618 TABLES 654-659.— ATOMIC AND MOLECULAR DATA 



Just a few years ago it was held that the universe was made up of 92 ele- 

 ments and that probably these elements were made of two elementary particles. 

 While most of these 92 elements had been identified and their properties 

 studied, there were several that had not been identified and thus very little 

 was known directly about their properties. 



As a result of a great amount of study and investigation, during the past 

 few years the number of known elementary particles has been extended to 

 seven or eight (see Table 720), and all the elements missing from the periodic 

 table (see Table 658) have been identified and some of their properties 

 studied. 199 In addition to this, the number of elements has been extended to 

 five or six beyond uranium and some of the properties of these elements have 

 been studied. (See Table 658.) 



It is now generally considered that the elements are made up of electrons, 

 protons, and neutrons. Each element now has three designations : the name ; 

 the atomic number, Z, i.e., the charge on the atomic nucleus and the mass num- 

 ber, A, which is the number of protons and neutrons that make up the nucleus 

 of the atom and extends from 1 for hydrogen (or the neutron) to 246 for the 

 isotope of californium. This mass number is not too definite since, in many 

 cases, several atoms have isotopes of the same mass number. 



Atoms of number greater than 83 and certain isotopes of eight atoms of 

 lower atomic number, are unstable in that they break down into other isotopes, 

 i.e., they are radioactive. (See Table 732.) There are in all about 1,220 

 different isotopes 199 that have been identified and have had some of their 

 properties studied. Of these only 274 are stable. A number of atoms 200 

 (Z = 43, 61, 85, 93, 94, 95, 96) are so unstable that they are not now found 

 on the earth. Two of the isotopes, A = 5, and 8, have so short a life that it is 

 almost impossible to detect them. A radioactive material with a life shorter 

 than about 10" 20 sec and longer than about 10 14 years will be unobservable 

 as such. 



The values given for certain physical dimensions of molecules, atoms, or 

 nuclei depend upon the definition of the particular dimension and the method 

 used in its calculation. Diameters may be calculated from Van der Waal's 

 equation, from viscosity, and from certain force relations. Some values are the 

 results of assuming the atom or nucleus to be a sphere. While these various 

 methods give results that do not differ too much, neither are the results in 

 good enough agreement for one to feel that the answer is final. The following 

 tables give some results of physical dimension obtained by various means of 

 calculation. 



199 Seaborg and Perlman, Rev. Mod. Phys., vol. 20, p. 585, 1948. 



200 Bethe, H. A., Elementary nuclear theory, John Wiley & Sons, Inc., 1947. 

 by permission. 



Reprinted 



TABLE 654.— CONVERSION FACTORS FOR UNITS OF MOLECULAR ENERGY* 



Units Erg/molecule 



1 erg/molecule = 1 



1 joule/mole = 1.660349X10" 

 1 cal/mole = 6.94690 X10" 

 1 electron-volt/ 



molecule = 1.601992X 10~ 12 96.4853X 10 

 1 wave No (cm" 1 ) = 1. 985776 XlO" 16 11.95999 



Joule/mole Cal/mole 



6.02283X10 16 1.43491 X10 1 



1 

 4.1840 



.239006 



2.30605 XlO 4 

 2.85851 



Electron-volt/ 

 molecule 



Wave No t 

 (cm- 1 ) 



6.2422 XlO 11 5.03581X10" 

 1.036427X10- 8 8.36121 XlO" 8 

 4.33641 XlO" 8 .349833 



1 

 1.239567X10" 4 



8.06734X10 3 

 1 



* This table adapted from data furnished by the National Bureau of Standards, 

 where the values given are for v = unity. 



t This means Av/molecule 



SMITHSONIAN PHYSICAL TABLES 



