Introduction to Atoms and Molecules

• Atoms are the fundamental building blocks of matter.

• Molecules are formed when two or more atoms chemically bond together.

• Atoms and molecules participate in chemical reactions while following specific laws.

Laws of Chemical Combination

The combination of elements follows definite laws, as discovered by early chemists. The two fundamental laws are:

1. Law of Conservation of Mass

“Mass can neither be created nor destroyed in a chemical reaction.”

Explanation:

• The total mass of reactants before the reaction is equal to the total mass of products after the reaction.

• This law was established by Antoine Lavoisier.

Diagram: Experimental Setup for Verification of Law of Conservation of Mass
(A sealed container with reactants undergoing a reaction, showing mass remains unchanged before and after the reaction.)

2. Law of Constant Proportions (Law of Definite Proportions)

“A given compound always contains the same elements in the same fixed proportion by mass, regardless of the source or method of preparation.”

Explanation:

• Water (H₂O) always contains hydrogen and oxygen in a fixed mass ratio of 1:8 (Hydrogen:Oxygen).

• This law was given by Joseph Proust.

Diagram: Example of Water Composition (H₂O)
(A representation showing H₂O molecules with two hydrogen atoms and one oxygen atom, illustrating the constant proportion of elements.)