Ionic & Covalent Bonding

Question 1

What does the octet ‘rule’ state?

Answer 1

It states that atoms reacting tend to reach an electron arrangement with eight outer electrons (i.e. a full outer shell).

Question 2

What are the exceptions to the ‘octet rule’?

Answer 2

• Some atoms with low amounts of protons tend to prefer an outer shell of only two electrons.
• D-block (transition elements in the middle of the table) tend not to obey this rule.
• The rule works inconsistently in the case of phosphorous and silicon atoms.

Question 3

Describe ‘water of crystallisation’.

Answer 3

Water that has been chemically combined (in definite proportions) in a crystalline compound.

Question 4

What is meant by the ‘valency’ of an element?

Answer 4

The number of bonds the elements forms when it reacts.

(e.g) Carbon forms four bonds when it reacts, so it has a valency of four.

Question 5

Name an element that has a variable valency.

Answer 5

Any of;

Chromium, Manganese, Iron, Copper.

Question 6

What is an ‘ionic bond’?

Answer 6

The electrostatic force of attraction between two oppositely charged ions.

Question 7

What type of structure does ionic bonding result in?

Answer 7

A rigid, 3D crystal lattice.

NOT a molecule…

Question 8

Name some characteristics of ionic substances.

Answer 8

• High melting point. (Strong bonds)
• No electrons free for conduction as a solid, doesn’t conduct. Will conduct as a liquid.
• Usually soluble in water due to polarity of water vs. charge on ions.

Question 9

Give an application of any ionic substance.

Answer 9

Sodium fluoride is added to water supplies in many countries to prevent tooth decay.

Question 10

What type of structures are formed due to covalent bonding?

Answer 10

Molecules.

Question 11

When is a covalent bond formed?

Answer 11

When two atoms share a pair of electrons.

Question 12

What’s the difference between ‘sigma’ and ‘pi’ bonds?

Answer 12

A sigma bond involves a bond between a p or s orbital (p-p, p-s, s-s) which overlap end-on.

A pi bond involves a bond between two p orbitals that overlap side-on.

-> Note that sigma bonds are stronger than pi bonds because there is a larger area of overlap between orbitals.

Question 13

What is the main difference between polar and non-polar molecules?

Answer 13

Non-polar molecules have an equal distribution of charge.

Polar molecules have an unequal distribution of charge.

Question 14

Which are stronger, intermolecular forces or intramolecular forces?

Answer 14

Intramolecular forces (within the molecule).

Question 15

Name some characteristics of covalent substances.

Answer 15

• Due to weak intermolecular (between molecules) forces, most are liquid or gas at room temperature.
• They usually do not conduct electricity in solution (neutral atoms rather than charged ions).
• They usually are not very soluble in water.

Question 16

What does ‘electronegativity’ refer to?

Answer 16

It is a measure of the relative attraction of an atom for shared pairs of electrons in a covalent bond.

Question 17

Explain why electronegativity values decrease down a group.

Answer 17

The atomic radius increases, as does the screening effect, meaning that the outer (bonded) electrons are less attracted to the nucleus.

Question 18

Explain why electronegativity increases across a group.

Answer 18

The nuclear charge increases, causing a smaller atomic radius, leading to a larger value for electronegativity.

Also, the screening effect does not increase across a group.

Question 19

What differences in electronegativity are used to predict the nature of bonds?

Answer 19

• If the atoms do not differ (diatomic) or the difference is extremely small, the bond can be predicted to be non-polar covalent.
• If the atoms differ by an amount larger than this (but less than about 1.7) the bond would be predicted to be polar covalent.
• If they differ by more than 1.7, they are likely ionically bonded.