## Atomic and Molecular Masses – Class 11 | Chapter – 2 | Physics Short Notes Series PDF for NEET & JEE

Atomic and Molecular Masses: Atomic and molecular masses are measures of the mass of atoms and molecules, respectively. The mass of an atom or molecule is determined by the sum of the masses of its constituent particles, including protons, neutrons, and electrons.

## Atomic and Molecular Masses

Atomic mass is usually expressed in atomic mass units (amu) or its multiples, such as kiloatomic mass units (kamu) and megatomic mass units (Mamu). The atomic mass of an element is the average mass of all the isotopes of that element, taking into account their relative abundances. Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei, and therefore have different atomic masses.

Molecular mass is the sum of the atomic masses of all the atoms in a molecule. Molecular mass is usually expressed in atomic mass units (amu) or its multiples, such as kiloatomic mass units (kamu) and megatomic mass units (Mamu). The molecular mass of a compound is equal to the sum of the atomic masses of all the atoms in the compound.

## Importance of Atomic and Molecular Masses

Atomic and molecular masses are important concepts in chemistry and physics because they are used to understand and predict the behavior of atoms and molecules in a wide variety of contexts. Some of the key applications of atomic and molecular masses include:

• Stoichiometry: Atomic and molecular masses are essential in the study of stoichiometry, which is the calculation of the amounts of reactants and products in chemical reactions. By knowing the atomic and molecular masses of the substances involved in a chemical reaction, scientists can determine the amount of each substance that is required to produce a given amount of product.
• Analysis of chemical compounds: The atomic and molecular masses of chemical compounds are used in analytical chemistry to determine the composition of a sample. For example, mass spectrometry is a technique that uses the atomic and molecular masses of ions to identify and quantify the chemical species in a sample.
• Prediction of physical and chemical properties: The atomic and molecular masses of atoms and molecules are used to predict their physical and chemical properties. For example, the boiling point and melting point of a substance are influenced by its molecular mass and the strength of the intermolecular forces between its molecules.
• Development of new materials: Understanding the atomic and molecular masses of different elements and compounds is essential in the development of new materials with specific properties. For example, the development of new catalysts for chemical reactions relies on a deep understanding of the atomic and molecular structures of the compounds involved.

## Atomic Mass of Different Elements

The Atomic Masses of the first 10 elements of the periodic table are:

Atomic Number Element Atomic Mass
1 Hydrogen 1.008 amu
2 Helium 4.0026 amu
3 Lithium 6.94 amu
4 Beryllium 9.0122 amu
5 Boron 10.81 amu
6 Carbon 12.011 amu
7 Nitrogen 14.007 amu
8 Oxygen 15.999 amu
9 Fluorine 18.998 amu
10 Neon 20.18 amu

## Molecular Mass Of Different Compounds

The Molecular Mass of Different Compounds are given as follows-:

Compound Formula Molecular Weight
Ammonia NH3 17.031 amu
Carbon monoxide CO 28.010 amu
Chlorine Cl2 70.905 amu
Nitrogen dioxide NO2 46.006 amu
Hydrogen Peroxide H2O2 34.015 amu
Sulfur dioxide SO2 64.065 amu
Ozone O3 47.998 amu
Carbon tetrachloride CCl4 153.822 amu
Benzene C6H6 78.114 amu
Ethylene C2H4

28.054 amu

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