Chemical compounds form when different elements bond, creating new substances. Compounds are categorized into ionic and molecular types based on how their atoms connect. Understanding this distinction is helpful because the type of bonding significantly influences a compound’s physical and chemical behavior, including its melting point, electrical conductivity, and solubility.
Understanding Ionic and Molecular Bonds
Ionic and molecular compounds differ fundamentally in the nature of their chemical bonds. Ionic bonds involve a complete transfer of electrons from one atom to another. This transfer forms charged particles called ions: positively charged cations (from lost electrons) and negatively charged anions (from gained electrons). Oppositely charged ions are strongly attracted by electrostatic forces, forming a stable ionic compound.
Molecular bonds (covalent bonds) form by sharing electrons between atoms. Unlike ionic bonds, atoms in a molecular compound share one or more pairs of electrons. This sharing creates a strong attractive force, holding atoms together as a discrete molecule. Shared electrons are attracted to the nuclei of both bonding atoms, forming the molecular bond.
Identifying Compounds by Elemental Makeup
The most direct way to distinguish compounds is by examining the elements in their chemical formula. Ionic compounds form when a metal reacts with a nonmetal. For instance, sodium chloride (NaCl) is ionic because sodium (Na) is a metal and chlorine (Cl) is a nonmetal. Calcium oxide (CaO) also consists of a metal (calcium) and a nonmetal (oxygen).
Conversely, molecular compounds form when two or more nonmetals bond together. Water (H₂O) is a molecular compound, as both hydrogen (H) and oxygen (O) are nonmetals. Carbon dioxide (CO₂) is another example, composed solely of carbon (C) and oxygen (O), both nonmetals. This rule, based on element position on the periodic table (metals on the left, nonmetals on the right, separated by a staircase line), provides a reliable initial classification.
The Role of Electronegativity in Bonding
Electronegativity explains the elemental makeup rule. Electronegativity measures an atom’s ability to attract shared electrons in a chemical bond. The difference in electronegativity between bonding atoms dictates bond type.
A large electronegativity difference, typically between a metal and nonmetal, causes the more electronegative nonmetal to strongly pull electrons from the metal. This pull results in complete electron transfer, forming ions and an ionic bond. In contrast, a small or negligible electronegativity difference, common between two nonmetals, leads to electron sharing rather than transfer. This sharing forms a molecular bond, where electrons are held equally between atoms.
Observable Properties and Special Cases
Beyond elemental composition, observable physical properties offer further clues. Ionic compounds exhibit high melting and boiling points, often existing as solids at room temperature due to strong electrostatic attractions between ions. They form crystalline solids and, while not conductive in solid state, become good conductors when melted or dissolved in water due to mobile ions.
Molecular compounds have lower melting and boiling points, often existing as gases or liquids at room temperature. Forces between individual molecules are weaker than within ionic lattices, requiring less energy to break. These compounds do not conduct electricity in any state because they consist of neutral molecules rather than mobile ions. Polyatomic ions, such as ammonium (NH₄⁺) or sulfate (SO₄²⁻), are a special consideration. While atoms within a polyatomic ion are covalently bonded (nonmetals to nonmetals), a compound containing a polyatomic ion and another ion (e.g., NH₄Cl or CuSO₄) is ionic due to electrostatic attraction between oppositely charged ions.