What are molecules? Give an account of the arrangement of the constituent atoms in the molecules.

A molecule is the smallest-particle of an element or of a compound which is stable under ordinary conditions and can exist freely and shows all the properties of that element or compound.
A molecule may be made up of one, two or more atoms. Molecules with only one atom are called monoatomic, e.g., helium, argon, neon.
Molecules with two atoms are diatomic. These may have similar or different atoms, e.g., chlorine (Cl₂), oxygen (O₂), hydrochloric acid gas (HCl). Similarly, there are molecules containing three atoms (H₂O, CO₂), four atoms (P₄, NH₃) and so on.
Arrangement of atoms in molecules:
The atoms in a molecule are chemically bonded in a definite pattern. This arrangement of atoms is given by the structural formula of the compound. For example, structural formula for water is H—O—H. This is a simple illustration for the structure of water. In fact water and all other molecules exhibit definite spatial arrangements depending on the nature of chemical bonding between atoms.
Fig. 3.3  shows molecular model for water, ammonia and sulphur.