Writing Lewis StructuresWriting Lewis StructuresWriting Lewis StructuresFormal ChargesFormal ChargesResonanceFree RadicalsIncomplete Valence ShellExpanded Valence ShellBond LengthChapter 8 1Writing Lewis Structures1. Find the sum of valence electrons of all atoms in the molecule or polyatomic ion.If it is an anion, add one electron for each negative charge.If it is a cation, subtract one electron for each positive charge.Chapter 8 2Writing Lewis Structures2. The central atom is the least electronegative element that isn’t hydrogen; connect the outer atoms to it by single bonds.3. Fill the octets of the outer atoms with lone pairs.4. Fill the octet of the central atom with lone pairs.Chapter 8 3Writing Lewis Structures5. If the number of electrons runs out before the central atom has an octet, form multiple bonds until it does.Chapter 8 4Formal ChargesWhen more than one different Lewis structure can be drawn, formal charges are used to decide which is the best.For each atom in the structure, count the electrons in lone pairs and half the electrons it shares with other atoms, then subtract this number from the number of valence electrons that atom has alone.Chapter 8 5Formal ChargesThe best Lewis structure is usually the one with the fewest nonzero formal charges or puts a negative formal charge on the most electronegative atom.Chapter 8 6ResonanceWhen a molecule or ion can be represented by two or more plausible Lewis structures that differ only in the distribution of electrons, the true structure is a composite of them, called a resonance hybrid.The different plausible structures are resonance structures.Chapter 8 7Free RadicalsFree radicals contain an odd number of electrons, so that at least one atom has an unpaired electron.Chapter 8 8Incomplete Valence ShellSome ions or molecules contain an atom with an incomplete valence shell, having fewer than eight electrons around it.Chapter 8 9Expanded Valence ShellSome ions or molecules contain an atom with an expanded valence shells, having more than eight electrons around it.Bond LengthChapter 8