C5 Chemicals in the natural enviroment

Revision cards for Chemicals in the natural enviroment, largely based on the Twenty First Century Science textbook and BBC Bitesize

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Ionic Compounds

Ionic substances form giant ionic lattic containing oppositely charged ions. They have high melting and boiling points, and conduct electricity when melted or dissolved in water.

Ionic Compounds

Ionic bonds form when a metal reacts with a non-metal. Metals form positive ions; non-metals form negative ions. Ionic bonds are the electrostatic forces of attraction between oppositely charged ions.
The oppositely charged ions are arranged in a regular way to form giant ionic lattice. Ionic compounds often form crystals as a result. The illustration shows part of a sodium chloride (NaCl) ionic lattice.

(http://d3j5vwomefv46c.cloudfront.net/photos/full/476680864.jpg?Expires=1324381574&Key-Pair-Id=APKAIYVGSUJFNRFZBBTA&Signature=TITVQcOho1O0n5sOtEtUaLiqDs5gCfSfeWjSwAnWxGT1UCKeaSQRRztgPAL2CztMq5M1Uau-bTScXs3wVt0E9rAxMKXR0zG5OChFGZRUuSJcEvZSOUuWnSyhbAs62JNtLOe4rC8p4PvS3G2fkDSJ6VX6wLlaYVdrQkylBu1dEgs_)

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Ionic Compounds (continued)

Properties of ionic compounds

  • High melting and boiling points - Ionic bonds are very strong - a lot of energy is needed to break them. So ionic compounds have high melting and boiling points.
  • Conductive when liquid - Ions are charged particles, but ionic compounds can only conduct electricity if their ions are free to move. Ionic compounds do not conduct electricity when they are solid - only when dissolved in water or melted.

Example: Soduium Chloride

  • High melting point:  800ºC
  • Non-conductive in its solid state, but when dissolved in water or molten NaCl will conduct electricity.
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Covalent compounds

Covalent compounds - simple molecules

Covalent bonds form between non-metal atoms. Each bond consists of a shared pair of electrons, and is very strong. Covalently bonded substances fall into two main types:

  1. simple molecules and
  2. giant covalent structures.

Properties of simple molecular substances

  • Low melting and boiling points - This is because the weak intermolecular forces break down easily.
  • Non-conductive - Substances with a simple molecular structure do notconduct electricity. This is because they do not have any free electrons or an overall electric charge.
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Covalent compounds - simple molecules

Simple Molecules

Diagram of carbon dioxide molecule. One atom of carbon shares four electrons with two atoms of oxygen (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_111.gif)

A molecule of carbon dioxide

These contain only a few atoms held together by strong covalent bonds. An example is carbon dioxide (CO2), the molecules of which contain one atom of carbon bonded with two atoms of oxygen.S

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Covalent compounds - simple molecules (continued)

Higher Tier Only

Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds. All have very strong bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. Simple molecular substances are gases, liquids or solids with low melting and boiling points.

                                    (http://d3j5vwomefv46c.cloudfront.net/photos/full/476689799.jpg?Expires=1324382721&Key-Pair-Id=APKAIYVGSUJFNRFZBBTA&Signature=MF38QLUZmvrA3FI6GEO1QBLtIhGPHtnEQCWMM-5bzK47S9UrlWWgYi3zMMGDX4SAJJt1sdq3JNh6rgipVJHDkngTea1DDeMyBiKgP9eG3UdKBmc9FqS4WyEAMUhOGozK-OP2ptg86WlbErb65CLOhVauyCN3K8P4EmEJwJVVQuQ_)

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Covalent compounds - giant covalent structures

Giant covalent structures

Giant covalent structures contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds. The atoms are usually arranged into giant regular lattices - extremely strong structures because of the many bonds involved. The graphic shows the molecular structure of diamond and graphite: two allotropes of carbon, and of silica (silicon dioxide).

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_55.jpg)

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