GENERAL REMARKS REGARDING ELEMENTS. ^3 



Looking at MendelejefF s ';able on page 64, it will be seen that ail 

 the elements are arranged in the order of their atomic weights, and 

 that the latter increase gradually by only a unit or a few units. 

 Moreover, the arrangement is such that eight groups and twelve 

 series are formed. The remarkable features of this classification 

 may thus be stated : Elements which are more or less closely allied 

 in their physical and chemical properties are made to stand together 

 in a group, as may be seen by pointing out a few of the more gen- 

 erally known instances as found in the groups I., II., and VII., the 

 first one containing the alkali metals, the second, the metals of the 

 alkaline earths, the last the halogens. 



There is, moreover, to be noticed a periodic repetition in the prop- 

 erties of the elements arranged in the horizontal lines from left to 

 right. Leaving out group VIII. for the present, we find that the 

 power of the elements to combine with oxygen atoms increases regu- 

 larly from the left to the right, whilst the power of the elements to 

 combine with hydrogen atoms increases from the right to left, as may 

 be shown by the following instances : 



I. II. Ill IV. V. VI. VII. 



Na 2 MgO A1 2 O 3 SiO 2 P 2 O 5 SO 3 C1 2 O T 



Hydrogen compounds unknown SiH^ PH 3 SH 2 C1H 



The oxides on the left show strongly basic properties, as illustrated 

 by sodium oxide ; these basic properties become weaker in the second, 

 and still weaker in the third group ; the oxides of the fourth group 

 show either indifferent, or but slightly acid properties, which latter 

 increase gradually in the fifth, sixth, and seventh groups. 



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 



