3.26 Sspd_chapter 1_part 11_solid state of matter_diifferent

Chapter 1_Part 11_Solid State of Matter_ Bonding.

1.10.3. BONDING IN SOLIDS

In Group IV elements there is 100% sharing of electrons leading to 100% covalent bond. 4 electrons in outer most orbit of an atom are shared with 4 electrons of the four neighboring atoms. In the process the given atom fulfills its octave condition of eight electrons as well the four neighboring atoms also fulfill their respective octave condition. This leads to the formation of very stable and strong covalent bonds. Covalent bond fully depends upon time sharing of outer most electrons with the neighboring atoms.

In contrast NaCl has ionic bond created by columbic attraction between positively charged Na(Group I) ion by virtue of losing its lone outer most electron to Cl(Group VII) which becomes negatively ionized. This ionic bond has 100% transfer of electron from Na to Cl.

1.10.3.1. COVALENT BOND.

Covalent bond of Group IV elements exhibit a very interesting feature. Though all elements of Group IV have covalent bonds they exhibit varying physical properties. As we move down from Carbon to Silicon to Germanium to Gray Tin and finally to Lead we find hardness, melting point and electrical conductivity changing over a wide range. Diamond , an allotrope of Carbon, is the hardest material and has very high melting point of 3550ºC whereas Tin is very soft and has low melting point. (It has just been announced that “Diamond is no longer nature’s hardest material”,Jessica Griggs, New Scientist, 16 ^th February,2009. Wurtzite Boron Nitride is 18% harder and Mineral Lonesdaleite is 58% harder than diamond.) Diamond is very good insulator, Si and Ge are semi-conductors and Tin is a good conductor. In fact one finds a gradual transition from a extreme valence bond in Diamond to metallic bond in Lead. Diamond, Silicon, Germanium, Silcon Carbide , tin and rutile form covalent bonds.

The gradual transition from extreme valence bond to metallic bond occurs because of gradual reduction in the energy band gap between Valence Band and Conduction Band. Energy Band Gap is 5.2eV in Diamond. Whereas it is 1.21eV in Si, 0.67eV in Ge, 0.08eV in Gray Tin and 0eV in Lead. This leads to increasing intrinsic carrier concentration and hence to increasing metallicity as we move from Diamond to Gray Tin. In Table 1.15 we make a comparative study of Group IV elements.

Table 1.15. Room Temperature Energy Band Gap(E g ), intrinsic carrier concentration (n i ) and resistivity(ρ)

1.10.3.2 IONIC BOND.

Just as elements of Group IV complete their octave condition (closed valence energy shell) by time sharing of electrons, elements of alkali group (Li, Na, K, Rb, Cs) and elements of Group VII, halogen Group(F, Cl, Br, I) complete their octave condition by exchanging electrons. Alkali metal releases its single electron in its outermost orbit to achieve the octave condition and becomes positively charged Cations. Halogen group acquires electron given up by Alkali metal to achieve the octave condition and becomes negatively charged Anion. By acquiring single electron from the alkali metal it completes 8 electrons in outermost orbit where it had 7 electrons from before. By completing this set of 8 electrons it fulfils the octave condition. These two oppositely charged ions bond together to form 100% ionic bond in alkali halide salts. Examples are NaCl,KF, LiF, MgO, CsCl and ZnS. Since both the ions have their electrons tightly bound hence ionic salts are insulators. Since ions are regularly arranged in solid hence they are crystalline in nature: fcc(face centered cube) in NaCl with coordination number 6 and bcc( body centered cube) in CsCl with coordination number 8. [fcc and bcc will be discussed in next section].