Chapter 3 



How Molecules Make Masses 



Atoms and molecules are so tiny that they seem quite remote. 

 Only under unusual experimental conditions do we deal with the 

 effects which they produce as individual bodies of matter. In the 

 practical affairs of life and in most scientific investigations as well 

 we are concerned with large numbers of atoms or molecules 

 which, when gathered together, form masses large enough to be 

 experimented upon, or even to become microscopically visible. 

 The smallest particle we can distinguish or "resolve" in micro- 

 scopes may contain many millions of the smaller molecules, or a 

 great number of macromolecules. 



The convenient shorthand used by chemists to describe a com- 

 pound, generally called its formula (little or concise form) is far 

 removed from what that molecule may actually be like under 

 natural conditions. When Ira Remsen of Johns Hopkins Uni- 

 versity presented his students with the customary formula for a 

 double salt, e.g., PtCl 4 . 2KC1, he would point to the period in the 

 formula and remark with a twinkle in his eye, "That period has 

 for many years been a full stop to thought. Don't let such devices 

 keep you from trying to find out what lies behind them." Even 

 the more informative "structural formulas" are only static dia- 

 grams, which give no indication of the dynamics of atomic and 

 molecular structure, though in some cases straight or curved arrows 

 are added to indicate oscillatory or resonance changes. 1 



Though three-dimensional isomers (stereoisomers) have been known 

 since the discovery by Pasteur of right- and left-handed tartaric acids, 

 the extent of this phenomenon and its biological importance are only 

 now being increasingly recognized. For example, Professor L. Zech- 

 meister of the California Institute of Technology 1 gives skeleton 

 models of the twenty stereoisomers of beta carotene. (See Figure 3). 

 Many of the naturally occurring carotenoids have a much larger 

 number of calculable stereoisomers, two having as many as 128. 



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