CHAPTER XXI 



CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, AND MANGANESE 



SULPHUR, selenium, and tellurium belong to the uneven series of the 

 Sixth group. In the even series of this group there are known chro- 

 mium, molybdenum, tungsten, and uranium ; these give acid oxides 

 of the type R0 3 , like SO 3 . Their acid properties are less sharply 

 defined than those of sulphur, selenium, and tellurium, as is the case 

 with all elements of the even series as compared with those of the 

 uneven series in the same group. But still the oxides Cr0 3 , Mo0 3 , 

 WOo, and even U0 3 , have clearly defined acid properties, and form 

 salts of the composition MO,nR0 3 with bases MO. In the case of the 

 heavy elements, and especially of uranium, the type of oxide, U0 3 , 

 is less acid and more basic, because in the even series of oxides the 

 element with the highest atomic weight always acquires a more and 

 more pronounced basic character. Hence UO 3 shows the properties of 

 A base, and gives salts UO 2 X 2 . The basic properties of chromium, 

 molybdenum, tungsten, and uranium are most clearly expressed in the 

 lower oxides, which they all form. Thus chromic oxide, Cr 2 O 3 , is as. 

 distinct a base as alumina, A1 2 O 3 . 



Of all these elements chromium is the most widely distributed, 

 and the most frequently used. It gives chromic anhydride, Cr0 3 , and 

 chromic oxide, Cr 2 3 two compounds whose relative amounts of 

 oxygen stand in the ratio 2:1. Chromium is, although somewhat 

 rarely, met with in nature as a compound of one or the other type. 

 The red chromium ore of the Urals, lead chromate or crocoisite 

 PbCrO 4 , was the source in which chromium was discovered, by 

 Vauquelin, who gave it this name (from the Greek word signifying 

 colour) owing to the brilliant colours of its compounds ; the chromatea 

 (salts of chromic anhydride) are red and yellow, and the chromic salts 

 (from Cr 2 O 3 ) green and violet. The red lead chromate is, however, a 

 rare chromium ore found only in the Urals and in a few other localities. 

 Chromic oxide, Cr 2 O 3 , is more frequently met with. In small quantities 

 it forms the colouring matter of many minerals and rocks for example, 



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