Atomic Mass And Molecular Mass

What is Atomic Mass?

Every particle of matter has some amount of mass associated with it whether small or large. Everything is made up of atoms. The mass of an atomic particle is called the atomic mass. This is commonly expressed as per the international agreement in terms of a unified atomic mass unit (amu).

The atomic mass of an atom is a feature that is closely related to its mass number. A single atom’s atomic mass is just its total mass, and it is commonly given in atomic mass units or amu. Carbon-12, an atom of carbon-containing six neutrons, has an atomic mass of 12 amu. Other atoms typically do not have round-number atomic masses. However, in general, an atom’s atomic mass will be very close to its mass number, with some variation in the decimal places.

Table of Contents

• Atomic Mass
• Atomic Mass of Elements
• What is Molecular Mass?
• Key Points
• Solved Example
• Frequently Asked Questions – FAQs

Atomic Mass

It can be best defined as 1/12 of the mass of a carbon-12 atom in its ground state. The mass of an atom can be accounted for by the sum of the mass of protons and neutrons which is almost equal to the atomic mass. This small change is due to the binding energy mass loss.

1 amu = 1.66 ×10⁻²⁴ g

• The atomic weight of an atom is a dimensionless number when it is divided by unified atomic weight or Daltons.
• This is called the relative isotopic mass.
• The atomic masses of elements vary from 1.008 amu for hydrogen up to 250 amu for elements which have a very high atomic number.
• The mass of molecules can be determined by adding the average atomic mass of each atom in the molecule.

Atomic Mass of Elements

The Atomic mass of some elements is tabulated below.

What is Molecular Mass?

The molecular mass of a compound is the sum of the atomic masses of the atoms that form the compound.

Molecular mass is the amount of mass associated with a molecule. It is also called molecular weight. It can be calculated by adding the mass of each atom multiplied by the number of atoms of the element present in the molecule. For example, water is made up of 2 hydrogen atoms and 1 oxygen atom. The mass of a water molecule is equal to the average atomic mass of hydrogen multiplied by two plus the atomic mass of oxygen. The molecular mass of elements depends upon the constituent atoms of the molecule.

The molecular weight of molecules can be determined by the following methods:

• Mass spectrometry: This method is generally used in determining the mass of small molecules. This is reported as a monoisotopic mass.
• Hydrodynamic method: The weight is determined as per Mark-Houwink relations. This method requires calibration; hence it is also described as a relative molecular weight determination method.
• Static Light Scattering: Molecular weight is determined from the amount of light scattered using the Zimm method.

Key Points

1. All matter is made up of atoms.
2. Elements are substances made up of only one type of atom.
3. Atoms have a central nucleus which contains protons and neutrons.
4. Electrons move in orbits around the nucleus.
5. Protons have a relative mass of 1 and a positive charge.
6. Neutrons have a relative mass of 1 and are neutral.
7. Electrons have negligible mass and a negative charge.

Recommended Videos

Introduction to Atomic and Molecular Weight

Solved Example

Question:
What is the molar mass of sulphur trioxide SO₃, a molecular compound?

By definition, the molar mass of SO₃ equals its molecular mass in grams. So the calculation of the molecular mass of SO₃ is required first.

Solution:
The molecular mass (or formula mass) of a compound is the sum of the atomic masses of all the atoms in the molecule. The molecular mass of SO₃ with 4 atoms in its molecule, is
1S = 1 x 32.07 amu = 32.07amu
3O = 3 x 16.00 amu = 48.00amu
The molecular mass of SO₃ = 80.07 amu

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