Atomic and molecular masses

1.0Atomic Mass

Atoms are extremely small particles, making their absolute masses challenging to measure. However, it is possible to determine their relative masses using a reference standard. Historically, the atomic mass unit was defined based on:

• The mass of one hydrogen atom (taken as unity).
• Later, 1/16th of the mass of an oxygen atom was used as a reference.
• Currently, the standard is 1/12th of the mass of a carbon-12 (C-12) atom.

The atomic mass of an element represents how many times the mass of one atom of the element is greater than the mass of one hydrogen atom.

2.0Atomic Mass Unit (AMU)

The atomic mass unit (amu) is defined as 1/12th of the mass of a carbon-12 atom. This unit is used to express atomic and molecular masses.

$1\text{ amu}=\frac{\text{Mass of one C-12 atom}}{12}$

Mathematically, it is expressed as:

$\text{Atomic mass of an element}=\frac{\text{Mass of one atom of the element}}{\frac{1}{12}\text{ of the mass of a C-12 atom}}$

Relative Atomic Mass

Atoms are extremely small, making their absolute masses challenging to measure. An element's relative atomic mass represents its mass in relation to its number of atoms. This scale helps compare the masses of different atoms.

However, their relative masses can be measured using a standard reference unit.

Historically, the atomic mass standard was:

• Initially, the mass of one hydrogen atom (taken as unity).
• Later, 1/16th of the mass of an oxygen atom was used.
• Currently, the standard is 1/12th of the mass of a carbon-12 (C-12) atom.

Gram Atomic Mass

The gram atomic mass of an element is its atomic mass expressed in grams.

For example, the atomic mass of an oxygen atom is 16 amu, so its gram atomic mass is 16 g.

3.0What is a Molecule?

A molecule is the smallest particle of matter that can exist independently while retaining the substance's chemical properties. It consists of two or more atoms that are chemically bonded together.

Molecules are classified into two types based on the nature of the atoms they contain:

• Homoatomic Molecules: These molecules consist of only one type of atom, meaning they are made up of a single element.
  Examples: H₂, O₂, N₂, S₈
• Heteroatomic Molecules: These molecules consist of atoms from different elements, forming compounds.
  Examples: NH₃, H₂O, CH₄

4.0Molecular Mass

The molecular mass of a compound is calculated by adding the atomic masses of the atoms that compose it.

Molecular mass 0is calculated by:

(Atomic Mass of Element×Number of Atoms in Molecule)

For example, in the case of water (H₂O):

• Hydrogen (H) has an atomic mass of 1 unit
• Oxygen (O) has an atomic mass of 16 units
  Thus, the molecular mass of water is:
  (2 × 1) + (1 × 16) = 18 units

5.0Methods to Determine Molecular Mass

1. Mass Spectrometry – Used for small compounds, it determines molecular mass by analysing ionized particles. Monoisotopic mass is often used for accuracy.
2. Hydrodynamic Methods (Houwink Relations) – This technique calculates molecular mass based on fluid dynamics and is used in polymer studies.
3. Static Light Scattering (Zimm Method) – Determines molecular weight by analyzing scattered light intensity.
4. Mass Photometry – A label-free technique that measures the molecular mass of biomolecules in solution. It uses interferometric scattered light microscopy, with mass being proportional to light contrast. This method helps analyze protein interactions, macromolecular assemblies (e.g., ribosomes), and oligomerization status.

Gram Molecular Mass

The gram molecular mass (or molar mass) is the molecular mass expressed in grams.

Example:

• Oxygen (O₂) has a molecular mass of 32 u
• Therefore, its gram molecular mass is 32 grams per mole

Formula Mass

Some compounds, such as sodium chloride (NaCl), do not consist of discrete molecules but rather form a three-dimensional lattice of ions. For such ionic compounds, we use formula mass instead of molecular mass.

Example: Sodium chloride (NaCl)

• Sodium (Na) atomic mass = 23.0 u
• Chlorine (Cl) atomic mass = 35.5 u

Thus, the formula mass of NaCl = 23.0 + 35.5 = 58.5 u