Comparing a simple molecular compound like water (\(\text{H}_2\text{O}\)) to an ionic compound such as table salt (sodium chloride, \(\text{NaCl}\)) illustrates the vast differences in chemical bonding. The mechanism of electron interaction dictates the resulting structure and behavior. Understanding these mechanisms explains why water is a liquid that does not conduct electricity and why salt is a solid crystal with a high melting point.
The Mechanism of Covalent Bonding
The bonding within a water molecule is an example of covalent bonding, involving the sharing of electrons between nonmetals like hydrogen and oxygen. Oxygen requires two electrons and each hydrogen requires one. They satisfy this need by sharing electron pairs, forming a discrete, individual molecule (\(\text{H}_2\text{O}\)).
The electron sharing in water is not equal, a phenomenon known as polarity. Oxygen has a much higher electronegativity, meaning it has a stronger pull on the shared electrons than hydrogen. This unequal sharing causes the electrons to spend more time closer to the oxygen nucleus, giving the oxygen side a slight negative charge (\(\delta-\)) and leaving the hydrogen sides with a slight positive charge (\(\delta+\)).
This charge separation creates a polar covalent bond. The water molecule is not linear; instead, it has a bent geometry due to two lone pairs of electrons on the oxygen atom. This bent shape prevents the partial charges from canceling out. The resulting internal polarity means the water molecule acts like a tiny magnet with distinct positive and negative ends.
The Mechanism of Ionic Bonding
In contrast to the sharing found in water, the formation of table salt (\(\text{NaCl}\)) involves the transfer of electrons, which is characteristic of ionic bonding. This process typically occurs between a metal, like sodium (\(\text{Na}\)), and a nonmetal, like chlorine (\(\text{Cl}\)).
Sodium (\(\text{Na}\)) possesses one valence electron and readily loses it. Chlorine (\(\text{Cl}\)), needing one electron to fill its outer shell, gains the electron transferred from sodium. This transfer results in the formation of charged particles: a positively charged cation (\(\text{Na}^+\)) and a negatively charged anion (\(\text{Cl}^-\)). The ionic bond is the powerful electrostatic force of attraction between these oppositely charged ions.
This attraction does not create individual molecules but instead pulls vast numbers of cations and anions together into a highly ordered, repeating structure known as a crystal lattice. In the sodium chloride lattice, every sodium ion is surrounded by chloride ions, and every chloride ion is surrounded by sodium ions, maximizing the attractive forces. The chemical formula \(\text{NaCl}\) therefore represents the simplest ratio of ions within this continuous three-dimensional network, not a discrete, isolated molecule.
Structural Differences and Physical Properties
The fundamental difference between the bond mechanisms—electron sharing that forms discrete molecules versus electron transfer that forms a lattice—leads to dramatic differences in the physical properties of water and salt. Covalent compounds like water consist of individual molecules that are held to neighboring molecules by relatively weak intermolecular forces. Ionic compounds, however, are held together by strong electrostatic forces extending throughout the entire crystal lattice.
Because the forces holding discrete water molecules together are weak, less energy is required to separate them, resulting in low melting and boiling points. Water is a liquid at room temperature, boiling at \(100^\circ\text{C}\). Conversely, the strong electrostatic attractions in the salt lattice require substantial energy to overcome, which is why ionic compounds are hard, brittle solids. Sodium chloride has a very high melting point of \(801^\circ\text{C}\).
Another distinguishing property is electrical conductivity. Pure water, composed of neutral molecules, does not conduct electricity effectively because it lacks free-moving charged particles. When salt is dissolved in water or melted, the strong ionic bonds are broken, freeing the \(\text{Na}^+\) and \(\text{Cl}^-\) ions. These mobile ions carry an electrical charge, making dissolved salt water a good conductor of electricity. In the solid state, however, ionic compounds do not conduct electricity because the charged ions are locked into fixed positions within the rigid crystal lattice.