Sodium chloride (NaCl) is not considered a molecule because it does not consist of discrete, covalently bonded units. Instead, it is classified as an ionic compound or a salt.
In an ionic compound like sodium chloride, the atoms are held together by ionic bonds, which are electrostatic attractions between positively and negatively charged ions. In the case of NaCl, sodium (Na) donates one electron to chlorine (Cl), forming positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). These ions are arranged in a crystal lattice structure rather than existing as individual, discrete units.
The ionic bond in sodium chloride is quite strong, and breaking it requires a substantial amount of energy. In contrast, molecules typically consist of covalent bonds, where atoms share electrons to form stable units held together by shared electron pairs. Covalent compounds, unlike ionic compounds, often exist as discrete units and can be represented by molecular formulas.
Therefore, while sodium chloride is a compound composed of atoms, it is not considered a molecule due to its ionic bonding and lack of discrete covalent units.