Molecules can easily consist of atoms from the exact same chemical element. For example, the oxygen gas we breathe consists of two oxygen atoms securely bonded together. Molecules can also consist of completely different chemical elements joined together.
Water is a famous example because it chemically combines hydrogen and oxygen atoms. The atoms inside a molecule share their outer electrons through strong connections called covalent bonds. This tight electron sharing keeps the atoms securely joined together in a specific geometric arrangement.
Breaking these strong covalent bonds requires a massive amount of inputted chemical energy. Students often struggle to distinguish true molecules from simple ions or crystal lattices. Ions carry a net electrical charge because they have either lost or gained electrons.
By strict chemical definition, molecules are always completely electrically neutral. Additionally, large ionic crystal structures like table salt are never considered molecules. They form continuous, repeating networks of charged particles rather than independent, discrete units.
However, biologists sometimes use the word molecule a bit more loosely in their daily work. They might describe massive, charged biochemical structures like DNA or folded proteins as molecules. In strict chemistry terms, a true molecule must feature covalent bonds and maintain zero overall charge.