The arrangement of ions in ionic compounds

• How do cations and anions of an ionic compound exist in its solid state?
• Let us explain this with sodium chloride as our example:
• Do you think that pairs of Na⁺Cl^- as units would be present in the solid crystal?

    If you think so, it is not correct. Remember that electrostatic forces are non directional. Therefore, it is not possible for one Na⁺ to be attracted by one Cl^- and vice-versa. Depending upon the size and charge of a particular ion, number of oppositely charged ions get attracted by it, but, in definite number. In sodium chloride crystal each Na⁺ is surrounded by 6 Cl^- and each Cl^-by six Na⁺ ions. Ionic compounds in the crystalline state consist of orderly arranged cations and anions held together by electrostatic forces of attractions in three dimensions. The crystal structure of sodium chloride is given below:

    NaCl is said to possess face centered cubic lattice crystal
structure (see figure-1).

   The number of ions of opposite charge that surround a given ion of given charge is known as the coordination number of that given ion.

    For example, in sodium chloride crystal, the coordination number of Na+ is 6 and that of Cl^- is also 6.

   In previous lesson you have learnt about variation of metallic and non metallic character of elements in a group or period of the periodic table. Recall the facts about metallic and non metallic character of elements.

    Generally elements of metals have tendency of losing electron to attain the octet in their valence shell. This property is called as the metallic character or electropositivity. Elements with more electropositive character form cations. Similarly non metals like oxygen(₈ O), fluorine ( ₉F) and chlorine (₁₇Cl) acquire electron configuration of elements of inert gases by gaining electrons this property is called the.non - metallic characteror electronegativity of the element. Elements with more electronegative character form anions.