Molecular and Ionic Compound Structure and Properties

2.7 VSEPR and Bond Hybridization

Essential Knowledge:

*These are taken directly from College Board

• Both Lewis diagrams and VSEPR theory must be used for predicting electronic and structural properties of many covalently bonded molecules and polyatomic ions, including the following: 

a. Molecular geometry 

b. Bond angles 

c. Relative bond energies based on bond order 

d. Relative bond lengths (multiple bonds, effects of atomic radius) 

e. Presence of a dipole moment 

f. Hybridization of valence orbitals of the molecule (SAP-4.C.2)  

• The terms “hybridization” and “hybrid atomic orbital” are used to describe the arrangement of electrons around a central atom. When the central atom is sp hybridized, its ideal bond angles are 180°; for sp2 hybridized atoms the bond angles are 120°; and for sp3 hybridized atoms the bond angles are 109.5°. (SAP-4.C.3) 
  
  

❌  AN UNDERSTANDING OF THE DERIVATION AND DEPICTION OF HYBRID ORBITALS WILL NOT BE ASSESSED ON THE AP EXAM.

Rationale: The course includes the distinction between sigma and pi bonding, the use of VSEPR to explain the shapes of molecules, and the sp, sp2 , and sp3 nomenclature. Additional aspects related to hybridization are both controversial and do not substantially enhance understanding of molecular structure.

❌  HYBRIDIZATION INVOLVING d ORBITALS WILL NOT BE ASSESSED ON THE AP EXAM. WHEN AN ATOM HAS MORE THAN FOUR PAIRS OF ELECTRONS SURROUNDING THE CENTRAL ATOM, STUDENTS ARE ONLY RESPONSIBLE FOR THE SHAPE OF THE RESULTING MOLECULE. 

Rationale: Current evidence suggests that main-group hybridization involving d orbitals does not exist, and there is controversy about the need to teach any hybridization. Until agreement is reached in the chemistry community, we will continue to include only sp, sp2 , and sp3 hybridization on the AP Exam.

• Bond formation is associated with overlap between atomic orbitals. In multiple bonds, such overlap leads to the formation of both sigma and pi bonds. The overlap is stronger in sigma than pi bonds, which is reflected in sigma bonds having greater bond energy than pi bonds. The presence of a pi bond also prevents the rotation of the bond and leads to structural isomers.  (SAP-4.C.3)  

❌  MOLECULAR ORBITAL THEORY IS RECOMMENDED AS A WAY TO PROVIDE DEEPER INSIGHT INTO BONDING. HOWEVER, THE AP EXAM WILL NEITHER EXPLICITLY ASSESS MOLECULAR ORBITAL DIAGRAMS, FILLING OF MOLECULAR ORBITALS, NOR THE DISTINCTION BETWEEN BONDING, NONBONDING, AND ANTIBONDING ORBITALS. 

Rationale: As currently covered in general chemistry college textbooks, molecular orbital theory is limited to homonuclear molecules in the second period.