Covalent Bonding: Orbitals, Exams of Geometry

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Covalent Bonding: Orbitals

Chapter 9

The Lewis structure of the methane molecule (left) and the tetrahedral molecular geometry of the methane molecule (right)

Valence Bond Theory and CH 4

• If bonds form from overlap of 1 2 s & 3 2 p orbitals on C with the 1 s orbital on each H atom, 2 problems result
  1. Two types of C—H bonds  1 s (H)— 2 s (C) & 1 s (H)— 2 p (C)
  2. Bond angles would be 90
• Facts: All 4 C—H bonds are identical with bond angles of 109.5

The valance orbitals on a free C atom: 2 s , 2 px , 2 py , and 2 pz

Formation of sp^3 Hybrid Orbitals

[combination of 1 s orbital with 3 p orbitals ( px , py , pz )]

The “native” 2 s and three 2 p atomic orbitals characteristic of a free carbon atom are combined to form a new set of four sp^3 orbitals. The small lobes of the orbitals are usually omitted from diagrams for clarity. The angle between the sp^3 orbital is 109.5

An Energy-Level Diagram Showing the

Formation of Four sp^3 Hybridization

The Shape of Ethylene (C 2 H 4 )

• Let’s look at the Lewis structure of C 2 H 4 (ethylene).
• What is the shape of an ethylene molecule?
  • around each carbon atom
• What are the approximate bond angles around the carbon atoms?
• Since the 2 s and 2 p orbitals of C do not have the re- quired arrangement, we need a set of hybrid orbitals
• Can we use the sp^3 orbitals?
  • No. Because sp^3 hybrid orbitals are at angles of 109.5o^ rather than the required 120o
• Thus in ethylene the C atoms must hybridize in a different manner
  • sp^2 Hybridization (^) 7

sp^2 Hybridization

• Involves combination of one s and two p

orbitals ( px and py )

• Gives a trigonal planar arrangement of a.o.
• One p orbital (usually pz ) is not used
  • Oriented  the plane of the sp^2 orbitals
• There are two types of bonds in ethylene:
  • Sigma () bond:
    • Electron pair is shared in an area centered on a line running between the atoms
  • Pi () bond (in double & triple bonds)
    • Formed from the 2 parallel, unhybridized p orbitals
    • Electron pair is shared in the space above and below the σ bond

An Orbital Energy-Level Diagram for

sp^2 Hybridization

sp^2 Hybridization

When an s and two p orbitals are mixed to form a set of three sp^2 orbitals, one p orbital remains unchanged and is perpendicular to the plane of the hybrid orbitals.

Pi (  ) Bonds in Ethylene

• A carbon-carbon double bond consists of a  and a  bond (4 electrons or 2 pairs).
• In the  bond the shared electrons occupy the space directly between the atoms.
• The  bond is formed from the unhybridized p orbital on the 2 C atoms.
• In a  bond the shared electron pair occupies the space above and below a line joining the atoms.

Formation of C=C Double Bond in Ethylene

Hybridization in Carbon Dioxide (CO 2 )

• Let’s look at the Lewis structure of CO 2.

• What is the shape of a carbon dioxide molecule?

• What are the bond angles?

• We need a new set of orbitals, sp hybrid orbitals
• sp hybridization
  • Involves combination of one s and one p orbital
  • Gives a linear arrangement of atomic orbitals
  • Two p orbitals are not used in hybridization
  • But used to form 2  bonds

O C O

Formation of sp Hybrid Orbitals

[combination of 1 s orbital with 1 p orbitals ( px )]

When one s orbital and one p orbital are hybridized, a set of two sp orbitals oriented at 180 degrees results.

The  Bonds in CO 2 Molecule

• The sp orbitals on carbon form  bonds with the sp^2 orbitals on the two oxygen atoms
• The remaining sp^2 orbitals on the oxygen atoms hold lone pairs.

The Orbitals Used to Form the Bonds in CO 2

• The carbon-oxygen double bonds each consist of one  bond and one  bond
• There are two  bonds ( py — py and pz — pz ) O (^) C O