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My book says:

As single ions of a metal are not associated in the solid with single ions of a non metal, separate units of ionic compounds do not exist. It is, therefore, wrong to talk of a molecule of an ionic compound.

I know ionic solids exist in form of crystal lattice but why can't we isolate a single molecule of ionic compound?

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2 Answers 2

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Ions are electrostatically bound. One need only separate them with integrated inverse squared distance work. If you mix multiple ionic compounds in solvent, a melt, gas phase, the original paired associations are spontaneously scrambled. Benzene plus xylene does not give you toluene. "Separating" benzene is not electrostatic attraction.

Quaternized glycine, $\ce{^{+}N(CH3)3-CH2-C(=O)O^{-}}$, is betaine. It is ionic, but the ions are covalently bound into a molecule. Where are the discrete formula units in alumina? You can draw lines to connect closest atoms, but those are not bonds. (Note fractional atom counts for atoms embedded in planes, edges, and corners.)

Alumina crystal structure

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    $\begingroup$ How is this unit cell $\ce{Al2O3}$? It seems to have six aluminum and six oxygen atoms. $\endgroup$ Nov 21, 2017 at 22:31
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    $\begingroup$ This oppose the Mithoron comment above. In principle molecular ionic entities exist. The might require exotic conditions but it is not the point. $\endgroup$
    – Alchimista
    May 13, 2021 at 10:01
  • $\begingroup$ I agree with Oscar Lanzi in an old message dated November $21, 2017$. The drawn unit cell contains 6 $\ce{Al}$ atoms and $2+1+3$ = $6$ $\ce{O}$ atoms. This cell is $\ce{AlO}$ and not $\ce{Al2O3}$. $\endgroup$
    – Maurice
    May 21, 2023 at 14:53
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Molecules that have strongly ionic bonds actually do exist. In the solid phase where we see most ionic compounds electrostatic bonding with a high density of ions causes the molecules to lose their identity, forming the familiar ionic lattice structures. The vapor phase is a different story. Ionic vapors typically consist of small neutral clusters. Such clusters are, of course, strongly polar, the polarity appearing as dipoles or quadrupoles (two oppositely oriented dipoles attract and join together to form the quadrupole).

Sodium chloride offers an example. Ron's answer to a pervious question identifies sodium chloride vapor as neutral $\ce{NaCl}$ dipoles combined with dimeric $\ce{Na2Cl2}$ quadrupoles.

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