2.2 - The covalent model

Question 1

Define covalent bond

Answer 1

A covalent bond consists of a shared pair of electrons between two non-metals. There are single, double, and triple bonds.

Question 2

Explain the properties of simple covalent structures

Answer 2

Most substances that contain simple covalent structures have low melting and boiling points meaning they are gases or liquids at room temperature.
This is because the covalent bonds within the molecules are strong whilst the bonds between the molecule are weak.

Question 3

Explain polar bonds

Answer 3

A bond is polar when the two elements sharing electrons share them unevenly due to their electronegativity values.
This creates a dipole, the more electronegative has become partially negative and the less electronegative atom has become partially positive.

Question 4

What happens when the difference in electronegativity increases?

Answer 4

The bond becomes more polar.

Question 5

List the bond continuum

Answer 5

Ionic - polar covalent - pure covalent

Question 6

Explain dative covalent bonds

Answer 6

Dative covalent bonds are formed when both electrons of the shared pair originate from the same atom.

Question 7

What makes a molecule polar?

Answer 7

A molecule possessing polar bonds doesn’t necessarily mean that the molecule as a whole will be polar. The molecule is polar if the electron densities are not symmetrical such that there is a net dipole movement.

Question 8

State VSEPR theory

Answer 8

Electron domains (bonding and nonbonding pairs) repel each other.

Question 9

List the electron group structures for four electron domains.

Answer 9

• 4 bonding groups - tetrahedral - 109.5
• 3 bonding groups - pyramidal - 107
• 2 bonding groups - bent - 105

Question 10

List the electron group structures for three electron domains

Answer 10

• 3 bonding groups - trigonal planar - 120
• 2 bonding groups - bent - <120

Question 11

List the electron group structures for two electron domains

Answer 11

• linear - 180

Question 12

Do bond pairs or lone pairs have more repulsion?

Answer 12

Lone pairs

Question 13

What is the difference between single and double bonds for repulsion purposes?

Answer 13

None; they are essentially the same for the purposes of repulsion.

Question 14

Explain Resonance Structures

Answer 14

When a lewis structure allows for two different but equally valid arrangement of electrons, resonance occurs. The electrons are said to be delocalized and are spread over two bonding orbitals.

Question 15

Define allotropes

Answer 15

Allotropes are compounds of the same element that differ in structure.

Question 16

List 3 allotropes of carbon

Answer 16

• Diamond
• Graphite
• Fullerenes (buckyballs and nanotubes)

Question 17

List the properties of diamond

Answer 17

• Each carbon is bonded to 4 other carbons
• High melting point due to strong covalent bonds (3550°)
• Extremely hard because its difficult to break atoms apart
• No conductivity because electrons are localized in specific bonds
• Insoluble in solvents because molecular bonds are stronger than any intermolecular forces

Question 18

List the properties of graphite

Answer 18

• Each carbon is bonded to 3 other carbons forming sheets of graphene that slide over each other.
• Conducts electricity due to delocalized electrons
• Slippery, can be used as lubricant, sheets can easily slip past each other.
• High melting point (slightly higher than diamond at 3653°)
• • Insoluble in solvents because molecular bonds are stronger than any intermolecular forces

Question 19

List the properties of fullerenes

Answer 19

• Buckyballs: spherical
• Nanotubes: tube shaped
• Both are:
• Very strong
• Conduct electricity with low resistance
• Free radical scavenger

Question 20

Explain the structure of silicon dioxide

Answer 20

• SiO2 is repeating as every oxygen bonds with a second Si atom
• Structurally tetrahedral

Question 21

Define network solids

Answer 21

Network solids are a special type of covalent compound where there are no molecules or discrete particles but rather, all the atoms are covalently bonded together into one giant crystal.