Additional Science

C2- Chemistry

Covalent bonds, covalent structures, metals,

nanoscience

This is the transfer of electrons between atoms.

The atoms become ions because they have lost or gained electrons.

This is the sharing of electrons between atoms.

There are three ways of representing these bonds.

X

Hydrogen = H2

This is a covalent bond.It is strong bond formed by the pair of shared electrons in the outer shell and electrostatic attraction of the nuclei.

H OX H

H H

Small covalent molecules you need to know about are…

Hydrogen = H2

Chlorine = Cl2 Oxygen = O2

Hydrochloric acid = HCl Water = H2O

Ammonia = NH3

Methane = CH4

H

O

H

HO

H

H

OH H

OH

H OH

HO

HH O

H

HO

H H

OH

Forces within the molecules (covalent bonds) are very strong. Forces between the molecules (intermolecular forces) are weak.It is these intermolecular forces which break when a substance melts or boils.

HO

H

HO H

HO

HH

OH

H OH

HO

H

H OH

HO

HH

OH

melting

Simple covalent molecules such as H2, Cl2, O2, HCl, H2O, NH3 and CH4 all have low melting and low boiling points because the intermolecular forces are weak, so they are relatively easy to break by heating.

These structures do not conduct electricity because molecules do not have overall charges.

Giant Covalent structures (or macromolecules)

Silicon dioxide (silica)

Diamond Graphite

Silicon dioxide (SO2)X

XX

XO

O

O

O

O

X

O

O

OO

O

O

O

X

SiliconOxygen Oxygen

A silicon atom makes covalent bonds with two oxygen atoms. This means it has a full outer shell. The oxygen atoms can make a further covalent bond with other silicon atoms, hence the formation of a giant covalent structure.

Diamond (C)

XX

OO

O

OO

O

X

OO

O

O

OO

O

O

OO

X

A carbon atom makes covalent bonds with four other carbon atoms. This means that all carbon atoms have formed covalent bonds so that they have a full outer shells of electrons.

Graphite (C)

X

XX

X

O

OO

O

O

OO

O

O

OO

O

I’m off

A carbon atom makes covalent bonds with three other carbon atoms. One of the electrons in carbon does not form a covalent bond with another carbon atom. This means carbon forms layers which can slide over each other due to the free electrons and weak intermolecular forces between the layers.

Silicon dioxide (silica)

Diamond Graphite

Giant covalent structures all have high melting and boiling points (due to the strong covalent forces between all atoms).Diamond is very hard, graphite is soft and slippery (due to the weak forces between layers).The delocalised electrons of graphite are able to conduct heat and electricity.

+-

+-

+-

+-

+-

+-

+-

+-

Metals are giant structures of atoms arranged in a regular pattern.The electrons in the highest occupied energy levels (outer shell) of metal atoms are delocalised and so free to move through the whole structure.

So really, metals are giant structures of positive ions with electrons between the ions holding them together by strong electrostatic attractions.

+-

+-

+-

+-

+-

+-

+-

+-

Delocalised electrons allow metals to conduct heat and electricity.

Layers of atoms mean that metal ions can slide over each other so metals can be bent and shaped.

Metal alloys consist of more than one type of metal atom.

The atoms distort the layers so they cannot slide over each other. This means alloys are harder than pure metals.

Shape memory alloys return to their original shape after being deformed, e.g. in braces

+- +

-

+-

+-

+-

+-

+- +-

+-

+-

+-

+-

+-

+-

Polymers are repeating chains of covalently bonded monomers.

The properties of polymers depend on the conditions they were made under.

Low density (LD) and high density (HD) poly(ethene)

Small number of particles

per volume

High number of particles

per volume

Ethene molecules joined in a polymer

These are made of the same monomers but the conditions they are made are different so they have different properties.

Low density (LD) poly(ethene)

Made using high pressure and with a trace of oxygen.Chains are branched and can’t pack together (hence the low density)

High density (HD) poly(ethene)

Made using a catalyst at 50oC and slightly raised pressure. They can pack closely (hence the high density).It is stronger than low density poly(ethene)

Intermolecular forces (forces between chains)

Weak Strong cross links

Softens and resets Does not meltThermosoftening

polymerThermosetting

polymer

Very, very small structures; 1- 100 nanometers (nm) in size

Only contain a few hundred atoms

Have different properties to the same materials in bulk.

Have a high surface area to volume ratio

Fullerene is an example of a nanoparticle made of hexagonal arrangements of carbon

Possible uses of fullerenes;

CatalystsDrug deliveryLubricantsReinforcing

Possible uses of other nanoparticles;

New computersNew catalystsNew coatingsSensorsStronger and lighter construction materialsNew cosmetics