Why is an ionic compound not considered to be a molecule?
Answer:
Because the ions aren't closely bound to one another. In a solid crystal lattice, they sort of hang around together because of electrostatic attraction. In solution, they don't even do that.
Because in an ionic compound you have ions interacting with each other through coulombic attractions and thus, there is not a molecule in the crystal lattice. In sodium chloride (NaCl), each sodium ion is surround by six chloride ions and each of those chloride ions is surrounded in turn by six other sodium ions, so you have an infinite lattice of alternating sodium and chloride ions. That is why NaCl is cubic, if you look at the table salt under a microscope, you can see that they are tiny cubes. So NaCl represents the empirical formula of sodium chloride, because in the end, there is one sodium ion per every chloride ion in the crystal lattice. Ionic compounds are very difficult to melt, they melt at relatively high temperatures. For NaCl, the temperature is around 820 degrees celcius. To vaporize NaCl, even higher temperatures are required. Once you get to the gas phase, then you can start taking about individual NaCl molecules in the gas phase, but in practice, nobody is interested in gaseous sodium chloride, except for theoretical studies in the determination of enthalpy of formation of compounds, which involve atoms in the gas phase.
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