What constitutes a molecule?

A molecule is defined as a group of two or more atoms that are chemically bonded together. This means that the correct answer highlights the fundamental aspect of a molecule, which can consist of atoms of the same element, such as O₂ (oxygen gas), or different elements, such as H₂O (water). The key characteristic is that these atoms are held together by covalent bonds, where they share electrons.

The other choices do not accurately represent what constitutes a molecule. A group of different elements (the first option) describes a compound, which is a specific type of molecule made from different elements. Furthermore, while a collection of protons and neutrons (the fourth option) refers to the components of atomic nuclei, it does not pertain to the concept of molecules, which involve bonded atoms rather than just nuclear particles. Lastly, saying that a molecule consists of two or more electrons (the third option) is incorrect as electrons are subatomic particles involved in bonding but do not constitute a molecule by themselves.