It is said that in a crystal lattice of NaCl, each cation is surrounded by six anions and each anion by six cations. Then what at the edges? I mean the lattice should carry some charge at the edges as it is not possible for cations at the edges to attach with more of anions.

Dear deeksha,

Sodium chloride is a specific example of a common structure type known as the "halite" structure. This lattice may also be viewed as a FCC lattice of anions (Z = 4), with smaller cations occupying all O_h holes (or vice versa).  By identifying and counting all of the O_h holes, it can be shown that the number of cations wholly within the unit cell is also 4.

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