Salt, known chemically as sodium chloride (NaCl), is an ionic compound. The bond holding the sodium and chlorine atoms together is ionic, not covalent. This classification is based on how the atoms interact at the subatomic level, specifically concerning the exchange and sharing of electrons. The resulting bond type is responsible for the physical properties observed in common table salt.
Understanding Ionic and Covalent Bonds
Chemical bonds are fundamentally categorized by the behavior of valence electrons, which are the outermost electrons involved in reactions. The two primary types of bonds are ionic and covalent, each representing an extreme on a bonding spectrum.
Ionic bonding involves the complete transfer of one or more valence electrons from one atom to another, typically between a metal and a non-metal. The atom that loses electrons becomes a positively charged ion (cation). The atom that gains electrons becomes a negatively charged ion (anion). The resulting bond is a strong electrostatic attraction between these oppositely charged ions.
Covalent bonding, in contrast, involves the sharing of valence electrons between atoms. This type of bond is generally formed between two non-metal atoms. The shared electrons are attracted to the nuclei of both atoms, holding the atoms together in a stable configuration. If the sharing of electrons is unequal due to differences in the atoms’ ability to attract electrons, the bond is considered polar covalent.
The determining factor for the type of bond formed is the difference in electronegativity between the two atoms. Electronegativity measures an atom’s ability to attract a shared electron pair. A large difference in this value, often greater than 1.7 on the Pauling scale, indicates a complete transfer of electrons and the formation of an ionic bond.
Sodium Chloride: The Definitive Ionic Compound
The bond in sodium chloride is a textbook example of ionic bonding due to the vast difference in the electron-attracting power of its elements. Sodium (Na) is an alkali metal with low electronegativity, easily giving up its single valence electron. Chlorine (Cl) is a non-metal with high electronegativity, strongly attracting an additional electron to complete its outer shell.
When these two elements react, the sodium atom transfers its valence electron to the chlorine atom. This electron transfer results in the formation of a positively charged sodium cation (Na+) and a negatively charged chloride anion (Cl-). The resulting ions achieve a stable, full electron shell, mimicking the structure of noble gases.
The resulting Na+ and Cl- ions are held together by a powerful electrostatic force of attraction. This attraction extends throughout the entire solid structure, not just between a single pair of ions. Consequently, the ions arrange themselves into a highly ordered, repeating pattern called a crystal lattice, which defines the structure of table salt. The calculated electronegativity difference between sodium and chlorine is approximately 2.2, confirming its ionic bond classification.
Physical Characteristics of Ionic Compounds
The ionic nature of sodium chloride gives rise to specific, observable physical characteristics. The strong electrostatic forces within the crystal lattice require significant energy to break, leading to a high melting point. Sodium chloride, for example, melts at approximately 801°C.
The orderly arrangement of positive and negative ions causes solid salt to be a poor conductor of electricity. In the solid state, the ions are fixed in place within the lattice and cannot move freely to carry a current. Ionic compounds are also brittle; a strong physical force can shift a layer of ions, causing like-charged ions to align and repel each other, which shatters the crystal along clean planes.
Salt is highly soluble in water, which is a polar solvent. Water molecules surround and pull the individual Na+ and Cl- ions away from the lattice structure, causing the salt to dissociate. Once dissolved, the ions are free to move, making the solution an excellent conductor of electricity. This ability to conduct current when dissolved or melted is a definitive property that confirms the compound’s ionic classification.