Lakhmir Singh Chemistry Class 9 Solutions Chapter 3 - Atoms and Molecules

Law of Chemical Combination

Atoms and molecules are functional and structural units of our world. Atoms and molecules define the physical and chemical properties of elements. This chapter covers the fundamentals of atoms and molecules.Chemical Reaction- this law defines products and reactants. The molecules that interact with one another to form a new compound are known as reactants, while the final compounds produced are known as products. 

1. Law of Conservation of mass- this law states how mass can neither be created nor destroyed, it can just be converted from one form to another. The amount of energy present at a specific time in the Universe remains the same. This means the mass of reactants will be equal to the mass of products.

2. Law of constant proportions- this law states that the proportions of each element in a pure chemical compound remain the same no matter the amount of state of the compound. 

Dalton’s Atomic Theory

This theory defines atoms as invisible particles that make up matter. The properties of all the atoms in a specific element are the same. The atoms of a specific element have the same chemical and physical properties but atoms of different elements have different chemical and physical properties. Different elements combine to form different molecules and different compounds. Atoms are invisible. Atoms cannot be created or destroyed. Any new compound is formed by the rearrangements of the existing elements.

Atomic Mass and Atomic Mass Unit

The atomic mass is equal to the summation of protons and neutrons present in an atom. The mass of a single atomic particle is known as atomic mass, it is expressed as a unified atomic mass unit(AMU) by international standards. 

Molecular mass, Mole concept, and Avogadro Number

The molecular mass of a molecule is the summation of the masses of elements present in it. Mole Concept is the most standard way used to express the amount of each element, compound, or molecule used or present in a chemical reaction. Avogadro number is the number used to represent the number of molecules present in one mole.  6.022×1023 molecules are the Avogadro number.