GENERAL REMARKS REGARDING ELEMENTS. 127 



While some elements show an exception, it may be stated that 

 most of the elements of group I. are univalent, of II. bivalent, of 

 III. trivalent, of IV. quadrivalent, of V. quinquivalent, of VI. 

 sexivalent, and of VII. septivalent. 



Properties other than those above mentioned might be enumerated 

 in order to show the regular gradation which exists between the 

 members of the various series, but what has been pointed out will 

 suffice to prove that there exists a regular gradation in. the properties 

 of the elements belonging to the same series, and that the same change 

 is repeated in the other series, or that the changes in the properties of 

 elements are periodic. It is for this reason that a series of elements 

 is called a period (in reality a small period, in order to distinguish it 

 from a large period, an explanation of which term will be given 

 directly). 



The 12 series or periods given in the following table show another 

 highly characteristic feature, which consists in the iact that the corre- 

 sponding members of the even (2, 4, 6, etc.) periods and of the uneven 

 (3, 5, 7, etc.) periods resemble each other more closely than the mem- 

 bers of the even periods resemble those of the uneven periods. Thus 

 the metals calcium, strontium, and barium, of the even periods, 4, 6, 

 and 8, resemble each other more closely than they resemble the metals 

 magnesium, zinc, and cadmium, of the uneven periods, 3, 5, and 7, the 

 latter metals again resembling each other greatly in many respects. 



It is for this reason that in the table the elements belonging to one 

 group are not placed exactly underneath each other, but are divided 

 into two lines containing the members of even and uneven periods 

 separately, whereby the elements resembling each other most are 

 made to stand together. 



In arranging the elements by the method indicated, it was found 

 that the elements mentioned in group VIII. could not be placed in 

 any of the 12 small periods, but that they had to be kept separately 

 in a group by themselves, three of these metals always forming aL 

 intermediate series following the even periods 4, 6, and 10. 



An uneven and even series, together with an intermediate series, 

 form a large period, the number of elements contained in a complete 

 large period being, therefore, 8 + 8 + 3 = 19. 



An apparently objectionable feature is the incompleteness of the 

 table, many places being left blank ; but it is this very point which 

 renders the table so highly interesting and valuable. 



Mendelejeff, in arranging his scheme, claimed that the places left 

 blank belonged to elements not yet discovered, and he predicted not 

 only the existence of these as yet missing elements, but also described 



