What is a molecule?

A molecule is defined as a group of two or more atoms that are chemically bonded together, specifically through covalent bonds. In this context, atoms can be of the same element, such as in an oxygen molecule (O₂), or of different elements, like in a water molecule (H₂O). The bonds formed between these atoms result in a stable structure that exhibits distinct chemical properties, which is fundamental to the formation of compounds and substances in chemistry.

The other options provide definitions that do not fully encompass the characteristics of a molecule. A single atom represents only one part of a chemical element, while an independent chemical species can refer to various forms of particles including ions, not exclusively molecules. Lastly, describing a molecule as a large structure like a compound overlooks that many molecules can be small and still be classified as such, regardless of their size in comparison to larger compounds. Thus, the essence of a molecule lies specifically in its formation of atoms held together by covalent bonds.