13.7 billion years ago Nuclei after three minutes
Hydrogen and helium nuclei form in the first three minutes, with perhaps another 300,000 years before they combine with electrons to form atoms

13 billion years ago Atoms after 300,000 years
As gravity exerts its pressure within parts of the expanding fireball, subnuclear particles merge into more complex elements

450 BC Four elements in nature say Greeks
Empedocles states that all matter is made up of four elemental substances - earth, fire, air and water

420 BC Matter consists of atoms
The Greek philosopher Democritus declares that matter is composed of indivisible and indestructible atoms

380 BC Four humours in human body say Greeks
A Greek text, attributed to Polybus, argues that the human body is composed of four humours

300 BC Epicurus brings pleasure into philosophy
Epicurus postulates a universe of indestructible atoms in which man himself is responsible for achieving a balanced life

250 BC Alchemy in Alexandria
The first alchemists, working in Alexandria, are also the world's first experimental chemists

1040 Gunpowder described
A Chinese manual on warfare includes the earliest known description of gunpowder

1648 Dutch chemist names gas
The Dutch chemist Jan Baptist van Helmont suggests that there are insubstantial substances other than air, and coins a name for them - gases

1662 Boyle finds law for gases
British chemist Robert Boyle defines the inverse relationship between pressure and volume in any gas (subsequently known as Boyle's Law)

1702 Mystery substance named phlogiston
German chemist Georg Stahl coins the name phlogiston for the substance believed to be released in the process of burning

1735 Cobalt discovered
Swedish chemist Georg Brandt discovers a new metallic element, which he names cobalt

1751 Nickel the very devil in copper ore
The Swedish chemist Alex Cronstedt identifies an impurity in copper ore as a separate metallic element, which he names nickel

1754 Black finds fixed air
Scottish chemist Joseph Black identifies the existence of a gas, carbon dioxide, which he calls 'fixed air'

1766 Cavendish misinterprets hydrogen
English chemist Henry Cavendish isolates hydrogen but believes that it is phlogiston

1773 Scheele keeps quiet about oxygen
Swedish chemist Carl Wilhelm Scheele isolates oxygen but does not immediately publish his achievement

1774 Priestley goes public with oxygen
English chemist Joseph Priestley isolates oxygen, but he believes it to be 'dephlogisticated air'

1787 Lavoisier classifies chemistry
French chemist Antoine Laurent Lavoisier publishes a system for classifying and naming chemical substances

1803 Dalton's Law
English chemist John Dalton reads a paper describing his Law of Partial Pressure in gases (discovered in 1801)

1803 Dalton's theory of atoms
At the end of his Partial Pressure paper, John Dalton makes brief mention of his radical theory of differing atomic weights

1807 Davy isolates sodium and potassium
English chemist Humphry Davy uses electrolysis to isolate the elements sodium and potassium

1809 Gay-Lussac's Law of Combining Volumes
French chemist Joseph Louis Gay-Lussac shows that when gases combine they do so in simple ratios by volume (later known as his Law of Combining Volumes)

1811 Avogradro's Law
Italian chemist Amedeo Avogadro publishes a hypothesis, about the number of molecules in gases, that becomes known as Avogadro's Law

1856 Perkin synthesizes chemical dye
English chemist William Henry Perkin accidentally creates the first synthetic die, aniline purple (now known as mauve)

1861 Crookes discovers thallium
English chemist and physicist William Crookes isolates a new element, thallium

1867 Nobel invents dynamite
Swedish chemist Alfred Nobel patents dynamite, making the volatile explosive nitroglycerine safer by combining it with kieselguhr

1869 Mendeleyev's periodic table
Dmitry Mendeleyev reads to the Russian Chemical Society in St Petersburg his formulation of the periodic table

1895 Ramsay isolates helium
Scottish chemist William Ramsay isolates the element helium

1898 Ramsay and Morris isolate krypton
British chemists William Ramsay and Morris Travers isolate the element c

1898 Ramsay and Morris isolate neon
British chemists William Ramsay and Morris Travers isolate the element neon

1898 Ramsay and Morris isolate xenon
British chemists William Ramsay and Morris Travers isolate the element xenon

1898 The Curies discover polonium
Marie Curie and her husband Pierre isolate a new element which they name polonium in honour of her native Poland

1898 The Curies discover radium
Marie and Pierre Curie isolate the element radium, working without any protection because unaware of the danger of radioactivity

1906 Soddy discovers isotopes
Frederick Soddy observes his first examples of chemically identical elements with differing atomic weights, to which he later gives the name isotopes

1913 Einstein discovers photochemical equivalence
Albert Einstein formulates the law of photochemical equivalence, a fundamental principle of chemical reactions induced by light

1913 Soddy coins the term 'isotope'
Frederick Soddy uses the term 'isotope' (Greek for 'same place') to describe observed anomalies in the periodic table

1922 Pauling investigates chemical bond
Linus Pauling, a graduate student at the California Institute of Technology, begins theoretical work on the nature of the chemical bond

1939 Pauling on the chemical bond
US chemist Linus Pauling publishes his collected discoveries on The nature of the chemical bond

1945 Hodgkin analyzes penicillin
British chemist Dorothy Hodgkin describes the molecular structure of penicillin

1949 Radiocarbon dating
The technique of radiocarbon dating is developed by US chemist Willard Libby

1951 Oral contraceptive
Syntex, a small chemical company in Mexico City, develops the first oral contraceptive

1961 Thalidomide disaster
The drug Thalidomide, synthesized in West Germany, is shown to have been the cause of severe defects in about 12,000 children born in 46 countries