Hydrogen, the lightest and most abundant element in the universe, exists in three distinct states of electrical charge: positive, negative, or neutral. This adaptability stems from the hydrogen atom’s simple structure, which consists of just one positively charged proton in its nucleus and one orbiting, negatively charged electron. Unlike most elements that tend to consistently gain or lose electrons, hydrogen’s chemical behavior depends entirely on the identity of the atom or molecule it bonds with. This flexibility allows it to participate in a vast array of chemical reactions.
Neutral Hydrogen Gas
The uncharged, neutral form of hydrogen is most commonly encountered as a diatomic gas, represented by the chemical formula \(\text{H}_2\). In this state, two hydrogen atoms bond together covalently, forming a molecule that is colorless, odorless, and highly flammable. The two atoms share their single electrons equally to complete their outer electron shell. This equal sharing results in a perfectly nonpolar bond, meaning the electrons are distributed symmetrically between the two nuclei. Consequently, the \(\text{H}_2\) molecule has no overall net electrical charge, maintaining its neutrality.
When Hydrogen Becomes Positive
Hydrogen takes on a positive charge when it loses its single electron, forming a cation known as the hydrogen ion, represented as \(\text{H}^+\). Since the common hydrogen isotope (protium) has no neutrons, the resulting \(\text{H}^+\) ion is simply a bare proton. This loss typically occurs when hydrogen is bonded to a highly electronegative element, such as oxygen in water (\(\text{H}_2\text{O}\)) or fluorine in hydrofluoric acid (\(\text{HF}\)). The hydrogen atom has an oxidation state of +1 in this positive form.
The positive ion is a fundamental component of acid-base chemistry, defining an acid. In water, the isolated proton is extremely reactive and immediately attaches itself to a water molecule to form the hydronium ion, \(\text{H}_3\text{O}^+\). The concentration of \(\text{H}_3\text{O}^+\) is measured on the pH scale, where a higher concentration corresponds to greater acidity. In biological systems, the movement of these protons across membranes is a primary mechanism for energy production, particularly in cellular respiration.
When Hydrogen Becomes Negative
Hydrogen can become negatively charged by gaining an extra electron, forming the hydride ion, represented as \(\text{H}^-\). The hydride ion is a hydrogen atom with two electrons orbiting its single proton, giving it a net charge of \(-1\). This state occurs when hydrogen reacts with elements significantly less electronegative than itself, primarily alkali metals and alkaline earth metals.
In compounds like sodium hydride (\(\text{NaH}\)) or calcium hydride (\(\text{CaH}_2\)), the metal atom readily gives up its electron to the hydrogen atom, creating an ionic compound. The hydride ion acts as a strong reducing agent in chemical reactions and is used in synthetic chemistry. However, the \(\text{H}^-\) ion is much less common and highly reactive in everyday environments like water. Hydrogen has an oxidation state of \(-1\) in the hydride ion.