AMIE INDIA Students Forum

[mandgm]

Metals: valence electrons are detached from atoms, and spread in an 'electron sea' that "glues" the ions together. Metals are usually strong, conduct electricity and heat well and are opaque to light (shiny if polished). Examples: aluminum, steel, brass, gold.

Semiconductors: the bonding is covalent (electrons are shared between atoms). Their electrical properties depend extremely strongly on minute proportions of contaminants. They are opaque visible light but transparent to the infrared. Examples: Si, Ge, GaAs.

Ceramics: atoms behave mostly like either positive or negative ions, and are bound by Coulomb forces between them. They are usually combinations of metals or semiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and carbides). Examples: glass, porcelain,
many minerals.

Polymers: are bound by covalent forces and also by weak van der Waals forces, and usually
based on H, C and other non-metallic elements. They decompose at moderate temperatures
(100 – 400 C), and are lightweight. Other properties vary greatly. Examples: plastics (nylon,
Teflon, polyester) and rubber.

Other categories are not based on bonding. A particular microstructure identifies composites,
made of different materials in intimate contact (example: fiberglass, concrete, wood) to achieve
specific properties. Biomaterials can be any type of material that is biocompatible and used,
for instance, to replace human body parts.

[hemlata]

Classification of materials

It is often useful to classify substances into different groups. All such classifications have their uses and drawbacks. This Download is an analysis of some of the ways we try to classify matter and some of the difficulties associated with each of them.

Here are some of them:
• Solids, liquids and gases. (Gives a room temperature perspective but most materials change state – is water a solid, a liquid or a gas? Is rock solid or liquid (lava), is carbon dioxide a gas or a solid (dry ice)?). See ‘Difficulties’ (para 1) at the end for more discussion on gases as ‘real matter’
• Elements mixtures and compounds. (Perhaps pure and impure/mixture is simpler, then classify pure into elements and compounds. Note the comments about ‘pure’ in para 2 ‘Difficulties’ - see below)
• Metals and non-metals. (Useful for elements but the non-metal category contains a wide range of different materials if compounds are included)
• Electrical conductors and insulators. (Both conductors and non-conductors contain a wide range of different substances)
• Things that burn in air/oxygen. (Inflammable/flammable; fuels.) and non-flammable materials. (Both categories contain a wide range of different sorts of substance, and some can go into both – for example most metals burn if they are finely divided, but are resistant to burning in bulk form)
• Food and non-food. (Again the non-food contains a wide range)
• Magnetic and non-magnetic. (Again the non-magnetic contains a wide range)
• Soluble or (almost) insoluble in water. (Again the insoluble contains a wide range)
• Acid and alkaline. (This excludes most materials)

Most of these classification systems have one category that is easy to recognise (magnetic, conductor, food, soluble etc) but the other is unsatisfactory. That is because we need more than two classes if we are to get a useful all-embracing system. Even solid liquid and gas is unhelpful – most substances are solid at room temperature, so we need to classify the solids further. A lot of effort can sometimes be wasted trying to force things into one category or another when it ceases to be helpful. (Almost everything will dissolve in water to some extent. Some substances burn much better than others. Many foods are harmful if eaten to excess.) – so we need to learn to use categorisations sensibly.

The structure triangle approach to classifying materials.

The classification system mentioned in download 2.1.1 “Matter and recycling” slides 15, 16 and 21 “2nd Idea: 5 structures only” is one that deserves some thought. It is explained in full in the CD Science Issues (2005), in Ross (2000) page63-4 and in Ross (1997).

All materials occur as solids if the temperature is low enough, and these solids only occur in 5 basic forms, depending on their elemental composition.
1. If they contain only metallic elements they will be metals.
2. If they are built from the more reactive non metals they will have a molecular structure and likely to be gases, liquids or volatile solids at room temperature (Volatile materials)
3. If they are giant structures built from carbon they are likely to be linear polymers (polymers including those from life)
4. If built from silicon they will have a three dimensional structure (Rocks).
5. Finally if they are made from a combination of metallic and non-metallic elements they are likely to be ionic (Salts).
Even children at key stage one will use this classification, though they are likely to split some categories:
Metals
Polymers: wood, plastic, fabric
Rocks: china, glass, rock
Volatile: Liquids, gases
Salts (they will have come across very few ionic materials)

The single question “What happens if you hit it with a hammer?” will distinguish these four [metals dent or bend, rocks and salts shatter, life polymers are elastic (the hammer bounces back), and volatiles will crumble or splash (Ionic materials/salts need more sophisticated testing to distinguish them from rocks)].

The structure triangle (Ross 1998 and 2000) extends this ‘natural’ classification system by considering the elements from which these structures have to be constructed.

You arrange the elements from Cs to F in order of their reactivity – from very reactive metal (Cs) through unreactive in-between elements like Si to very reactive non-metals (F). This forms the top edge of the triangle – see picture below. Caesium reacts with Fluorine to form the salt CsF, this becomes the lower apex of the triangle. All other two-element compounds now appear by drawing lines from the two elements, parallel to the triangle sides (the lines from Cs and F to meet at the apex). These two lines will meet somewhere within this triangle. The structure types summarised above appear naturally – two metals reacting retain their metallic structure, two non-metals form small self contained molecules of the volatile group, a metal and non-metal (if reactive enough) form salts, like CsF, and the middle ground is occupied by the giant covalent structures dominated by Silicon (rocks) and carbon (polymers & life)