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CHEMICAL BONDING AND MOLECULAR STRUCTURE 105 he ounting is based on the assumption 4.1.5 Limitations of the Octet Rule that the atom in the molecule owns one he o tet rule though useful is not uni ersal electron of each shared pair and both the It is quite useful for understanding the electrons of a lone pair. structures of most of the organic compounds et us onsider the o one mole ule and it applies mainly to the second period he ewis stru ture of may be drawn as: elements of the eriodi table here are three ty es of e e tions to the o tet rule The incomplete octet of the central atom In some compounds, the number of electrons surrounding the central atom is less than eight his is es e ially the ase with elements ha ing less than four alen e ele trons am les are i l e and l . he atoms ha e been numbered as 2 and he formal harge on • he entral atom mar ed 1 i e and ha e and alen e ele trons 2 only ome other su h om ounds are l l and . • he end atom mar ed Odd-electron molecules 1 In molecules with an odd number of electrons 2 li e nitri o ide and nitrogen dio ide • he end atom mar ed 2 the o tet rule is not satisfied for all the 1 atoms 2 en e we re resent along with the formal charges as follows: The expanded octet lements in and beyond the third eriod of the eriodi table ha e a art from s and p orbitals d orbitals also a ailable for bonding In a number of compounds of these elements there are more than eight alen e ele trons We must understand that formal charges around the entral atom his is termed as do not indicate real charge separation within the e anded o tet b iously the o tet rule the molecule. Indicating the charges on the does not apply in such cases. atoms in the ewis stru ture only hel s in ome of the e am les of su h om ounds ee ing tra of the alen e ele trons in are and a number of the molecule. Formal charges help in the coordination compounds. selection of the lowest energy structure from a number of ossible ewis stru tures for a gi en s e ies Generally the lowest energy structure is the one with the smallest formal charges on the atoms. The formal charge is a factor based on a pure covalent view of bonding in which electron pairs are shared equally by neighbouring atoms. Reprint 2026-27