L8 - Chemical Bonding (2)

Question 1

Describe the importance of covalent bonds.

Answer 1

• Bonding in organic compounds = building blocks of life
• Determines the shape of molecules
• Responsible for interactions between proteins and receptors / drugs
• Can cause permanent enzyme inhibition

Question 2

What does polarity of a bond determine?

Answer 2

Intermolecular forces

Question 3

Polarity of a ____ and polarity of a ________ are different.

Answer 3

Polarity of a bond and polarity of a molecule are different.

Question 4

What is a covalent bond?

Answer 4

Bond formed by the electrostatic attraction between oppositely charged nuclei through the sharing od electrons.

Question 5

Why are covalent bonds formed between atoms?

Answer 5

Both atoms aim to attain a stable configuration of the nearest inert gas by sharing one, two or three electron pairs between the combining atoms.

Question 6

Where are the shared pair of electrons in a covalent bond found?

Answer 6

Shared pair of electrons are localised in a definite space between the nuclei of the 2 atoms.

Question 7

What type of bond is a covalent bond?

Answer 7

Directional

Question 8

What atoms is a covalent bond established between?

Answer 8

Established between the atoms of same / different non-metallic elements.

Question 9

What happens when atomic orbitals overlap?

Answer 9

Molecular orbitals are formed.

Question 10

Describe the steps for forming correlation diagrams (molecular orbital diagrams).

Answer 10

1) Determine the electronic structure of the atoms involved

2) Construct a correlation diagram:
- Y-axis is energy
- X-axis has atoms on either side with molecule in middle
- Orbitals are placed on x-axis depending on energy
- nσg formed between bonding electrons
- nσu* formed between anti-bonding electrons
- Hexagon style shape made by joining energy levels and bonds /anti-bonds
- Electrons placed on bonds and anti-bonds in the centre

3) Determine the electronic structure of the molecule
4) Calculate: Bond Order = Bonding Electrons - Anti-bonding Electrons
5) Value will provide information on the stability of the bond

Question 11

What are molecular orbitals?

Answer 11

Linear combinations of atomic orbitals

Question 12

When atoms interact to form molecules, the overall number of ________ must remain the same.

Answer 12

When atoms interact to form molecules, the overall number of orbitals must remain the same.

Question 13

How do electrons fill molecular orbitals?

Answer 13

In the same way that they do atomic orbitals; into the lowest energy level first.

Question 14

Describe a single covalent bond.

Answer 14

1 pair of electrons shared

Question 15

Describe a double covalent bond.

Answer 15

2 pairs of electrons shared

Question 16

Describe a triple covalent bond.

Answer 16

3 pairs of electrons shared

Question 17

What are Lewis structures used for?

Answer 17

Method of describing covalent bonding in polyatomic molecules.

Question 18

What is shown in Lewis structures?

Answer 18

Show valence electrons as dots / crosses.

Question 19

Describe the steps for drawing Lewis structures.

Answer 19

1) Draw the electron configuration of the atoms involved
2) Count the valence electrons for all atoms to determine the total number of electrons in the molecule
3) Use pairs of electrons to form a single bond between atoms (bonding pairs)
4) Arrange the remaining electrons around atoms (lone pairs) to satisfy the octet rule
5) If you run out of electrons, use multiple bonds to complete octets

Question 20

Hydrogen does not follow the octet rule. What rule does it follow?

Answer 20

The duet rule

Question 21

What factors favour covalent bonding?

Answer 21

• High ionisation energies
• Equal electron affinities
• Equal / similar electronegativity
• High nuclear charge
• Small atomic size
• Number of valence electrons (e.g. gaining / losing 4 valence electrons is very hard, so carbon forms covalent bonds)

Question 22

Which elements form covalent bonds?

Answer 22

Elements on the right hand side of the periodic table

Question 23

Which atoms form covalent bonds?

Answer 23

Same / different kinds of atoms with high electronegativity

Question 24

Describe the features of covalently bonded molecules.

Answer 24

• Don’t ionise

- Equal attraction to electrons to complete octets

Question 25

Define Polar Bond.

Answer 25

Bond between 2 atoms of significantly different electronegativity.

Question 26

Define Non-Polar Bond.

Answer 26

Bond between 2 atoms of equal (or very similar) electronegativity.

Question 27

Describe the features of Polar Bonds.

Answer 27

• Electrons shared unequally
• Separation of charge between one atom and the other
• Greater electron density around the more electronegative atom
• Has an overall dipole (due to partial charges on atoms)

Question 28

Describe the features of Non-Polar Bonds.

Answer 28

• Electrons are shared evenly
• Average bonding pair is located halfway between the 2 atoms
• No overall dipole

Question 29

What do Polar Bonds affect?

Answer 29

• Reactivity of the bond

- Types of intramolecular forces between molecules

Question 30

Summarise the different types of bonds, with reference to electronegativity.

Answer 30

• Pure non-polar covalent bond: No electronegativity difference between 2 atoms
• Polar covalent bond: Small electronegativity difference between 2 atoms (difference < 0.5)
• Ionic bond: Large electronegativity difference between 2 atoms (difference > 0.5)

Question 31

Describe the features of covalent compounds.

Answer 31

• Lattice of strong covalent bonds
• Hard, brittle, insoluble
• High m.p.

Question 32

Compare the physical states of Covalent Compounds and Ionic Compounds.

Answer 32

• Covalent Compounds: Gases, liquids, solids

- Ionic Compounds: Crystalline solids

Question 33

Compare the m.p. and b.p. of Covalent Compounds and Ionic Compounds.

Answer 33

• Covalent Compounds: Depends on size and intermolecular bonding
• Ionic Compounds: High m.p. and b.p.

Question 34

Compare the conductivity of Covalent Compounds and Ionic Compounds.

Answer 34

• Covalent Compounds: Mostly poor

- Ionic Compounds: Good when molten

Question 35

Compare the solubility of Covalent Compounds and Ionic Compounds.

Answer 35

• Covalent Compounds: Depends on intermolecular bonding

- Ionic Compounds: Many are soluble in water, but not in non-polar liquids

Question 36

Define Allotrope.

Answer 36

Each of two or more different physical forms in which an element can exist.

Question 37

State the 3 allotropes of carbon.

Answer 37

• Graphite
• Diamond
• Silicon Dioxide

Question 38

Describe the electrical conductivity of Graphite, Diamond and Silicon Dioxide.

Answer 38

• Graphite: Conducts electricity (in one direction if pure) as delocalised electrons carry charge
• Diamond: Does not conduct electricity
• Silicon Dioxide: Semi-conductor (conducts electricity under some conditions; is a good medium for the control of electric current)

Question 39

Describe the bonding in Graphite.

Answer 39

Weak intermolecular forces between sheets, allows layers to slide over each other = soft / slippery, lubricant

Question 40

How many bonds can Group 6 elements form?

Answer 40

Up to 4 strong bonds between atoms

Question 41

Describe the properties of Giant Covalent Structures.

Answer 41

• Contain many atoms similar to an ionic lattice
• Strong, hard, non-conductive
• Insoluble in all solvents

Question 42

Describe how covalent compounds are named.

Answer 42

• 2 word names
• Second element has -ide ending
• Prefix indicates number of atoms (not valence)
• First vowel often dropped to avoid ‘ao’ and ‘co’ combination