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Chemistry HL IB 2020-22 > 4.3 Covalent structures > Flashcards

4.3 Covalent structures Flashcards

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1
Q

Octet rule

A

States that most stable arrangement for an atom is to have 8 electron in outermost energy level or configuration of a noble gas

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2
Q

What are exceptions to octet rule?

A
  • 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+
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3
Q

Incomplete octet

A

When an atom is electron deficient

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4
Q

Why is BF3 stable even though it’s electron deficient?

A

Boron has formed max. no of bonds possible

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5
Q

Why is BeCl2 stable?

A

Because Be has only 2 electrons to give

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6
Q

What are Lewis structures and what must they show?

A

They represent bonding in a molecule and must show bonding and non-bonding electrons

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7
Q

Calculate no. of bonding electrons for CN-

A

C= 4e-
N= 5e-
-= 1-
Compound has 2 elements so must have 16 to be stable so there are 6 bonding electrons

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8
Q

What are resonance structures?

A

Formed when there are more than 1 Lewis structures for the same molecule/polyatomic ion

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9
Q

CO3 2- has how many resonance structures and why?

A

3 as double bond can be on any CO bond

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10
Q

What is the actual structure for a molecule called?

A

Resonance hybrid structure

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11
Q

Are C-O bonds identical in strength and length in resonance hybrid structures?

A

Yes they are identical however they are intermediate in strength and length between bond.

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12
Q

What is an intermediate bond?

A

One that is between a single and a double bond in length and strength

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13
Q

How are intermediate bonds drawn?

A

As dotted lines

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14
Q

What does the circle in benzene molecules represent and how do they help?

A

Delocalized electrons that exist because they are shared between more than 2 nuclei. These help give stability

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15
Q

What is a net dipole moment?

A

Sum of all bond dipoles in a molecule

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16
Q

Which molecules have net dipole moment?

A

Polar molecules

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17
Q

What affects molecular polarity?

A
  1. Presence of polar bonds

2. Molecular geometry of molecule

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18
Q

Are molecules with polar bonds always polar?

A

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

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19
Q

Which structures generally don’t have net dipole especially when same atoms are bound to center?

A

Tetrahedral and trigonal planar

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20
Q

Is HCl polar?

A

Yes. There is no net dipole

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21
Q

Is BF3 polar?

A

No. It’s trigonal planar and net dipoles cancel out

22
Q

CHCl2 is tetrahedral so why is it still polar?

A

It has different molecules bound to center with different electronegativity so bonds don’t cancel

23
Q

What is VSEPR theory?

A

Valence shell electron pair repulsion theory. It predicts the shape of molecules

24
Q

What type of shape will a molecule adopt?

A

One that minimizes repulsion

25
Q

What do electron domains consist of?

A

Bonding and non-bonding electrons

26
Q

Are single, double and triple bonds counted as multiple electrons domains?

A

No. Just a single domain

27
Q

Which causes more repulsion: Lone pairs or bonding domains?

A

Lone pairs

28
Q

What is electron domain geometry?

A

Total number of electron domains around a central atom

29
Q

How is molecular geometry different from electron domain geometry?

A

It often shows extra repulsion between a bonding and non-bonding pair

30
Q

What is the electron domain geometry when there are 2 electron domains and what are the bond angles?

A

Linear

Bond angle= 180

31
Q

Examples of linear molecules

A

CO2 and C2H2

32
Q

What are the molecular geometries when there are 3 electron domains and what are the bond angles?

A 
(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
33
Q

What are the molecular geometries when there are 4 electron domains and what are the bond angles?

A 
(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
34
Q

Why do bond angles become smaller as lone pairs increase?

A

There is stronger repulsion between lone pairs and bonding pairs

35
Q

H3O + ion has a lone pair on oxygen. What is its molecular and electron domain geometry?

A

Electron domain: Tetrahedral
Molecular:
Trigonal pyramidal

36
Q

What are 4 different allotropes of carbon?

A
  1. Diamond
  2. Graphite
  3. Fullerene
  4. Graphene
37
Q

Allotropes

A

Different forms of same element in same physical state. They have different physical properties

38
Q

Diamond: Structure

A

1 carbon atom bonded to 4 other carbons in a tetrahedral arrangement

39
Q

Diamond: Properties and reason

A 
Strong covalent bonds 
-Hard 
-High M.P. and B.P.
-Insoluble 
No delocalized electrons 
-Poor electrical conductor
40
Q

Graphite: Structure

A

1 carbon bonded to 3 carbon atoms in trigonal planar arrangement.
Layered structure= Carbon atoms are in fused hexagonal ring

41
Q

Graphite: What are layers held by?

A

Weak London dispersion forces

42
Q

Graphite: Properties and reason

A 
Free delocalized electrons 
-Able to conduct electricity 
Layers slide 
-Soft 
Electrons reflect light 
- Dark shiny grey unlike colorless diamond
  • Also insoluble
43
Q

Fullerene (C60): Structure

A
  • 1 carbon to 3 carbon atoms, trigonal planar

- 20 hexagons and 12 pentagons of carbon i.e. truncated icosahedron

44
Q

Fullerene: Why is it a worse electrical conductor than graphite?

A

It has delocalized electrons but cannot jump between different fullerene. It’s also spherical

45
Q

Fullerene: Is it soluble?

A

Insoluble in water but soluble in organic compounds like benzene

46
Q

Fullerene: Why is it (C60) not considered a macromolecule

A

It is large but carbon number is not fixed

47
Q

Graphene

A

Single layers of graphite

48
Q

Graphene: Bond angle and structure

A

Trigonal planar and bond angle of 120

49
Q

Graphene: Properties

A
  • High tensile strength (1000x steel)

- High conductivity (electric and thermal)

50
Q

Graphene: Uses based on properties

A
  • 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
51
Q

Graphene: Why is it the most chemically reactive?

A

Carbon atoms on edges have unoccupied bonds

Chemistry HL IB 2020-22 (19 decks)

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