Lewis concept of ccovalency of an element involved octet rule. Later on it was found that may elements in their compounds e.g. BeF_(2),BF_(3) etc, have incomplete octet whereas PCl_(5), SF_(6) etcc. Have expanded octet. This classical concept also failed in predicting the geometry of molecules. Modern concept of covalence was proposed in terms of valence bond theory. Hybridization concept along with v.B. Theory successfully molecules was explained by VSEPR concept. Finally molecular orbital theory was proposed to explain many other molecules. Which statements are correct for CO^(+) and N_(2)^(-) according to M.O. Theory: (I) Both have same configuration. (II) Bond order for CO^(+) and N_(2)^(+) are 3.5 and 2.5 (III) Bond order of CO^(+) and N_(2)^(+) are same. (IV) During the formation of N_(2)^(@) from N_(2) bond length increases. (V) During the formation of CO^(+) from CO, the bond length decreases.