IB SL Structure 2.2: Covalent bonding

Explain what happens with electrons in covalent bonding

In covalent bonding, atoms share electrons.

Covalent bonding (definition)

is the electrostatic attraction between the shared pair of electrons and the nuclei of the atoms making up the bond

Lewis structure of CO₂

Lewis structure of PCl₃

Lewis structure of NO₃^-

Lewis structure of BF₃

Lewis structure of H₂O

What are the steps to draw a Lewis structure

1. Identify the central atom

2. Calculate the total number of valence electron pairs

3. Connect all atoms to the central atom with single bonds (1 single bond = 1 electron pair)

4. Assign lone pairs on the side atoms, so that the octet (4 pairs) is achieved on the side atoms

5. If there are electron pairs left, assign them to the central atom

6. If the central atom does not have an octet (has less than 4 electron pairs), move lone pairs from the side atoms to form double bonds

Why are single bonds weaker than triple?

• single bonds have one shared electron pair (as opposed to 3 pairs in triple bonds)

• the electrostatic attraction between shared electrons and nuclei is weaker in a single bond.

Why are single bonds longer than triple?

• single bonds have one shared electron pair (as opposed to 3 pairs in triple bonds)

• so the electrostatic attraction between shared electrons and nuclei is weaker in a single bond, so the distance between nuclei is larger.

A dative covalent bond (coordinate bond)

is a covalent bond in which both electrons come from the same atom

State VSEPR assumptions

1. Electron domains in the valence shell of the central atom in a molecule repel each other taking position to minimise these repulsions

2. The repulsion strength:

lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair

State the shape and bond angle around an atom that has:

• 2 atoms bonded to the central atom

• 0 lone pairs

Shape: linear

Bond angle: 180⁰

Electron domain geometry: linear

State the shape and bond angle around an atom that has:

• 2 atoms bonded to the central atom

• 1 lone pairs

Shape: V-shaped/bent

Bond angle: 117⁰

Electron domain geometry: trigonal planar

State the shape, bond angle, and electron domain geometry around an atom that has:

• 2 atoms bonded to the central atom

• 2 lone pairs

Shape: V-shaped/bent

Bond angle: 104.5⁰

Electron domain geometry: tetrahedral

State the shape and bond angle around an atom that has:

• 3 atoms bonded to the central atom

• 0 lone pairs

Shape: trigonal planar

Bond angle: 120⁰

Electron domain geometry: trigonal planar

State the shape and bond angle around an atom that has:

• 3 atoms bonded to the central atom

• 1 lone pairs

Shape: trigonal pyramidal

Bond angle: 107⁰

Electron domain geometry: tetrahedral

State the shape and bond angle around an atom that has:

• 4 atoms bonded to the central atom

• 0 lone pairs

Shape: tetrahedral

Bond angle: 109.5⁰

Electron domain geometry: tetrahedral

What are the conditions for a molecule to be polar?

 A molecule is polar when BOTH conditions are true:

1. The molecule has polar bonds

2. The molecule has such a shape that the dipoles do not cancel each other out.

What are 2 structures that covalent substances can form?

1. Simple molecules

2. Network (giant) covalent

Explain why CO₂ has a low melting point

• CO₂ is covalent

• CO₂ has a simple molecular structure

• Weak intermolecular forces need to be broken 

• This requires little energy

Explain why diamond has a high melting point

• Diamond has a giant (network) covalent lattice

• A lot of strong covalent bonds must be be broken

• This requires a lot of energy

Describe the structure of graphite

• Giant (network) covalent

• Each C atom is bonded to 3 other C atoms by covalent bonds.

• Delocalised electrons

• Forms layers 

Explain why graphite is soft

• No covalent bonds between layers

• So layers can slide over each other

Explain why graphite conducts electricity

It has delocalised electrons that can flow and carry charge

Describe the structure of graphene 

• Giant (network) covalent

• Each C atom is bonded to 3 other C atoms

• Forms a layer (just like graphite, but it’s only 1 layer)

• Delocalised electrons

Describe the structure of diamond

• Ginat (network) covalent

• each C atom is bonded to 4 other C atoms by covalent bonds

• No delocalised electrons

Describe the structure of fullerene

• Simple covalent (molecular)

• In fullerene, each C atom is bonded to 3 other C atoms by covalent bonds.

• Hexagons and pentagons of C atoms.

• Conducts electricity on the surface of the molecule.

• Hollow inside

Describe the structure of silicon dioxide

• Giant (network) covalent

• Each Si atom is bonded to 4 O atoms

• Each O atom is bonded to 2 Si atoms

• No delocalised electrons

Describe the structure of silicon dioxide

• Giant (network) covalent

• No delocalised electrons

What are the 4 types of intermolecular forces

• London

• Dipole-induced dipole

• Dipole-dipole

• Hydrogen bonding

What IMF are between CH₄ molecules?

London

What IMF are between PCl₃ molecules?

London, dipole-dipole

What IMF are between NH₃ molecules?

London, dipole-dipole, H-bonding

Paper chromatography

Paper chromatography is used to separate mixtures of soluble substances. These are often coloured substances such as food colourings, inks, dyes or plant pigments.

Rf formula