A water molecule is electrically neutral overall, but the distribution of charge within it is far from uniform. The chemical formula for water, \(\text{H}_2\text{O}\), indicates a balance of two hydrogen atoms and one oxygen atom, and the total number of protons equals the total number of electrons. This perfect balance ensures that a single, isolated water molecule carries no net electrical charge. The common misunderstanding about water’s charge arises because the electrons are not shared equally between the atoms, leading to a profound condition known as polarity. This uneven sharing creates localized regions of positive and negative electrical character, even though the molecule as a whole remains neutral.
The Molecular Structure of Water
The basic unit of water is a molecule composed of one oxygen atom bonded to two hydrogen atoms. The atoms are held together by polar covalent bonds, which involve the sharing of electron pairs between the oxygen and hydrogen nuclei. The molecular geometry of water is not linear but is instead bent or V-shaped. This unique shape results from the oxygen atom having two pairs of unshared, or “lone,” electrons. The repulsive forces from these lone pairs push the hydrogen atoms closer together, resulting in a bond angle of approximately \(104.5\) degrees.
Why Water is Neutral Overall But Polar
The polarity of the water molecule is a direct consequence of the difference in electronegativity between oxygen and hydrogen. Electronegativity is an atom’s ability to attract shared electrons in a covalent bond. Oxygen is significantly more electronegative than hydrogen, meaning it has a much stronger pull on the shared electrons.
The oxygen atom draws the electron density of the covalent bonds closer to its nucleus, which creates a region of slight negative charge, often symbolized as \(\delta^-\). Conversely, this electron displacement leaves the hydrogen atoms with less electron density, giving each of them a slight positive charge, symbolized as \(\delta^+\). These are partial charges, not full ionic charges, and they are equal and opposite, maintaining the molecule’s overall electrical neutrality.
Because the molecule has distinct positive and negative ends, it is defined as a polar molecule, possessing a net dipole moment. The bent shape of the molecule prevents the partial charges from canceling each other out. If the molecule were linear, the bond dipoles would oppose each other perfectly, resulting in a non-polar molecule.
The Consequences of Polarity
The polarity of water allows it to engage in strong intermolecular attractions that dictate its properties. The partial positive charge on a hydrogen atom of one molecule is drawn to the partial negative charge on the oxygen atom of a neighboring molecule, forming a hydrogen bond. The collective action of these bonds gives water its high boiling point and cohesive properties.
This polarity also makes water an exceptional solvent. The charged ends of the water molecule can effectively surround and separate ions and other polar molecules. For instance, when table salt dissolves, the negative oxygen ends cluster around the positive sodium ions, while the positive hydrogen ends cluster around the negative chloride ions, pulling the crystal lattice apart. This process of surrounding charged particles is known as solvation, and it is fundamental to the transport of nutrients and chemical reactions.
Water’s Natural Ionization
A tiny fraction of water molecules spontaneously dissociate in a process called autoionization. This reaction involves one water molecule donating a proton (a hydrogen nucleus) to a neighboring water molecule. The result is the formation of a positively charged hydronium ion (\(\text{H}_3\text{O}^+\)) and a negatively charged hydroxide ion (\(\text{OH}^-\)). In pure water at a standard temperature of \(25\) degrees Celsius, the concentration of these positive and negative ions is extremely low and exactly equal. The concentration of both \(\text{H}_3\text{O}^+\) and \(\text{OH}^-\) is \(1.0 \times 10^{-7}\) moles per liter, which corresponds to a neutral \(\text{pH}\) of \(7\).