From atoms to a molecule

Discrete means a separate unit. Two hydrogen atoms sharing an electron pair form one hydrogen molecule, $H_2$. There are still two atoms, but they now belong to one bonded particle. Two unbonded hydrogen atoms would be two separate particles, not a molecule.

[DIAGRAM: asset_name: covalent_bonding_to_molecules - diagram 1; asset_slug: 035_1_29_covalent_bonding_results_in_the_formation_of_molecules_diagram1; file: 035_1_29_covalent_bonding_results_in_the_formation_of_molecules_diagram1.png; recommended_method: deterministic_drawn; description: Title: Covalent bonding forms molecules. Deterministic monochrome teaching diagram. Shows separate atoms becoming one molecule by covalent bonding, then two examples: an element molecule labelled hydrogen molecule, H2, and a compound molecule labelled water molecule, H2O. Uses simple atom circles and bond lines only, no dot-and-cross electrons. Students should learn that covalent bonding joins atoms into molecules and that molecules may contain one element or more than one element.]

Covalent bonds join the atoms within each molecule. For example, a sample containing three separate water molecules contains three particles, each with its own bonded group of atoms. It is not one nine-atom molecule.

This lesson concerns simple molecular substances. Covalent bonding can also form giant networks, such as diamond, which have no separate small molecules; those structures are studied later.

Molecules of elements and compounds

A molecule can contain atoms of the same element or atoms of different elements. Its formula tells you the atoms in one molecule: a subscript applies to the element just before it, and no subscript means one atom.

A compound contains different elements chemically bonded together. Oxygen is a molecule but not a compound because both its atoms are oxygen. Water is both molecular and a compound. Conversely, not every compound consists of molecules: an ionic compound such as sodium chloride forms a lattice of ions.