Size of a Molecule To measure a very small size like that of a molecule 108 m to 10 10 m we have to adopt special methods We cannot use a screw gauge or similar instruments Even a microscope has certain limitations An optical microscope uses visible light to look at the system under investigation As light has wave like features the resolution to which an optical microscope can be used is the wavelength of light A detailed explanation can be found in the Class XII Physics textbook For visible light the range of wavelengths is from about 4000 to 7000 1 10 0 m Hence an optical 1 angstrom microscope cannot resolve particles with sizes smaller than this Instead of visible light we can use an electron beam Electron beams can be focussed by properly designed electric and magnetic fields The resolution of such an electron microscope is limited finally by the fact that electrons can also behave as waves You will learn more about this in class XII The wavelength of an electron can be as small as a fraction of an angstrom Such electron microscopes with a resolution of 0 6 have been built They can almost resolve atoms and molecules in a material In recent times tunnelling microscopy has been developed in which again the limit of resolution is better than an angstrom It is possible to estimate the sizes of molecules A simple method for estimating the molecular size of oleic acid is given below Oleic acid is a soapy liquid with large molecular size of the order of 109 m The idea is to first form mono molecular layer of oleic acid on water surface We dissolve 1 cm of oleic acid in alcohol to make a solution of 20 cm Then we take 1 cm