14.1 Further aspects of covalent bonding (HL)
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Sigma bonds
Formed by direct head-on/axial overlap of atomic orbitals
Which covalent bonds have sigma bonds?
All covalent bonds have atleast one sigma bond
Electron density in a sigma bond
It is concentrated in region directly between nuclei
What type of symmetry does sigma bonds have?
Cylindrical symmetry along bond axis
Which orbitals can sigma bonds be formed between?
Can be formed between s and s orbitals or s and p orbitals
Pi bond
Formed by sideways overlap of two unhybridized p orbitals
If a carbon has formed double bonds like in alkenes, which bonds does it consist of?
One sigma and one pi bond
What bonds does a carbon triple bond consist of?
One sigma and 2 pi bonds
How are single bonds different from double?
There is no free rotation around double bonds like single bonds and doing so would require bond breaking.
Where is electron density concentrated in pi bonds?
Above and below plane of nuclei of bonding atoms
What does the strength of C-C bond (346 kJmol-1) and C=C bond (614 kJmol-1) tell us about strength of sigma and pi bonds?
It tells us sigma is stronger than pi. If sigma bond is 346 kJmol-1, that means pi bond in C=C bond is only 268kJmol-1
Why does sigma bond have more strength than pi bond?
Extra strength is due to greater overlap of orbitals whereas in pi bonds, they cannot overlap as much
What are non-equivalent Lewis structures and give example
It is when one molecule can have different number of multiple bonds eg. CO2 can have 2 double bonds or 1 single and 1 triple bond
Formal charge
It determines which Lewis structure is preferred
What does formal charge ignore?
Ignores difference in electronegativity between atoms and assumes 100% covalency
What is the equation for formal charge?
No. of valence electrons - 1/2(no. of bonding electrons) - no. of non-bonding electrons
Which formal charge is preferred?
The one closest to 0
Formal charge for both structures of CO2 and which is preferred
• Double bonds: - C= 4-4-0=0 - Both oxygens= 6-2-4=0 • 1 single and 1 triple bond - C=4-4-0=0 - Single O= 6-1-6= 1- - Triple O= 6-3-2= 1+ So DOUBLE bond CO2 is preferred
What if two structures have same formal charge?
Most electronegative atom with negative formal charge is preferred
Why can elements in Period 3 and beyond have expanded octets?
They have d orbitals which can be used for bonding
What does expanded octets result in?
5 or 6 electron domains instead of 2-4
What are molecules with 6 domains?
Octahedral
What are molecules with 5 domains?
Trigonal bipyramidal
Molecular geometry of octahedral molecule without lone pairs + bond angle
• Octahedral • 4 will be on plane and 1 domain will be on top and bottom each • Bond angle in plane =90 • Bond angle from top-bottom= 180