain nuclear reactions can release enormous amounts of energy.

7. What is the relationship between atomic mass and the strength of gravitational attraction between atoms?

The gravitational attraction between atoms is directly proportional to their masses, as described by Newton's law of universal gravitation. However, at the atomic scale, gravitational forces are extremely weak compared to electromagnetic and nuclear forces. The effect of atomic mass on gravitational attraction becomes significant only when dealing with enormous numbers of atoms, such as in astronomical bodies. In most chemical and physical processes at the molecular level, gravitational effects due to atomic mass are negligible.

8. How does the concept of atomic mass fraction help in understanding the composition of the universe?

Atomic mass fraction is a key concept in astrophysics and cosmology for understanding the composition of the universe. It represents the proportion of the total mass of a system (like a star, galaxy, or the entire universe) that is made up of a particular element. By analyzing spectral data and using our knowledge of atomic masses, scientists can determine the relative abundances of elements in celestial bodies. This information is crucial for understanding stellar evolution, galactic chemistry, and the overall composition and history of the universe.

9. Why is it important to use atomic mass rather than mass number in chemical calculations?

Using atomic mass in chemical calculations is crucial because it represents the average mass of atoms as they occur in nature. This average accounts for the different isotopes of an element and their natural abundances. Using mass numbers, which are whole numbers specific to individual isotopes, would lead to inaccurate results in calculations involving bulk quantities of elements or compounds.

10. How does the periodic table represent atomic masses?

The periodic table typically shows atomic masses below the element symbol. These values are weighted averages of the masses of all naturally occurring isotopes of each element, rounded to a certain number of decimal places. The atomic masses in the periodic table are essential for various chemical calculations and provide a quick reference for the relative masses of elements.

11. What is the formula for calculating molecular mass?

The formula for calculating molecular mass is:

12. How does the concept of molar mass relate to atomic and molecular mass?

Molar mass is numerically equal to the atomic or molecular mass, but it represents the mass of one mole of a substance. It's expressed in grams per mole (g/mol). For elements, the molar mass is equal to the atomic mass in g/mol. For compounds, it's equal to the molecular mass in g/mol. This concept bridges the gap between the microscopic world of atoms and molecules and the macroscopic world of measurable quantities.

13. Why is understanding atomic and molecular mass important in stoichiometry?

Understanding atomic and molecular mass is crucial in stoichiometry because these concepts form the basis for calculating the quantities of reactants and products in chemical reactions. They allow chemists to convert between the number of particles (atoms or molecules) and measurable masses, enabling precise predictions and measurements in chemical processes.

14. How do isotopes affect the calculation of atomic mass?

Isotopes significantly affect the calculation of atomic mass. Each isotope of an element has a different number of neutrons and thus a different mass. The atomic mass is calculated as a weighted average of these isotopic masses, with the weighting based on the natural abundance of each isotope. This is why elements with multiple naturally occurring isotopes often have atomic masses that are not whole numbers.

15. What is the significance of Avogadro's number in relation to atomic and molecular mass?

Avogadro's number (6.022 × 10^23) represents the number of particles in one mole of a substance. It provides a link between the microscopic world of atoms and molecules and the macroscopic world of measurable quantities. When we consider atomic or molecular mass in grams, Avogadro's number of those atoms or molecules will have a mass in grams numerically equal to the atomic or molecular mass in amu.

16. How does the concept of isotopic abundance influence atomic mass?

Isotopic abundance plays a crucial role in determining atomic mass. The atomic mass of an element is calculated by multiplying the mass of each isotope by its fractional abundance and then summing these products. For example, if an element has two isotopes with masses of 10 and 11 amu, occurring in a 3:1 ratio, the atomic mass would be ((3/4 × 10) + (1/4 × 11)) = 10.25 amu. This weighted average approach accounts for the natural occurrence of different isotopes.

17. Why is the atomic mass of carbon-12 significant in defining the atomic mass unit?

Carbon-12 is significant because it serves as the standard for defining the atomic mass unit (amu). One amu is defined as exactly 1/12th of the mass of a carbon-12 atom. This definition provides a consistent reference point for measuring the masses of other atoms and molecules. Carbon-12 was chosen due to its stability and the precision with which its mass could be measured.

18. How does the mass spectrometer contribute to our understanding of atomic and molecular masses?

Mass spectrometers play a crucial role in determining accurate atomic and molecular masses. They separate ions based on their mass-to-charge ratio, allowing precise measurement of the masses of different isotopes and molecules. This technology has enabled scientists to refine atomic mass values, discover new isotopes, and accurately determine the composition of complex molecules, greatly enhancing our understanding of atomic and molecular structure.

19. What is the difference between average atomic mass and exact atomic mass?

Average atomic mass is the weighted mean of the masses of all naturally occurring isotopes of an element, considering their relative abundances. Exact atomic mass, on the other hand, refers to the mass of a specific isotope of an element. For example, the average atomic mass of chlorine is about 35.45 amu, while the exact atomic mass of chlorine-35 is 34.97 amu and that of chlorine-37 is 36.97 amu.

20. How does the concept of atomic mass fraction relate to molecular formulas?

Atomic mass fraction represents the proportion of a compound's total mass contributed by a particular element. It's calculated by dividing the total mass of an element in a compound by the compound's molecular mass. This concept is useful in determining empirical and molecular formulas, as it helps in understanding the relative contributions of different elements to a compound's overall composition.

21. Why is it important to consider significant figures when working with atomic and molecular masses?

Significant figures are crucial when working with atomic and molecular masses because they indicate the precision of the measurement or calculation. The number of significant figures in atomic masses reflects the uncertainty in isotopic compositions and measurements. When performing calculations involving these masses, it's important to carry through the appropriate number of significant figures to ensure the final result accurately represents the precision of the data used.

22. How does the concept of molecular mass apply to ionic compounds?

For ionic compounds, we use the term formula mass instead of molecular mass, as these compounds don't form discrete molecules. The formula mass is calculated similarly to molecular mass – by summing the atomic masses of all atoms in the formula unit of the ionic compound. For example, the formula mass of NaCl is the sum of the atomic masses of one sodium atom and one chlorine atom.

23. What role does atomic mass play in the arrangement of elements in the periodic table?

Atomic mass played a crucial role in the historical development of the periodic table. While elements are now primarily arranged by atomic number (number of protons), atomic mass still influences their positioning. In most cases, elements with higher atomic numbers also have higher atomic masses. However, there are a few exceptions (like tellurium and iodine) where the order based on atomic mass differs from that based on atomic number, reflecting the complex interplay of nuclear structure and stability.

24. How does the concept of atomic mass relate to nuclear binding energy?

Atomic mass is closely related to nuclear binding energy. The actual mass of an atom is slightly less than the sum of the masses of its constituent protons, neutrons, and electrons. This mass difference, known as the mass defect, is converted to binding energy according to Einstein's equation E=mc². Elements with higher binding energies per nucleon tend to have atomic masses that deviate more from whole numbers, reflecting the greater stability of their nuclei.

25. Why is it necessary to use relative atomic masses in chemical calculations rather than absolute masses?

Relative atomic masses are used in chemical calculations because they provide a convenient and practical way to compare the masses of different atoms. Using absolute masses (in kilograms, for example) would involve extremely small numbers that are difficult to work with. Relative atomic masses, being dimensionless ratios, simplify calculations and allow for easy comparison of atomic and molecular quantities across different elements and compounds.

26. How does the discovery of new isotopes affect the atomic mass of an element?

The discovery of new isotopes can potentially affect the atomic mass of an element, especially if the new isotope is found to occur naturally and in significant abundance. The atomic mass would need to be recalculated to include the mass and abundance of the new isotope. However, for most elements, the discovery of new isotopes is unlikely to significantly change the atomic mass, as the most abundant isotopes are typically already known and accounted for.

27. What is the relationship between atomic mass and nuclear stability?

There's a complex relationship between atomic mass and nuclear stability. Generally, nuclei with certain numbers of protons and neutrons (called "magic numbers") are more stable and have higher binding energies per nucleon. This stability is reflected in their atomic masses, which tend to deviate less from whole numbers. Elements with atomic masses farther from these stable configurations are often less stable and may be radioactive.

28. How does the concept of atomic mass apply to artificially created elements?

For artificially created elements, especially those that are very short-lived, the concept of atomic mass is applied differently. Since these elements don't exist in nature and often have very few atoms created at a time, there's no natural isotopic abundance to average. Instead, the mass of the most stable or most easily produced isotope is often used. These masses are typically whole numbers (the mass number) until more precise measurements can be made.

29. How does isotopic composition vary between terrestrial and extraterrestrial samples, and how does this affect atomic mass calculations?

Isotopic composition can vary between terrestrial and extraterrestrial samples due to different formation conditions and histories. For instance, some meteorites have different isotopic ratios compared to Earth rocks. This variation can lead to slightly different atomic masses for elements depending on their source. When calculating atomic masses, scientists typically use terrestrial samples as the standard, but understanding these variations is crucial for fields like cosmochemistry and planetary science.

30. What is the concept of "atomic weight" and how does it differ from atomic mass?

"Atomic weight" is an older term that is often used interchangeably with atomic mass, but there is a subtle difference. Atomic mass specifically refers to the mass of an atom, usually expressed in atomic mass units (amu). Atomic weight, historically, was a dimensionless quantity expressing how many times heavier an atom is than 1/12th of a carbon-12 atom. In modern usage, the term "relative atomic mass" is preferred over "atomic weight" to avoid confusion with weight as a force.

31. How do atomic and molecular masses relate to the concept of density?

Atomic and molecular masses are directly related to density. Density is mass per unit volume, and the mass component of this ratio is determined by the atomic or molecular mass of the substance. Generally, elements or compounds with higher atomic or molecular masses tend to have higher densities, although the arrangement of atoms or molecules (crystal structure in solids, intermolecular forces in liquids) also plays a crucial role in determining the overall density.

32. What is the importance of understanding atomic and molecular mass in the field of spectroscopy?

Understanding atomic and molecular mass is crucial in spectroscopy for several reasons. Mass spectrometry, a type of spectroscopy, directly measures mass-to-charge ratios, allowing for the identification of elements and molecules. In other forms of spectroscopy, like infrared or Raman spectroscopy, the masses of atoms affect the vibrational frequencies of molecules, influencing the observed spectra. Knowledge of atomic and molecular masses helps in interpreting these spectra and identifying unknown compounds.

33. How does the concept of atomic mass apply to isotopic labeling in scientific research?

Isotopic labeling involves replacing specific atoms in a molecule with their isotopes. This technique relies on the difference in atomic mass between isotopes. For example, replacing hydrogen with deuterium (heavy hydrogen) in a molecule slightly increases its mass. This mass difference can be detected using mass spectrometry, allowing researchers to track specific atoms or molecules in biological systems or chemical reactions. Understanding atomic mass is crucial for designing and interpreting these experiments.

34. How does the concept of reduced mass, which involves atomic masses, apply to molecular vibrations?

Reduced mass is an important concept in understanding molecular vibrations. It's calculated using the masses of the atoms involved in the vibration. For a diatomic molecule, the reduced mass μ is given by μ = (m1*m2)/(m1+m2), where m1 and m2 are the masses of the two atoms. This concept allows for the simplification of the two-body problem into a one-body problem, making it easier to calculate vibrational frequencies. Understanding atomic masses is crucial for accurately determining reduced masses and, consequently, predicting and interpreting molecular vibrational spectra.

35. Why is it important to distinguish between atomic mass and isotopic mass in nuclear chemistry?

Distinguishing between atomic mass and isotopic mass is crucial in nuclear chemistry. Atomic mass is an average value considering all naturally occurring isotopes of an element, while isotopic mass refers to the mass of a specific isotope. In nuclear reactions, it's often necessary to know the exact mass of the specific isotopes involved, as these can behave differently in reactions. Using the average atomic mass instead of the specific isotopic mass could lead to significant errors in calculations of reaction energies and products in nuclear processes.

36. What role do atomic and molecular masses play in the field of nanotechnology?

In nanotechnology, atomic and molecular masses are crucial for several reasons. They affect the properties of nanomaterials, including their reactivity, electrical and thermal conductivity, and optical properties. The mass of atoms and molecules influences how they behave at the nanoscale, affecting phenomena like quantum confinement. Additionally, precise knowledge of masses is essential for designing and synthesizing nanomaterials with specific properties, as well as for characterizing these materials using techniques like mass spectrometry.

37. How does the concept of atomic mass relate to the field of isotope geochemistry?

Isotope geochemistry heavily relies on the concept of atomic mass. Different isotopes of an element have slightly different masses, which can lead to fractionation (separation) during