4.3 Covalent structures Flashcards
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
What do electron domains consist of?
Bonding and non-bonding electrons
Are single, double and triple bonds counted as multiple electrons domains?
No. Just a single domain
Which causes more repulsion: Lone pairs or bonding domains?
Lone pairs
What is electron domain geometry?
Total number of electron domains around a central atom
How is molecular geometry different from electron domain geometry?
It often shows extra repulsion between a bonding and non-bonding pair
What is the electron domain geometry when there are 2 electron domains and what are the bond angles?
Linear
Bond angle= 180
Examples of linear molecules
CO2 and C2H2
What are the molecular geometries when there are 3 electron domains and what are the bond angles?
(E.G and M.G) Trigonal planar - Bond angle = 120 (M.G) V shaped - Depends on presence of lone pairs - Stronger repulsion - Bond angle= slightly less than 120
What are the molecular geometries when there are 4 electron domains and what are the bond angles?
(E.G and M.G) Tetrahedral -Bond angles = 109.5 (M.G) Trigonal pyramidal -One lone pair -Bond angles= 107.8 (M.G) V-shaped - Two lone pairs -Bond angles= 104.5
Why do bond angles become smaller as lone pairs increase?
There is stronger repulsion between lone pairs and bonding pairs
H3O + ion has a lone pair on oxygen. What is its molecular and electron domain geometry?
Electron domain: Tetrahedral
Molecular:
Trigonal pyramidal
What are 4 different allotropes of carbon?
- Diamond
- Graphite
- Fullerene
- Graphene
Allotropes
Different forms of same element in same physical state. They have different physical properties
Diamond: Structure
1 carbon atom bonded to 4 other carbons in a tetrahedral arrangement
Diamond: Properties and reason
Strong covalent bonds -Hard -High M.P. and B.P. -Insoluble No delocalized electrons -Poor electrical conductor
Graphite: Structure
1 carbon bonded to 3 carbon atoms in trigonal planar arrangement.
Layered structure= Carbon atoms are in fused hexagonal ring
Graphite: What are layers held by?
Weak London dispersion forces
Graphite: Properties and reason
Free delocalized electrons -Able to conduct electricity Layers slide -Soft Electrons reflect light - Dark shiny grey unlike colorless diamond
- Also insoluble
Fullerene (C60): Structure
- 1 carbon to 3 carbon atoms, trigonal planar
- 20 hexagons and 12 pentagons of carbon i.e. truncated icosahedron
Fullerene: Why is it a worse electrical conductor than graphite?
It has delocalized electrons but cannot jump between different fullerene. It’s also spherical
Fullerene: Is it soluble?
Insoluble in water but soluble in organic compounds like benzene
Fullerene: Why is it (C60) not considered a macromolecule
It is large but carbon number is not fixed
Graphene
Single layers of graphite
Graphene: Bond angle and structure
Trigonal planar and bond angle of 120
Graphene: Properties
- High tensile strength (1000x steel)
- High conductivity (electric and thermal)
Graphene: Uses based on properties
- Behaves as semi metal so suitable for electronic device
- Addition of 1% of graphene to plastic helps conduct electricity
- Permeable so suitable for desalination and water purification
Graphene: Why is it the most chemically reactive?
Carbon atoms on edges have unoccupied bonds
