126 PRINCIPLES OF CHEMISTRY. 



between the atomic weights will hardly be suspected by looking at 

 the figures, but will be noticed at once by adding together the atomic 

 weights of the first and last elements and dividing this sum by 2, 

 when the atomic weights (very nearly, at least) of the middle mem- 

 bers of the series are obtained. Thus : 

 8S.2 + 125.9 _ go 55 . 31.8 + 126.6 __ 79 _ 2 . 7.0 + 38.8 = 22>g . 39.8 + 136.4 = g8 _ L 



Mendelejeff's periodic law. 1 The relationship between atomic 

 weights and properties has been used for arranging all elements sys- 

 tematically in such a manner that the existing relation is clearly 

 pointed out. Of the various schemes proposed, the one arranged by 

 Mendelejeff may be selected as most suitable to show this relation. 



Looking at Mendelejeff's table on page 128 it will be seen that all 

 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 nine 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 groups and VIII. for the present, we find 

 that the power of the elements to combine with oxygen atoms 

 increases regularly from the left to the right, while the power of the 

 elements to combine with hydrogen atoms increases from the right to 

 the left, as may be shown by the following instances : 



I. II. III. IV. V. VI. VII. 



N^O MgO A1 2 3 SiO 2 P 2 O 5 SO 3 Cl a O 7 



Hydrogen compounds unknown SiH 4 PHs 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. 



i The consideration of this law should be postponed until the student has become acquainted 

 with the larger number of important elements. 



