Ionic crystal
= Ionic crystal
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An <ionic crystal> repeats an arrangement of <ions> in three dimensions. A <unit cell> describes that repetition, whereas a <formula unit> describes composition. The two accounting devices answer different questions. <Coulomb's law> is not restricted to an arbitrarily drawn line connecting one <cation> to one <anion>: every <ion> contributes to the <electric field> experienced by the others.