p17p35ujimn9ejc9cap81i1j7e4.pdf

C BONDING GENERALIZATIONS BASED ON PERIODIC TRENDS IN THE ELECTRONEGATIVITY
Table 1.4. Selected properties of the oxides of group IV elements.
compound common name T m °C electroneg.
CO 2 dry ice 57 (at 5.2 atm) 1.0
SiO 2 quartz 1610 1.7
GeO 2 1090 1.7
SnO 2 cassiterite 1630 1.7
TiO 2 rutile 1830 2.0
ZrO 2 zirconia 2700 2.1
Table 1.5. Selected properties of three isoelectronic polar-covalent solids.
group(s) material ionicity ( f ) band gap T m °C
IV Ge 0 % 0.7 eV 1231
III–V GaAs 4 % 1.4 eV 1510
II–VI ZnSe 15 % 2.6 eV 1790
I–VIII CuBr 18 % 5.0 eV 492
Finally, the variation of melting temperature and band gap with the ionicity
fraction in isoelectronic solids that exhibit partial ionic and covalent bonding is
illustrated in Table 1.5. The band gap is the separation, in energy, between the
highest filled electron energy level in the crystal and the lowest empty electron
energy level. Radiation at energies equal to or greater than the band gap will be
absorbed by the solid and promote electrons to higher energy unfilled states.
Thus, the band gap is a quantitative parameter that influences the appearance of
the solid. Since visible light varies in energy from 1.7 to 3.0 eV, nondefective
solids with band gaps greater than 3.0 eV transmit all visible light and are thus
transparent and colorless. Solids with band gaps less than 1.8 eV are opaque. If
the band gap is much less than 1.7 eV (but greater than zero), the crystal will be
black. From Table 1.5, we can see that the band gap increases with ionicity and
we can infer that compounds with greater than 50% ionicity should have large
band gaps and, therefore, be colorless. This is a simple explanation for why most
ceramics (ionically bound materials) are colorless and most semiconductors
(covalently bound materials) are black.
In conclusion, a brief survey of properties demonstrates that the periodicity
of electronegativity and metallicity leads to a periodicity of bonding type.
Because certain properties (electronic, optical) are also linked to bonding type,
we see that there is also a periodicity of properties, as implied by Fig. 1.3.
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