What is a molecule defined as?

A molecule is defined as a chemical combination of two or more atoms. This definition reflects the concept that molecules are formed when atoms bond together through chemical bonds, which can be covalent, ionic, or metallic in nature. Molecules can consist of two or more atoms of the same element, as seen in diatomic molecules like O2 (oxygen) or H2 (hydrogen), or they can be made up of different elements, such as H2O (water) which contains both hydrogen and oxygen atoms.

Understanding the structure of molecules is fundamental in chemistry, as they are the smallest units that retain the chemical properties of a substance. Molecules can vary significantly in size and complexity, ranging from simple diatomic molecules to large organic compounds composed of hundreds of atoms.

The other choices do not accurately define a molecule. A single atom describes just one element without bonds to other elements. A collection of electrons refers to subatomic particles that do not constitute a molecule by themselves. A solid form of a chemical element describes a particular state of matter but does not encompass the broader definition of a molecule, which includes various combinations of atoms in different states.