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1906
 
    
Frederick Soddy observes his first examples of chemically identical elements with differing atomic weights, to which he later gives the name isotopes       
1913
 
    
Albert Einstein formulates the law of photochemical equivalence, a fundamental principle of chemical reactions induced by light       
1913
 
    
Frederick Soddy uses the term 'isotope' (Greek for 'same place') to describe observed anomalies in the periodic table       
1922
 
    
Linus Pauling, a graduate student at the California Institute of Technology, begins theoretical work on the nature of the chemical bond       
1939
 
    
US chemist Linus Pauling publishes his collected discoveries on The nature of the chemical bond       
1945
 
    
British chemist Dorothy Hodgkin describes the molecular structure of penicillin       
1949
 
    
The technique of radiocarbon dating is developed by US chemist Willard Libby       
1951
 
    
Syntex, a small chemical company in Mexico City, develops the first oral contraceptive       
c. 1961
 
   
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      
2011  August 5
 
 
NASA announces that it has photographic evidence from the Mars Reconnaissance Orbiter of possible liquid water on the planet in warm seasons