Y11 Chemistry - Covalent Bonding

What is a covalent bond?

The intramolecular force of attraction between two (nonmetal) atoms and one or more pairs of shared (bonding) electrons. Covalent bonds are directional.

What is a molecule?

A discrete group of atoms covalently bonded together.

Do polyatomic ions contain covalent bonds?

Yes, however they are not molecules.

What are some examples of covalent substances?

Cl₂, O₂, N₂, CH₄, H₂O, HBr, NH₃, HNO₃, H₂SO₄, N₂O

Why can’t covalent molecules conduct electricity?

There are no free charge carriers, all the electrons are localised to 1 (lone pairs) or 2 (bonding pairs) atoms. Covalent bonding does not involve ions. The only exceptions are acidic and basic covalent substances such as NH₃, HNO₃ and H₂SO₄, which ionise on contact with water and so conduct electricity in their aqueous form.

Why are covalent molecules soft and weak, with low-moderate melting and boiling points?

They’re only held together by intermolecular forces. Only these (typically) weak intermolecular bonds need to be broken to melt or boil a covalent substance, which also makes the molecular substances soft and weak.

When you boil a covalent molecule, do the covalent bonds break?

No, only the intermolecular forces are broken

What is a covalent network?

A covalent network is when atoms form covalent bonds with multiple neighbours, who do the same, forming a continuous array of covalently bonded atoms.

What are some elements and compounds that form covalent networks?

C (diamonds, graphite, fullerenes), B, Si, SiO₂ (silica, quartz, sand), SiC

What are the general properties of covalent network compounds?

Nonconductor of electricity - With the exception of graphite, in most covalent networks electrons are held within bonds or individual atoms.

Very hard and brittle - With the exception of graphite, in most covalent networks the entire structure is bonded with strong directional intramolecular covalent bonds which give these substances their hardness.

Very High Melting/Boiling Point - There are string bonds throughout the whole structure, meaning it takes a lot of energy to break them.

Allotropes

Different forms of the same element, e.g. O₂ and O₃

How are carbon atoms bonded in diamonds?

Each carbon atom bonds with 4 of its neighbours in a 3D tetrahedral structure.

What are diamonds used for?

Jewellery, Cutting and drilling tools.

How are carbon atoms bonded in graphite?

Each carbon atom bonds with 3 of its neighbours in a 2D trigonal planar structure known as a layer lattice. This creates sheets called Graphene that stack on top of each other to form Graphite with weak dispersion forces holding them together. Each carbon atom has one electron delocalised in this structure.

What is graphite used for?

Lubrication, pencils, electrodes.

How do the differences in structure explain the different properties of diamonds and graphite.

Both have covalent bonds, and so a high melting/boiling point. However, Graphite can conduct electricity due to its delocalised electrons, which diamonds can not. Diamonds are very rigid, hard and brittle as they are covalent all throughout, whereas the weak dispersion forces between

What is a Buckyball (Buckminsterfullerene)?

C₆₀, an allotrope of Carbon, with the carbon atoms arranged in a ball shape made up of 20 hexagons and 12 pentagons. It is a black or brown powdery solid that is a semiconductor and sublimes at 800K.

What is a carbon nanotube (CNT)?

A tiny structure that resembles graphene rolled into a cylinder. It can be open-ended or have a hemispherical cap similar to C₆₀. They are very strong and can conduct electricity.

What is a multi-walled nanotube (MWNT)?

Multiple carbon nanotubes of varying diameters nested within each other.

Which elements will not be able to form an octet when bonding covalently?

Elements with less than 4 valence electrons, e.g. Boron (3).