Phosphorus, elemental nitrogen halides

The radius of an atom helps to determine how many other atoms can bond to it. The small radii of Period 2 atoms, for instance, are largely responsible for the differences between their properties and those of their congeners. As described in Section 2.10, one reason that small atoms typically have low valences is that so few other atoms can pack around them. Nitrogen, for instance, never forms penta-halides, but phosphorus does. With few exceptions, only Period 2 elements form multiple bonds with themselves or other elements in the same period, because only they are small enough for their p-orbitals to have substantial tt overlap (Fig. 14.6). 

It is easy to reduce anhydrous rare-earth halides to the metal by reaction of more electropositive metals such as calcium, lithium, sodium, potassium, and aluminum. Electrolytic reduction is an alternative in the production of the light lanthanide metals, including didymium, a Nd—Pr mixture. The rare-earth metals have a great affinity for oxygen, sulfur, nitrogen, carbon, silicon, boron, phosphorus, and hydrogen at elevated temperature and remove these elements from most other metals. 

The members of class (b) are located in a small region in the periodic table at the lower right-hand side of the transition metals. In the periodic table of Figure 6-11, the elements that are always in class (b) and those that are commonly in class (b) when they have low or zero oxidation states are identified. In addition, the transition metals have class (b) character in compounds in which their oxidation state is zero (organometallic compounds). The class (b) ions form halides whose solubility is in the order F > Cr > Br > 1 . The solubility of class (a) halides is in the reverse order. The class (b) metal ions also have a larger enthalpy of reaction with phosphorus donors than with nitrogen donors, again the reverse of the class (a) metal ion reactions. 

Not only is the sensitivity of the AFD much higher for phosphorus than for all the other elements, but also the selectivity is much better. While the selectivity of phosphorus compounds as compared with carbon compounds may be in the order of 10 to 10 , the selectivity for halides is between 10 and 10 , nitrogen occupies the same range, and sulfur and arsenic are more than one order of magnitude lower. 

Other elements, such as halides, various metals and metalloides (iron, manganese, sodium, magnesium, zinc, mercury), nitrogen and phosphorus were also cycled through main Earth s components as a results of volcanism, hydrothermal venting and tectonic shifting of the crust. We can see the summary ofprebiotic element cycles in Figure 15. 

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