4.3 Covalent structures
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Octet rule
States that most stable arrangement for an atom is to have 8 electron in outermost energy level or configuration of a noble gas
What are exceptions to octet rule?
• Atoms that are stable with less than 8 electrons like H, Al, Be, B are stable (in compounds) • Expanded octets: More than 8 electrons in valence shell (Elements in period 3+
Incomplete octet
When an atom is electron deficient
Why is BF3 stable even though it’s electron deficient?
Boron has formed max. no of bonds possible
Why is BeCl2 stable?
Because Be has only 2 electrons to give
What are Lewis structures and what must they show?
They represent bonding in a molecule and must show bonding and non-bonding electrons
Calculate no. of bonding electrons for CN-
C= 4e- N= 5e- -= 1- Compound has 2 elements so must have 16 to be stable so there are 6 bonding electrons
What are resonance structures?
Formed when there are more than 1 Lewis structures for the same molecule/polyatomic ion
CO3 2- has how many resonance structures and why?
3 as double bond can be on any CO bond
What is the actual structure for a molecule called?
Resonance hybrid structure
Are C-O bonds identical in strength and length in resonance hybrid structures?
Yes they are identical however they are intermediate in strength and length between bond.
What is an intermediate bond?
One that is between a single and a double bond in length and strength
How are intermediate bonds drawn?
As dotted lines
What does the circle in benzene molecules represent and how do they help?
Delocalized electrons that exist because they are shared between more than 2 nuclei. These help give stability
What is a net dipole moment?
Sum of all bond dipoles in a molecule
Which molecules have net dipole moment?
Polar molecules
What affects molecular polarity?
• Presence of polar bonds 2. Molecular geometry of molecule
Are molecules with polar bonds always polar?
No. If they have polar bonds and are symmetrical, it will be non-polar as bond dipoles cancel each other out. Bonds can be polar but molecule may not be
Which structures generally don’t have net dipole especially when same atoms are bound to center?
Tetrahedral and trigonal planar
Is HCl polar?
Yes. There is no net dipole
Is BF3 polar?
No. It’s trigonal planar and net dipoles cancel out
CHCl2 is tetrahedral so why is it still polar?
It has different molecules bound to center with different electronegativity so bonds don’t cancel
What is VSEPR theory?
Valence shell electron pair repulsion theory. It predicts the shape of molecules
What type of shape will a molecule adopt?
One that minimizes repulsion