Editing Hadean Eon (section)

=== Formation of Molecular Hydrogen ===
The vast majority of the contents of the interstellar medium is gaseous '''H'''2''',''' a '''molecule''' - a structure consisting of more than one atom - in this case, two '''hydrogen atoms''' bound together ('''H-H''').  Molecular hydrogen forms when two hydrogen atoms share their '''electrons''' through a '''covalent bond''', a <u>chemical</u> process that occurs at the low energies typical in cold, dense regions of the interstellar medium. The resulting molecule consists of two hydrogen nuclei ('''protons''') and two electrons. It's the most basic molecular structure possible and is a stable, neutral molecule. In contrast with a <u>nuclear</u> process (for example, the creation of '''helium'''), within the hydrogen molecule each nucleus/electron pair retains its identity as a separate hydrogen atom; they are simply bonded together. The protons (nuclei) do not fuse into a different element with a heavier nucleus, as they would in a higher-energy <u>nuclear</u> reaction.

Electrons do not have fixed '''orbits''' in the way planets orbit a star; their positions are described by a '''probability distribution''' within '''electron clouds'''. When two hydrogen atoms come close together, their electron clouds merge to form a region of shared electron density (the covalent bond, when the <u>1s orbitals</u> of each '''hydrogen''' atom overlap). This combination of electron density in the space between the nuclei holds the atoms together into a molecule of hydrogen gas. 

The 'stability' of molecular hydrogen is due to the fact that both hydrogen atoms achieve a full '''valence shell''' (for hydrogen: two electrons). Most atoms have a maximum of eight electrons that can occupy their outermost (valence) shell, and they generally seek to stabilize by 'filling' that shell with electrons. For most elements this is called the <u>octet rule</u> - but hydrogen has its own analog, the <u>duplet rule</u>, as its valence shell is naturally more compact, and can hold a maximum of two '''valence electrons'''. The behavior of an atom, particularly its ability to form chemical bonds with other atoms, is largely determined by the number of electrons in this outer shell. Hydrogen is the simplest element, with just one proton and one electron; its electron resides in the <u>1s orbital</u>, the closest orbital to the nucleus. Because hydrogen is in the first row of the periodic table, its <u>1s orbital</u> can hold a maximum of two electrons. By sharing its single electron with another atom that also needs additional electrons to complete its own valence shell, hydrogen can effectively achieve a stable electronic arrangement resembling that of the '''noble gases''', which are naturally stable and nonreactive due to their full valence shells.

The formation of '''H'''2 from two hydrogen atoms is energetically favorable and '''exothermic''' (it releases energy). However, in space, there's a challenge: <u>conservation of momentum</u> and <u>conservation of energy</u>. The newly formed H2 molecule must find a way to release the excess energy from the bond formation to become stable; on <u>Earth</u>, this energy would be dissipated quickly into the surrounding environment which is rich with atoms, but in the low-density environment of space, there's often nothing nearby to absorb it. Without a third body to take away the energy, the two '''hydrogen''' atoms would simply bounce off each other without forming a stable molecule (this is why dust often acts as a suitable 'surface' upon which these reactions can occur).

When a chemical bond forms, such as an H-H bond in an H2 molecule, energy is released because the system goes from a higher '''energy state''' (separate hydrogen atoms) to a lower energy state (the '''bonded''' molecule). This is a fundamental principle of chemistry - systems tend to move toward lower energy states, which are more stable. Separated hydrogen atoms have '''potential energy''' due to their mutual '''electrostatic''' potential and the potential of their electrons in the field of another nucleus. When they come close enough to share their electrons and form a covalent bond, they create a molecule with lower potential energy than the sum of the two atoms taken separately.

The bond formation process releases energy, primarily in the form of '''heat''', which can be absorbed by a dust grain, causing it to vibrate more intensely. This process not only allows the dust to act as a '''heat sink''' but also to catalyze the formation of molecular hydrogen. As these interactions occur on a vast scale, they collectively facilitate the formation and thermal regulation of molecular clouds, rich in molecular hydrogen and dust, precursors to star formation. The energy transfer to dust grains and subsequent infrared radiation that results contribute to the cloud's '''thermal balance''', essential for its evolution and the birth of new stars. This dynamic interplay of physical and chemical processes underpins the formation of molecular clouds in the interstellar medium, marking them as fertile grounds for the complex chemistry that leads to star and planet formation.