The water molecule is fundamental to life on Earth, consisting of one oxygen atom bonded to two hydrogen atoms. This atomic structure is responsible for water’s unique properties, such as its ability to dissolve many substances and remain liquid across a wide temperature range. The force that locks these atoms together to form a single, stable molecule is the covalent bond.
The Primary Force: Covalent Bonding
The atoms within a water molecule are held together by the sharing of electrons. Both hydrogen and oxygen are non-metals, and forming a covalent bond allows them to achieve greater stability by filling their outermost electron orbits. A single hydrogen atom has one electron but needs two to complete its valence shell, while an oxygen atom has six valence electrons and seeks a total of eight.
To satisfy these requirements, the oxygen atom shares one electron with each hydrogen atom, and each hydrogen atom shares its single electron with the oxygen. This arrangement results in two distinct covalent bonds within the H₂O molecule, where a pair of electrons is mutually held by the oxygen and one hydrogen atom. Through this sharing mechanism, all three atoms achieve a full and stable outer electron shell. The strength of these internal bonds is substantial, requiring a significant input of energy to break them apart.
The Result: Molecular Polarity
The sharing of electrons is not equal. This unequal distribution is due to electronegativity, which is an atom’s ability to attract a shared electron pair toward itself. The oxygen atom possesses a higher electronegativity than the hydrogen atoms.
Because oxygen pulls the shared electrons closer to its nucleus, the electrons spend more time near the oxygen atom. This unequal distribution causes the oxygen end of the molecule to acquire a slightly negative charge (represented as delta minus). Conversely, the hydrogen ends, having their electrons pulled slightly away, develop a slight positive charge (represented as delta plus). This separation of charge defines water as a polar molecule, giving it distinct positive and negative regions.
Intermolecular Attraction: Hydrogen Bonds
The polarity created by the unequal sharing of electrons leads to attractive forces between separate water molecules. The slightly positive hydrogen end of one water molecule is electrostatically drawn to the slightly negative oxygen end of a neighboring water molecule. This specific attraction is known as a hydrogen bond, and it is an intermolecular force.
Hydrogen bonds are significantly weaker than the covalent bonds that hold the H and O atoms together within a single molecule. For instance, it takes about 25 times more energy to break the covalent bond than it does to break the intermolecular hydrogen bond. Despite being weak, the sheer number of hydrogen bonds formed in liquid water is responsible for many unique characteristics, such as its high boiling point and surface tension. These forces continually break and reform, allowing water molecules to slide past one another while remaining loosely connected in the liquid state.