4.3 Covalent structures

Question 1

Octet rule

Answer 1

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

Question 2

What are exceptions to octet rule?

Answer 2

• 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+

Question 3

Incomplete octet

Answer 3

When an atom is electron deficient

Question 4

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

Answer 4

Boron has formed max. no of bonds possible

Question 5

Why is BeCl2 stable?

Answer 5

Because Be has only 2 electrons to give

Question 6

What are Lewis structures and what must they show?

Answer 6

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

Question 7

Calculate no. of bonding electrons for CN-

Answer 7

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

Question 8

What are resonance structures?

Answer 8

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

Question 9

CO3 2- has how many resonance structures and why?

Answer 9

3 as double bond can be on any CO bond

Question 10

What is the actual structure for a molecule called?

Answer 10

Resonance hybrid structure

Question 11

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

Answer 11

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

Question 12

What is an intermediate bond?

Answer 12

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

Question 13

How are intermediate bonds drawn?

Answer 13

As dotted lines

Question 14

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

Answer 14

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

Question 15

What is a net dipole moment?

Answer 15

Sum of all bond dipoles in a molecule

Question 16

Which molecules have net dipole moment?

Answer 16

Polar molecules

Question 17

What affects molecular polarity?

Answer 17

1. Presence of polar bonds

2. Molecular geometry of molecule

Question 18

Are molecules with polar bonds always polar?

Answer 18

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

Question 19

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

Answer 19

Tetrahedral and trigonal planar

Question 20

Is HCl polar?

Answer 20

Yes. There is no net dipole

Question 21

Is BF3 polar?

Answer 21

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

Question 22

CHCl2 is tetrahedral so why is it still polar?

Answer 22

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

Question 23

What is VSEPR theory?

Answer 23

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

Question 24

What type of shape will a molecule adopt?

Answer 24

One that minimizes repulsion

Question 25

What do electron domains consist of?

Answer 25

Bonding and non-bonding electrons

Question 26

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

Answer 26

No. Just a single domain

Question 27

Which causes more repulsion: Lone pairs or bonding domains?

Answer 27

Lone pairs

Question 28

What is electron domain geometry?

Answer 28

Total number of electron domains around a central atom

Question 29

How is molecular geometry different from electron domain geometry?

Answer 29

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

Question 30

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

Answer 30

Linear | Bond angle= 180

Question 31

Examples of linear molecules

Answer 31

CO2 and C2H2

Question 32

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

Answer 32

``` (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 ```

Question 33

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

Answer 33

``` (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 ```

Question 34

Why do bond angles become smaller as lone pairs increase?

Answer 34

There is stronger repulsion between lone pairs and bonding pairs

Question 35

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

Answer 35

Electron domain: Tetrahedral Molecular: Trigonal pyramidal

Question 36

What are 4 different allotropes of carbon?

Answer 36

1. Diamond 2. Graphite 3. Fullerene 4. Graphene

Question 37

Allotropes

Answer 37

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

Question 38

Diamond: Structure

Answer 38

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

Question 39

Diamond: Properties and reason

Answer 39

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

Question 40

Graphite: Structure

Answer 40

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

Question 41

Graphite: What are layers held by?

Answer 41

Weak London dispersion forces

Question 42

Graphite: Properties and reason

Answer 42

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

Question 43

Fullerene (C60): Structure

Answer 43

- 1 carbon to 3 carbon atoms, trigonal planar | - 20 hexagons and 12 pentagons of carbon i.e. truncated icosahedron

Question 44

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

Answer 44

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

Question 45

Fullerene: Is it soluble?

Answer 45

Insoluble in water but soluble in organic compounds like benzene

Question 46

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

Answer 46

It is large but carbon number is not fixed

Question 47

Graphene

Answer 47

Single layers of graphite

Question 48

Graphene: Bond angle and structure

Answer 48

Trigonal planar and bond angle of 120

Question 49

Graphene: Properties

Answer 49

- High tensile strength (1000x steel) | - High conductivity (electric and thermal)

Question 50

Graphene: Uses based on properties

Answer 50

- 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

Question 51

Graphene: Why is it the most chemically reactive?

Answer 51

Carbon atoms on edges have unoccupied bonds