Hydrogen, the first element on the periodic table, possesses a unique atomic structure that dictates its variable electrical charge. It contains a single, positively charged proton in its nucleus and a single, negatively charged electron orbiting it. This simple composition means hydrogen’s charge state is highly dependent on the atoms it interacts with, allowing it to exist in three distinct forms: a neutral atom, a positive ion, or a negative ion. The presence of isotopes like deuterium and tritium, which contain one or two neutrons, does not alter this fundamental charging behavior.
The Neutral Hydrogen Atom
The most basic form of hydrogen is the neutral atom, which carries a net electrical charge of zero. This neutrality arises from the perfect balance between the single positive charge of the proton and the single negative charge of the orbiting electron. This neutral form acts as the baseline for understanding how hydrogen gains or loses charge in chemical reactions. Because the single electron is held relatively loosely, the neutral hydrogen atom readily seeks stability by either losing, gaining, or sharing this electron. The zero-charge state of the atom only persists when it is isolated or when it forms a nonpolar bond with another hydrogen atom, such as in the diatomic gas \(\text{H}_2\).
The Positive Hydrogen Ion (\(\text{H}^{+}\))
Hydrogen most commonly adopts a positive charge when it loses its sole electron, forming the hydrogen ion (\(\text{H}^{+}\)). The resulting particle is literally a bare proton, with a charge of \(+1\) and virtually no electron cloud surrounding it. This positively charged species is extremely reactive and rarely exists in isolation in chemical systems.
In aqueous solutions, the positive hydrogen ion immediately attaches itself to a water molecule to form the hydronium ion (\(\text{H}_3\text{O}^{+}\)). This formation occurs because the bare proton is strongly attracted to the lone pairs of electrons on the oxygen atom of water. The concentration of these positive ions in a solution is the chemical property measured by the \(\text{pH}\) scale.
A high concentration of \(\text{H}^{+}\) ions signifies an acidic solution, corresponding to a low \(\text{pH}\) value. Acids, like hydrochloric acid, function by readily releasing these positive ions into the solution. The proton’s mobility in water is exceptionally high, allowing it to be rapidly transferred between water molecules, a mechanism that underlies the fast nature of acid-base reactions.
The Negative Hydrogen Ion (Hydride)
Hydrogen can gain an extra electron, resulting in a negative ion called hydride, symbolized as \(\text{H}^{-}\). The hydride ion has a charge of \(-1\) and possesses two electrons orbiting the single proton. This state is less common in everyday chemistry because the hydrogen atom does not have a strong natural tendency to attract a second electron.
The formation of the negative hydride ion primarily occurs when hydrogen bonds with highly electropositive elements, such as alkali metals or alkaline earth metals. For example, in the compound sodium hydride (\(\text{NaH}\)), the sodium atom readily gives up its electron to the hydrogen atom, forming an ionic compound consisting of the positive sodium ion (\(\text{Na}^{+}\)) and the negative hydride ion (\(\text{H}^{-}\)). This type of compound is known as an ionic hydride.
Hydrides are powerful reducing agents in chemical reactions due to the ease with which the extra electron can be donated to another chemical species.
Hydrogen in Covalent Bonds (Partial Charges)
When hydrogen forms a covalent bond, it shares its electron with another atom, resulting in partial charges instead of full ionic charges. The distribution of the shared electrons is governed by a property called electronegativity, which is an atom’s ability to attract electrons within a bond. This unequal sharing creates a separation of charge, giving hydrogen a partial positive (\(\delta+\)) or partial negative (\(\delta-\)) character.
If hydrogen bonds with an atom that has a higher electronegativity, such as oxygen or fluorine, the shared electrons are pulled closer to the partner atom. This leaves the hydrogen atom with a partial positive charge (\(\delta+\)), as seen in the water molecule (\(\text{H}_2\text{O}\)). The hydrogen atoms are slightly positive, while the oxygen atom is partially negative.
Conversely, if hydrogen bonds with an atom that has a lower electronegativity than itself, such as carbon in certain organometallic compounds, the shared electrons are slightly pulled away from the partner atom. This causes the hydrogen atom to acquire a partial negative charge (\(\delta-\)). These partial charges are always smaller than the full \(+1\) or \(-1\) charges of the ions, but they are significant enough to determine the molecule’s chemical reactivity and physical properties, such as polarity.