# Lecture 15: Resonance Structures and VSEPR

• ﻿$OCN^-$﻿ Lewis Structure
• Because ﻿$O$﻿ is more electronegative than ﻿$N$﻿
• When ﻿$OCN^-$﻿ takes up ﻿$H^+$﻿ it goes to O﻿$^-$﻿

### Comparing Resonance Structures

• Double Bond - Bond length is shorter and stronger than a single bond
• Negative sign once on the right and once on the left
• The Bonds - Double and single are always switching between them
• The lengths are the same for both

### VSEPR Method

• Count all ﻿$VS$﻿ ﻿$e^-$﻿ of the central atom, ﻿$A$﻿
• Add ﻿$e^-$﻿ from the ligands, ﻿$X$﻿
• Compensate for ionic charge
• Divide by 2 = Steric number, ﻿$SN$﻿
• Distribute ﻿$e^-$﻿ pairs so as to minimize ﻿$e^-$﻿ pairs are not equivalent
• If the ﻿$SN=4$﻿ the basic shape is tetrahedral
• If we have only 3 atoms (like ﻿$NH_3$﻿) ﻿$\rightarrow$﻿ shape is pyramid
• ﻿$Ax_3E$﻿ ﻿$\rightarrow$﻿ Triangular Pyramid Structure
• Electron count for arriving at Steric Number (﻿$SN$﻿) ﻿$AX_n$﻿ (﻿$A$﻿ is the central atom, ﻿$X$﻿ is atom bonded to it) ﻿$E$﻿ is the number of lone pairs
• Example #1
• ﻿$CCl_4$﻿

• Example #2
• ﻿$BF_3$﻿

• Central Atom ﻿$\rightarrow$﻿ Look at the value electrons
• When connected to central atom, go by roles for each group