Hydrogen is the most abundant chemical element in the universe, making up approximately 75% of all normal matter. Despite its prevalence, it is rarely found as a solitary atom in its natural, uncombined state on Earth. Instead, under typical conditions, hydrogen exists as a diatomic molecule, represented by the chemical formula H2.
The Unstable Hydrogen Atom
A single hydrogen atom is the simplest of all atoms, consisting of one positively charged proton in its nucleus and one negatively charged electron orbiting it. This electron occupies the innermost electron shell. Atoms strive for a stable electron configuration, usually by having a full outer electron shell. For hydrogen, this stability is achieved by having two electrons in its outermost shell, known as the “duet rule”.
A lone hydrogen atom, with its single electron, does not satisfy this duet rule and possesses an incomplete outer shell. This makes the individual hydrogen atom highly reactive and inherently unstable. Such an atom readily seeks a more stable state by interacting with other atoms.
The Path to Stability: Covalent Bonding
Two unstable hydrogen atoms overcome their instability by forming a chemical bond. They achieve this stability through covalent bonding.
In a covalent bond, atoms share electrons rather than transferring them. When two hydrogen atoms approach, their single electrons are shared. This sharing creates a single covalent bond, often represented as H-H, forming the H2 molecule.
The shared pair of electrons effectively orbits both hydrogen nuclei, allowing each hydrogen atom to complete its electron duet. This electron sharing significantly lowers the overall energy of the system, resulting in a stable H2 molecule. The formation of H2 from two reactive hydrogen atoms is an exothermic process, meaning it releases energy.
Characteristics of H2 Gas
The formation of the H2 molecule dictates the physical and chemical properties of hydrogen gas. Molecular hydrogen is a colorless, odorless, and tasteless gas. It is highly flammable and has the lowest density of all gases, making it significantly lighter than air.
The strong covalent bond within each H2 molecule contributes to hydrogen’s inertness at room temperature; a considerable amount of energy is required to break this bond. H2 gas finds various applications. It is explored as a clean-burning fuel in fuel cells and combustion engines, used in the production of ammonia for fertilizers, and for hydrogenating oils in industrial processes.