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Anonymous asked in 科學及數學化學 · 10 years ago

AL Chemistry

PbCl2 is an ionic compound while PbCl4 exists as simple molecules.

Why PbCl4 is not existed as an ionic compund?

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  • 10 years ago
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    The outer valency shell of element Lead is  6s26p2  Lead has oxidation    number of +4 and +2.There's nothing surprising about the normal Group 4 oxidation state of +4. The typical oxidation state shown by elements in Group 4 is  +4, found in compounds like CCl4  and SiCl4.All of the elements in thegroup 4 have the outer electronic structure ns2npx1npy1,where n varies from 2 (for carbon) to 6 (for lead). The oxidation state of  +4 is where all these outer electrons are directly involved in the bonding.You need to think about why carbon normally  forms four covalent bonds rather than two. There are only two unpaired  electrons. Before carbon forms bonds, though, it normally promotes one of the s electrons to the empty p orbital. That leaves 4 unpaired electrons which (after hybridisation) can go on to form 4 covalent bonds. In PbCl4 , the lead atom acts like a normal carbon atom. 4 unpaired electrons form 4 covalent bonds with chlorine.As you get closer to the bottom of the Group 4, there  is an increasing tendency for the  s2  pair not to be used in the bonding. This is often known as the  inert pair effect  and is dominant in lead chemistry. Because of large size of the lead atom, its valency electrons are much less strongly held than those of carbon. Thus lead loses two 6p  electrons from its ground state configuration and is quite ready to form the Pb++ ion , leaving the 6s pair unused – “inert pair”.  PbCl2,is an ionic compound, composed of one Pb++ and two Cl-  ions. 

    The lead(II)oxidation state is the more stable, and there is a strong tendency for lead(IV)compounds to react to give lead(II) compounds. Lead(IV) chloride decomposes atroom temperature to give lead(II) chloride and chlorine gas.

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