2.2 : Electrons, bonding and structure - covalent and dative bonding

Question 1

What is a covalent bond?

Answer 1

A covalent bond is the electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.

Question 2

Suggest one difference between ionic and covalent bonding

Answer 2

Ionic attractions act in all directions whereas covalent bonds only act in one direction.

Question 3

What is a lone pair?

Answer 3

An outer shell pair of electrons that is not involved in chemical bonding. A lone pair gives a concentrated region of negative charge around the atom.

Question 4

How is a covalent bond formed?

Answer 4

By the overlap of two orbitals

Question 5

What quantity is used as a measure of the strength of covalent bonds?

Answer 5

Average bond enthalpy

Question 6

What is a dative covalent bond?

Answer 6

A covalent bond formed when the shared pair of electrons has been provided by one of the bonding atoms only.

Question 7

What ions are responsible for the reactions of acids?

Answer 7

H3O+, the hydronium ion

Question 8

Why is the octet rule not always followed?

Answer 8

• not enough electrons to reach the octet, eg, Be and B
• more than 4 electrons may pair up in bonding - expansion of the octet - eg, elements in groups 15-17 period 3

Question 9

Describe what happens to BF3 when bonding, in terms of the octet rule

Answer 9

B has 3 outer shell electrons. Each F atom has 7 outer shell electrons.
- 3 covalent bonds can be formed
- each of B’s 3 outer electrons is paired, so there are now 6 electrons in it’s outer shell
- each of the F atoms now has 8 outer electrons, achieving an octet
- B does not achieve an octet

Question 10

Describe what happens to SF6 when bonding, in terms of the octet rule

Answer 10

S has 6 outer shell electrons. Each F atom has 7 outer shell electrons.
- so 6 covalent bonds can be formed
- each of S’s 6 electrons are paired, so S now has 12 outer electrons - it no longer follows the octet rule - it has expanded the octet
- each of the 6 F atoms now has 8 outer electrons, achieving the octet.

Question 11

Describe a better version of the octet rule

Answer 11

• unpaired electrons pair up
• the max. number of electrons that can pair up is equivalent to the number of electrons in the outer shell

Question 12

What are the two types of covalent structure and some examples of each?

Answer 12

1. simple molecular structure : Ne, H2, O2, N2, H2O
2. giant covalent lattice : diamond, graphite, SiO2

Question 13

Explain the properties of simple molecular structures

Answer 13

• low melting and boiling points because the molecules are held together by weak intermolecular forces, so only a small amount of energy is needed to break them
• simple molecular structures do not conduct electricity because there are no charged particles free to move
• simple molecular structures are generally soluble in non-polar solvents, e.g. hexane

Question 14

Explain the properties of giant covalent structures

Answer 14

• high melting and boiling points because lots of energy is needed to break the strong covalent bonds within the lattice
• giant covalent structures do not conduct electricity as there are no charged particles free to move, except for graphite which is able to conduct as it has delocalised electrons between the layers which are able to move freely and carry charge through the structure
• giant covalent structures are insoluble as the covalent bonds in the lattice are too strong to be broken by either polar or non-polar solvents

Question 15

What is a giant covalent lattice?

Answer 15

A giant covalent lattice is a 3D structure of atoms that are all bonded together by strong covalent bonds