Orbitals & Bonding

• How can two p atomic orbitals combine?

  In additive and subtractive ways

  
• What does an additive combination of p orbitals form?

  A bonding molecular orbital with lower energy

  
• What does a subtractive combination of p orbitals form?

  An antibonding orbital with higher energy

  
• What is the molecular formula for ethene?

  H2C=CH2
• How do p orbitals combine in terms of molecular orbitals?

  They form π and π* molecular orbitals

  
• What happens to the energy difference between HOMO and LUMO as conjugation increases?

  The energy difference decreases
• Why is the energy difference between HOMO and LUMO important?

  It is important for spectroscopy
• What type of spectroscopy provides information about compounds with conjugated double bonds?

  UV & visible spectroscopy
• What does UV excitation of 1,3-butadiene promote?

  Promotion from HOMO to LUMO
• What is the energy of UV light required for 1,3-butadiene promotion?

  217 nm

  
• What happens when 1,3-butadiene is irradiated with UV light at 217 nm?

  It absorbs the UV light

  
• What is plotted on the chart when UV light is absorbed by 1,3-butadiene?

  Absorbance (A) versus wavelength (nm)
• Which elements can promote an electron to increase bonding capability?

  Group 2, 3, and 4 elements
• Which elements can promote into vacant d orbitals?

  Phosphorus and sulfur
• What does promotion allow an element to do?

  Increase the number of bonds it can make
• How many covalent bonds can carbon form without promotion?

  Two covalent bonds
• Are there two kinds of C—H bonds in methane?

  No, all C—H bonds are identical

  
• What happens when one paired 2s electron is promoted in carbon?

  All four electrons can participate in bonding

  
• What does Pauling’s theory state about hybridization in carbon?

  One s-orbital and three p-orbitals hybridize

  
• What do sp3 hybrid orbitals result from?

  Combination of 2s and three 2p orbitals

  
• What is the energy level of sp3 hybrid orbitals compared to original orbitals?

  Lower than 2p but higher than 2s

  
• How are sp3 orbitals arranged around the nucleus?

  Unsymmetrical with one larger lobe
• What is the C-H bond strength in methane?

  439 kJ/mol (105 kcal/mol)

  
• What is the structure of ethane in terms of hybridization?

  Both carbons are sp3 hybridized

  
• What is the C-C bond strength in ethane?

  377 kJ/mol (90 kcal/mol)

  
• What is the arrangement of sp2 orbitals in ethene?

  They lie in a plane

  
• What is the bond angle in sp2 hybridized carbon atoms?

  About 120°

  
• How is a σ bond formed in ethene?

  By end-on overlap of two sp2 orbitals

  
• How is a π bond formed in ethene?

  By side-to-side overlap of two unhybridized p orbitals

  
• What is the C=C bond strength in ethene?

  611 kJ/mol (146 kcal/mol)

  
• What is the structure of ethyne in terms of hybridization?

  Each carbon is sp hybridized

  
• How many sp hybrid orbitals are created in ethyne?

  Two sp hybrid orbitals

  
• What is the geometry of sp hybrid orbitals in ethyne?

  Linear geometry
• What is the C≡C bond strength in ethyne?

  837 kJ/mol (200 kcal/mol)

  
• What does VSEPR theory account for in molecular geometry?

  Repulsion between different types of electron pairs

  
• What is the order of repulsion effects in VSEPR theory?

  Lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair
• How does VSEPR theory explain bond angles in methane?

  Accounts for tetrahedral arrangement of electron pairs

  
• What bond angle is observed in water due to VSEPR theory?

  104.5° bond angle
• What is the molecular geometry of carbon dioxide?

  Linear

  
• What is the molecular geometry of boron trifluoride?

  Trigonal planar