All materials – by definition – are made of matter, and chemists are profoundly interested in the nature of material stuff.


The Four Aristotelian Elements

Western chemistry grew up around old alchemical ideas of Earth, Air, Fire, and Water – the Aristotelian elements – a concept that originated with the ancient Greeks and others, here.

Read more about the  Four Elements  in Carmen Giunta's  Elements and Atoms: Case Studies in the Development of Chemistry ,  here .  Fathi Habashi of Laval University discusses  Zoroastra and The Theory of Four Elements ,  here , and  Cambodia's Four Elements,   here .

Read more about the Four Elements in Carmen Giunta's Elements and Atoms: Case Studies in the Development of Chemistryhere.

Fathi Habashi of Laval University discusses Zoroastra and The Theory of Four Elementshere, and Cambodia's Four Elements, here.

The Year 1800: Organic & Inorganic Matter

In the year 1800 some 27 chemical elements were known:


Ideas had moved on from the four Aristotelian elements, and it was thought that there were two distinct types of matter: organic and inorganic.

  • Organic matter was associated with living things (biological origin: flora, forna, food, us) and was assumed to possess a vital-force, an indefinable characteristic that separated living organisms and materials derived from living organisms from inanimate inorganic matter:
  • Inorganic matter is of geological origin: minerals, rock, sea, air


New Ideas: Urea and an Unanswerable Challenge the Vital-Force Theory

In the nineteenth century chemical knowledge increased dramatically:

In 1804, Dalton proposed that matter was constructed from identical, indivisible atoms which combined with each other in constant, or stoichiometric, proportions.

In 1828 the classical distinction between organic and inorganic matter was resolved as evidence accumulated that organic materials could be synthesised in the laboratory from inorganic, non-living sources.

The crucial step occurred when the German chemist Friedrich Wohler heated ammonium cyanate, an inorganic salt, and produced the substance urea, that was identical to organic urea isolated from the urine of animals, and so was organic. Wohler's synthesis of an organic chemical from inorganic starting materials was an unanswerable challenge to the vital-force theory.

By the end of the nineteenth century chemical methodology had become very sophisticated.

  • Scientists understood that atom types could be classified and grouped into a periodic table of chemical elements, and by 1900 the only non-radioactive s, p or d-block element that remained to be discovered was rhenium, Re.
  • Common pharmaceutical preparations such as opium, tobacco and coca were shown to have active ingredients that were discrete molecular entities (morphine, nicotine & cocaine, respectively) that could be purified to white crystalline materials (usually as the .HCl salt) of known chemical composition.

Four points:

  • Most materials obtained from nature – organic OR  inorganic – are chemically complex, heterogeneous mixtures or composite materials: e.g. granite and wood.
  • It is now generally recognised that many classically defined inorganic materials, such as limestone, coal and the oxygen in our atmosphere are actually of biological origin, produced over geological time scales.
  • The modern chemical classification system says that to be "organic" a substances possess carbon hydrogen (C-H) chemical bonds, and that "inorganic" substances do not possess C-H bonds. Under this system, oxygen is inorganic. And, ironically, so is urea!
  • In popular culture – and at the grocery store – the term organic has another, different meaning. Organic crops are grown without man-made fertilisers or pesticides.


The Chemical Classification of Matter

Many chemistry textbooks provide a diagram In their introductory sections showing how matter can be classified into mixtures and pure substances, and then to heterogeneous and homogeneous mixtures, elements and compounds:


Matter, the stuff from which our physical world is formed, presents to us as various types of material. On a first analysis, the possible phases are:

  • gaseous, such as air
  • liquid, such as water
  • solid, such as rock

However, for classification purposes it is useful to divide materials into:

  • mixtures: variable composition
  • pure substances: stoichiometric composition

Physical techniques, such as: distillation, filtration, crush-&-sort, selective dissolution, chromatography, etc., can be used to separate the individual components of a mixture into chemically pure substances, and contrawise, physical methods such as turbulent mixing can be used to blend pure substances together into mixtures.


The Chemical Classification of Matter: Updated

However, the above graphic is a little over simple for our purposes, and can be usefully expanded to a classification system is both derived from and is compatible with the classification system employed in The Chemical Thesaurus reaction chemistry database:


We will not go through each of the above boxes in turn.



Mixtures can be sub classified into four types: homogeneous, heterogeneous, colloidal and composite.

Homogeneous Mixtures

Homogeneous Mixtures can all be regarded as solutions, and they can form in various ways:

  • A mixture of two or more gases: air
  • A gas dissolved in a liquid: soda water
  • A mixture of two or more miscible liquids: water and methanol
  • A solid fully dissolved in a liquid: 1.0 molar NaCl (aq)

By definition, any region of a homogeneous solution will be chemically identical to any other region so sampling is not an issue. A common way to insure that a homogeneous mixture remains homogeneous is by turbulent mixing.

Heterogeneous Mixtures

Heterogeneous Mixtures are agglomerates. In the natural world, nearly all matter is heterogeneous, apart from air, fresh clear water and various minerals such as quartz, rock salt, sulfur etc.

However, scale is important: a 1.0 m3 sample of air will be homogeneous but the atmosphere as a whole is heterogeneous. Poorly stirred solutions where there is chemistry occurring, even simple heating, are liable to become heterogeneous.

Generally, chemists dislike heterogeneous mixtures and heterogeneous materials. This is because chemists are interested in the composition of a particular piece of matter and how it behaves chemically. But, by definition, the composition of a heterogeneous material varies from region to region, where the distance between regions may range from microns to kilometres.

A farmer may want to know the boron levels because boron is an important trace element for crop growth. Somebody will have to take samples from all over the farm, perform chemical analysis of all the samples and perform a statistical analysis of the data because the soil is heterogeneous, both under a microscope and over the area of the farm: boron levels will vary from field to field. On the other hand, if the farmer wants to know the pH of the swimming pool only a single sample is required because the pool can be assumed to be be homogeneous.

Chemists go to great lengths to transform heterogeneous matter into homogeneous matter. They grind and sort, but the favoured methods are fractional distillation, dissolution, selective precipitation and filtration.


Colloids are defined thus:

"A colloid is a heterogeneous mixture composed of tiny particles suspended in another material. The particles are larger than molecules but less than 1 µm in diameter. Particles this small do not settle out and pass right through filter paper. Milk is an example of a colloid. The particles can be solid, tiny droplets of liquid, or tiny bubbles of gas; the suspending medium can be a solid, liquid, or gas (although gas-gas colloids aren't possible)."

Colloids often appear to be homogeneous in bulk, but when are examined under a microscope are observed to be heterogeneous. Chemists must treat colloids as heterogeneous and process colloids to homogeneous before analysis.


Most real world solid materials are composites:

  • Inorganic materials like rock are composite. Granite is a mixture of of feldspar (65-90%) , quartz (10 to 60%) and biotite or mica (10 to 15%). 
  • Wood is an organic composite of consisting of cellulose and lignin.
  • Yeast in block form looks rather like a pure substance, but it is of course an extraordinarily complex, living biomaterial.
  • Glass Reinforced Plastic, GRP, is a composite of glass fibre in a crosslinked polymer resin.
  • Many industrial chemical products may have names that make then appear to be pure substances, but are actually highly complex mixtures of: active ingredient, binder, stabilisers, accelerators, lubricants, etc. For example, as a product aspirin is a tablet consisting of many components including the active ingredient acetylsalicylic acid plus calcium carbonate, magnesium stearate, etc., and these ingredients may change with time. Likewise, dynamite is not a substance, but a mixture of nitroglycerine, kieselguhr (diatomaceous earth), stabilisers, etc.


Pure Substances

Pure Substances, as noted by John Dalton, have a fixed stoichiometric composition with simple ratios of atoms:

  • Neon: Ne
  • Table salt: NaCl
  • Oxygen: O2
  • Carbon dioxide: CO2
  • Glucose: C6H12O6
  • etc.

Chemical Elements

Elemental substances are collections of atoms with the same proton number. Most elements consist of a mixture of isotopes. This is not usually an issue, however. Yes, isotopes can be separated – enriched or depleted – in various ways, but this is not an issue for this web book.

It is difficult to say how exactly many elements there are because:

  • There are 81 non-radioactive elements.
  • All elements heavier than barium, Ba, atomic number 83 are radioactive; as are technetium, Z = 43, and promethium, Z = 61.
  • Some radioactive elements have isotopes with half-lives of a billion years or more, and these still exist on Earth: U-235 and U-238 are well known examples.
  • Elements atomic number 93 to 118 are all known, but they must be prepared synthetically and may exist for microseconds or less. 
  • It is often said that there are 92 naturally occurring elements.

Binary Compounds

Binary compound substances consist of just two elements with the constraint [here] that they possess just one type of strong bond present: metallic, ionic or covalent.

Note that this local definition includes methane, CH4, as a binary because it has only C-H bonds, but not ethane, CH3CH3, or the other hydrocarbons which possess both C-H and C-C bonds.

The Tetrahedron of Structure, Bonding & Material Type shows that pure elements and substances consisting of only two elements but with only one type of strong chemical bond exhibit four extremes of material type: metals, ionic salts, molecular van der Waals & network covalent, or they are intermediate between these four extremes.


Ternary and polyelemental compound substances

Ternary and polyelemental compound substances include chloromethane, CH3Cl, methanol, CH4OH, and glucose, C6H12O6. There substances have multiple types of chemical bonds of varying polarity.

Chemical Substance Types

Network covalent materials 

Network covalent materials have atoms arranged in an extended lattice of strong, "shared electron pair" covalent bonds. Materials are generally hard, refractory solid substances, poor electrical conductors, and they are not soluble in any solvent. They hery high melting point (>1500°C) and are chemically intractable materials. Examples include: diamond, boron, silica, gemstones, ceramics, etc.

Metallic Materials

Metallic elements are (in the Drude model) a lattice of cations immersed in a sea of mobile valence electrons that are delocalised over the entire crystal. Electrons are the agents responsible for the conduction of electricity and heat. Metals have a characteristic lustre, are often ductile and exhibit a huge range of melting points, from mercury, -39°C, to tungsten at 3200°C. All the elements on the left hand side of the periodic table are metals. Indeed, as the pure elemental substance the majority of the elements present as metals.


Alloys are "a partial or complete solid solution of one or more elements in a metallic matrix", from here. It can only be determined if an alloy is heterogeneous, homogeneous or stoichiometric by microscopic, physical and chemical examination. 

Molecular Materials

Molecular substances consist of discrete molecules that are held together internally by strong intramolecular (within molecule) "shared electron pair" covalent bonds, but when forming condensed solid or liquid phases, the molecules interact via weak intermolecular (between molecule) van der Waals forces:

  • There are several types of van der Waals attraction: dipole/dipole, dipole/induced-dipole and spontaneous-dipole/induced-dipole. It is tempting to consider these forces to be of different strengths, but it is the distance range that is more important. The spontaneous-dipole/induced-dipole attractions – also known as London dispersion forces (LDF) – are surprisingly strong but only act at very short range: the surfaces of even neutral, non-polar molecules like methane are 'sticky'.
  • All molecules have London dispersion forces and the strength increases with the size/surface area of the molecule. This logic is used to explains the increasing boiling and sublimation temperatures of the halogens: F2 < Cl2 < Br2 < I2
  • Some molecules have dipole-dipole attractions as well, which increase the total amount of interaction between the molecules.
  • Consider the compound iodine chloride, ICl and the elemental molecule bromine, Br2. Both a molecular mass close to 160 and both are 70-electron systems, but ICl is polar and Br2 is non-polar, yet they have rather similar boiling points of 97° and 59° respectively. This shows that the dipole/dipole attraction makes only a minor contribution.
  • Molecular materials may also be hydrogen bonded, where a hydrogen bond involves a proton being shared between two Lewis bases, usually with oxygen, nitrogen or fluorine atomic centres, as discussed here.

Molecular materials exhibit a vast array of properties, but they are generally mechanically weak, have low electrical conductivity, have low melting and boiling points, and/or a susceptibility to sublime. Molecular materials usually soluble in (or miscible with) non-polar solvents. Hydrogen bonded molecular solids are often soluble in water.

Binary Ionic Salts 

Binary ionic salts, like sodium chloride, NaCl, have a crystal lattice with anions electrostatically attracted to adjacent cations and cations electrostatically attracted to adjacent anions. Simple ionic materials are insulators as solids, but are electrical conductors when molten and when dissolved in aqueous solution. Ionic materials may be soluble in water (and sometimes in dipolar aprotic solvents such as DMSO), but they are insoluble in non-polar solvents like hexane. Simple ionic materials have moderately high melting points, usually 300-1000°C.

Molecular & complex salts

Molecular & complex salts have a crystal lattice anions and cations electrostatically attracted to each other, but the cations and anions are compound entities. Some properties of molecular and complex salts are dominated by the ionic nature of the material. For example, substances are more soluble in water than organic solvents, indeed, many complex ions are only stable in aqueous solution. Other properties are dominated by the molecular nature of the ions. For example, melting points tend to be low or substances decompose on heating. Solubility is often pH dependent. Examples include:

  • sodium acetate Na+ CH3COO
  • ammonium nitrate [NH4]+[NO3]
  • hexaaquacopper(II) chloride [Cu(H2O)6]2+ 2Cl

Intermediate materials

Intermediate materials are between ionic, molecular and network. Examples include metal oxides, such as magnesium oxide and calcium oxide, as well as metal sulfides and phosphides. 


Polymers consist of a large number of identical monomer components linked together in a chain, and there maybe cross linking between chains. Properties such as melting point and crystallinity are determined more by chain length and the degree of cross linking than by the nature of the monomer entities or their bonding.

  • Polymers consisting of long chains, such as low density polyethylene, are essentially molecular and are often thermoplastic and melt on heating.
  • Extensively crosslinked polymers, such as the and melamine-formaldehyde are network covalent materials that do not melt. Light fittings and electrical plugs are normally made from such polymers.


Glass, as defined by Wikipedia:

"A uniform amorphous solid material, usually produced when a suitably viscous molten material cools very rapidly, thereby not giving enough time for a regular crystal lattice to form."

An interesting video of melting glass in a microwave oven. The secret is to make a spot red hot first.


Minerals As defined by Wikipedia:

"Minerals are natural compounds formed through geological processes. The term mineralencompasses not only the material's chemical composition but also the mineral structures. Minerals range in composition from pure elements and simple salts to very complex silicates with thousands of known forms. The study of minerals is called mineralogy. "

Not all minerals are pure chemicals, as a chemist understands the term. Brimstone is very pure elemental sulfur, but very few minerals are able to pass the chemist's pure substance of uniform composition test as most are mixtures and/or vary in composition between geographic location.

  • crude oil
  • natural gas
  • fresh water
  • sea water
  • air

Minerals are of crucial important to chemists because – ultimately – all chemical substances are obtained from biological or geological sources.

Transient & Hypothesised Entity Types

Atomic ions

Atomic ions are ions of single atoms:

Na+, K+, Ca2+, Cl, S2–, etc.

All ions require a counter ion to maintain electrical neutrality.

Molecular and complex ions

Molecular and complex ions are ionic compound entities:

CH3COO [NH4]+ [Cu(H2O)6]2+

All ions require a counter ion to maintain electrical neutrality.

Free radicals

Free radicals, or simply radicals, are neutral molecular species with a single unpaired electron in their valence shell. Radicals are discussed in more detail here.

Excited state species

Excited state species are transient atomic or molecular entities formed by moving a ground state electron to a higher energy orbital. The behaviour of the excited state species will be very different to the ground state species.

  • Excited state sodium atoms emit light of precise wavelength, here.
  • Ground state singlet oxygen has a different spectrum of reactivity compared with excited state triplet oxygen, here.

Allotropes, Polymorphs & Particle Size

From the Wikipedia: "Allotropy, or allotropism, is the property of some chemical elements to be able to take two or more different structural forms that exhibit quite different physical properties and chemical behaviours."

  • Carbon can exist as: graphite, the thermodynamically stable form of carbon under standard conditions (25°C, 1.0 atm), diamond, fullerenes including: C60 buckyballs, SWCN (single walled carbon nanotubes), and many others.

  • Oxygen can exist as dioxygen, O2, (a diradical which can exist in distinct singlet and triplet forms) and ozone, O3.

  • Allotropes of: carbon, phosphorus, oxygen, nitrogen, sulfur, selenium, boron, silicon, arsenic, antimony, polonium, tin, iron, titanium, strontium, plutonium, ytterbium, terbium, promethium, curium, americium, berkelium and californium are discussed on the Wikipedia page.

From the Wikipedia: "Polymorphism is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to elemental solids. Together with polymorphism the complete morphology of a material is described by other variables such as crystal habit, amorphous fraction or crystallographic defects.

  • Silica, SiO2, has many polymorphs, including: α-quartz & β-quartz, tridymite, cristobalite, coesite, stishovite and silica gel.
  • Walter McCrone stated that every compound has different polymorphic forms, and that, in general, the number of forms known for a given compound is proportional to the time and money spent in research on that compound.

The chemistry of a solid substance can be strongly influenced by the particle size.

  • Magnesium oxide, MgO, is a refractory (heat resistant) material that is used to form the fire bricksthat line furnaces and kilns. The chemistry of a sample of MgO consisting of a fine power will be rather different to the chemistry of a solid block of fire brick. 
  • Colloidal gold, wikipedia, or nanogold, is a suspension of sub-micrometre-sized particles of gold in a water. The liquid is usually either an intense red colour (for particles less than 100 nm), or a dirty yellowish colour (for larger particles).