Topic D: Models of Bonding and Structure - Covalent

Question 1

Definition of covalent bond

Answer 1

Electrostatic attraction between a shared pair of electrons and the nuclei of the atoms being bonded

Question 2

What are some exceptions to the octet rule

Answer 2

Molecules whose central atoms have fewer than 8 electrons after bonding (usually compounds in Group 2 and 3)

Molecules whose atoms have more than 8 electrons after bonding (usually compounds in period 3 onwards)

Molecules with an off number of electrons

Question 3

What factors affect bond strength

Answer 3

Bond length
Bond multiplicity
Bond polarity

Question 4

How does bond length affect bond strength

Answer 4

The longer the bond length, the further from the shared pair of electrons the nuclei are, thus the bond energy and strength will be lower

Question 5

How does bond multiplicity affect bond strength

Answer 5

Bond multiplicity refers to the presence of multiple covalent bonds between 2 atoms
The presence of multiple bonds increase the overall bond strength between the 2 atoms

Question 6

How does bond polarity affect bond strength

Answer 6

An increase in bond polarity means an increase in the ionic character of the bond, which usually results in an increase in bond strength and energy

Question 7

What is a coordination bond

Answer 7

A covalent bond in which the shared pair of electrons is contributed by only one of the bonded atoms. This atom donates both electrons to the shared pair

Question 8

What are the criteria for the formation of a coordination bond

Answer 8

Donnor atom must possess an unshared pair of electrons
Acceptor atom must possess an empty orbital in its valence shell

Question 9

How is the VSEPR theory applied

Answer 9

Any atom in a molecule or ion bonded to more than one atom is known as the central atom (eg in H2O, O is the central atom)

Each bonded atom or lone pair of electrons joined to the central atom constitutes an electron domain

A multiple bond is to be considered as 1 electron domain for the purpose of determining the overall geometry

Electron pairs around the central atom arrange themselves as far as possible to minimise repulsion

Question 10

What is the order of the strength of repulsion in the VSEPR theory

Answer 10

From strongest to weakest
Lone pair - lone pair repulsion
Lone pair- bond pair repulsion
Bond pair - bond pair repulsion

Question 11

Definition of electronegativity

Answer 11

The relative attraction that an atom has for the shared pair of electrons in a covalent bond

Question 12

How does electronegativity change going across a period

Answer 12

Going across a period, the nuclear charge increases but the shielding effect remains relatively constant, so the effective nuclear charge increases
Thus atoms get smaller, resulting in a decreased distance between bonding electrons and the nuclei
Electrostatic attraction between the bond and nuclei increases, so electronegativity increases

Question 13

How does electronegativity change going down a group

Answer 13

Going down the group, atoms get larger, resulting in an increased distance between bonding electrons and the nuclei
The increase in distance results in a decrease in electrostatic attraction between bonding electrons and the nuclei of the atoms, so electronegativity decreases

Question 14

How does the difference in electronegativity of two atoms affect its polarity

Answer 14

When the difference in electronegativity increases, the polarity of the covalent bond increases
It is possible for the difference in electronegativity to increase until a stage that the covalent bond turns into an ionic bond

Question 15

How are electrons shared in a polar bond

Answer 15

Electrons are shared unequally because of the difference in electronegativity
Valence electron density distorts towards the atom with the higher electronegativity

Question 16

How does a dipole moment form

Answer 16

In a polar covalent molecule, the atom with the higher electronegativity will have a higher electron density and thus a partial negative charge
The atom with the lower electronegativity will have a lower electron density and thus a partial positive charge
The separation of charges creates a dipole

Question 17

What are the 3 types of dipoles

Answer 17

Permanent dipole
Instantaneous induced dipole (temporary)
Induced dipole

Question 18

What is a permanent dipole

Answer 18

Exists in all polar molecules as a result of the difference in electronegativity of bonded atoms

Question 19

What is an instantaneous induced dipole (temporary)

Answer 19

In non-polar molecules, there are no permanent dipoles because electron clouds are symmetrically distributed around the molecule
However due to fluctuations of electron clouds, positive and negative charges can arise temporarily, giving rise to instantaneous induced dipoles

Question 20

What is an induced dipole

Answer 20

When a molecule having a dipole comes close to a non-polar molecule, a temporary dipole will be induced on the non-polar molecule

Question 21

What is the criteria for a covalent molecule to be polar

Answer 21

There must be at least one polar bond
The polar bonds must not cancel, as there should be a net dipole

Question 22

What are physical properties of giant covalent structures

Answer 22

Hard
High melting and boiling points
Insoluble in polar solvents and water
Low electrical conductivity

Question 23

What are the properties of simple covalent structures

Answer 23

Usually soft or brittle
Low melting and boiling points
Solubility in water and organic solvents depends on its polarity
Low electrical conductivity

Question 24

What are the 3 types of intermolecular forces

Answer 24

London Dispersion force
Dipole-dipole forces
Hydrogen bonding

Question 25

How are London dispersion forces formed

Answer 25

The electron density on a non-polar molecule is usually evenly distributed.

However, the electrons in a molecule are in continual motion, so temporary fluctuations in the electron density of the molecule result in a non-symmetrical electron cloud and an instantaneous induced dipole.

This temporary dipole can induce a similar dipole in neighbouring molecules so that molecules attract each other

Question 26

Why are London dispersion forces weak

Answer 26

These attractions tend to be temporary and fluctuating in nature, thus making them generally weak compared to covalent bonds

Question 27

What factors affect the strength of London dispersion forces

Answer 27

When the number of electrons increases, the size of the electron cloud also increases, causing London dispersion forces to become stronger.

When the size of the molecule increases, the size of the electron cloud also increases, causing London dispersion forces to become stronger.

Question 28

How are dipole-dipole forces formed

Answer 28

There is an electrostatic attraction between the oppositely charged ends of two polar molecules with permanent dipoles

This is called the dipole-dipole forces

Question 29

What factors affect the strength of dipole-dipole forces

Answer 29

The polarity of the molecule
The greater the electronegativity of between the bonded atoms, the larger the bond dipoles and the stronger the attraction between oppositely charged dipoles

Question 30

What are the requirements for hydrogen bonding to occur

Answer 30

For one molecule, a hydrogen atom is attached to a highly electronegative atom such as N, O or F
For the other molecule, there is at least one lone pair of electrons on the electronegative atom

Question 31

How does hydrogen bonding occur

Answer 31

When a hydrogen atom is bonded to a highly electronegative atom such as N, O or F, the bonding electron pair is drawn towards the more electronegative atom

The electron density of the hydrogen is reduced and it becomes slightly positively charged. The highly electronegative atom of another molecule possessing lone pairs of electrons can share its electron density with the positively charged hydrogen atom

Question 32

What does the number of hydrogen bonds formed per molecule depend on

Answer 32

Number of lone pairs available on the electronegative atom
Number of H atoms covalently bonded to the electronegative atom

Question 33

What is the order of the strength of intermolecular forces of attraction from strongest to weakest

Answer 33

Hydrogen bonding
Dipole-dipole
London dispersion

Question 34

What are factors affecting the degree of polarisation

Answer 34

Charge and size of cation
Charge and size of anion

Question 35

How does the charge and size of cation affect the degree of polarisation

Answer 35

The higher the cationic charge, the greater is the polarising power
The smaller the ionic radius of the cation, the greater the polarising power

Question 36

How does the charge and size of anion affect the degree of polarisation

Answer 36

The higher the anionic charge, the greater the polarisability
The larger the ionic radius of the anion, the greater the polarisability