Hydrogen has an atomic number of one, consisting of a single positively charged proton and a single negatively charged electron. This structure gives hydrogen unique flexibility, allowing it to adopt different charge states. The charge hydrogen carries in a chemical compound is not fixed, but depends entirely on the identity of its bonding partner.
The Neutral Hydrogen Atom
A neutral hydrogen atom (\(\text{H}\)) maintains a net charge of zero because the atom’s single positive proton perfectly balances the charge of its one negative electron. In this balanced state, the atom is stable.
While isolated neutral atoms are rare, this state is commonly observed in the diatomic hydrogen gas molecule (\(\text{H}_2\)). In this molecule, two hydrogen atoms share their electrons equally, resulting in a non-polar bond, which preserves the overall neutral state.
The Positive Hydrogen Ion
The positive hydrogen ion (\(\text{H}^+\)) forms when the neutral atom loses its sole electron. Since the most common isotope of hydrogen has no neutrons, the resulting \(\text{H}^+\) ion is simply a bare proton. This positive ion is highly reactive, seeking to regain an electron.
In aqueous solutions, the \(\text{H}^+\) ion does not exist independently due to its high reactivity. It immediately associates with a water molecule (\(\text{H}_2\text{O}\)) to form the hydronium ion (\(\text{H}_3\text{O}^+\)). This proton transfer defines an acid according to the Brønsted-Lowry theory.
The concentration of these ions determines the acidity of a solution. The pH scale measures acidity and is a logarithmic measure related to the concentration of \(\text{H}^+\) ions. A higher concentration of \(\text{H}^+\) ions corresponds to a lower pH value and a more acidic environment.
The Negative Hydrogen Ion
The negative hydrogen ion, known as the hydride ion (\(\text{H}^-\)), forms when a hydrogen atom gains a second electron, resulting in a net charge of negative one. By accepting this extra electron, the hydrogen atom achieves a stable, filled outer electron shell configuration, resembling helium.
The formation of the hydride ion is uncommon compared to the positive ion. It occurs almost exclusively when hydrogen bonds with highly electropositive elements, such as the alkali and alkaline earth metals, which readily give up their electrons to form ionic compounds.
Compounds containing the hydride ion include sodium hydride (\(\text{NaH}\)) and calcium hydride (\(\text{CaH}_2\)). In these ionic compounds, the \(\text{H}^-\) species acts as a strong base or a reducing agent. The negative charge on the hydride ion makes it chemically opposite to the acidic proton.
How Bonding Partners Determine Hydrogen’s Charge
The charge hydrogen adopts in a compound is governed by electronegativity, a chemical property measuring an atom’s tendency to attract a shared pair of electrons. Hydrogen’s electronegativity value sits near the middle of the scale, allowing it the capacity to either lose or gain electron character.
When hydrogen bonds with an element that has a much higher electronegativity, such as oxygen or chlorine, the shared electrons are pulled closer to the partner atom. This unequal sharing causes hydrogen to take on a partial positive charge, effectively acting as the \(\text{H}^+\) ion.
Conversely, when hydrogen bonds with a highly electropositive metal, the metal has a much weaker pull on electrons. The metal transfers its electron to the hydrogen atom, resulting in the formation of the \(\text{H}^-\) ion. The identity of the bonding partner determines whether hydrogen is positive or negative in the final compound.