62 PRINCIPLES OF CHEMISTRY. 



classification of all elements is the one dividing them into two groups 

 of metals and non-rnetals. 



^Metals are all elements which have that peculiar lustre known as 

 metallic lustre ) which are good conductors of heat and electricity; 

 which, in combination with oxygen, form compounds generally 

 showing basic properties ; and which are capable of replacing hy- 

 drogen in acids, thus forming salts. 



Non-metals or metalloids are all elements not having the above- 

 mentioned properties. Their oxides in combination with water gen- 

 erally have acid properties. In all other respects the chemical and 

 physical properties of non-metals differ widely. Their number 

 amounts to 14, the other 55 elements being metals. 



Natural groups of elements. Besides classifying all elements 

 into metals and non-metals, certain members of both classes exhibit 

 so much resemblance in their properties, that many of them have 

 been arranged into natural groups. The members of such a natural 

 group frequently show some connection between atomic weights and 

 properties. 



Chlorine, 354 Sulphur, 32 Lithium, 7 Calcium, 40 



Bromine, 80 Selenium, 78.8 Sodium, 23 Strontium, 87 



Iodine, 126.5 Tellurium, 125 Potassium, 39 Barium, 137 



Each three elements mentioned in the above four columns resemble 

 each other in many respects, forming a natural group. The relation 

 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 : 



35.4 + 126.5 32 + 125_ 7g5 . 7 + 39_ 23 . 40 + 137__ g85 



2222 



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. 



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

 with the larger number of important elements. 



