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Covalent Bonding and Electronegativity Chapter 9… (9.5) A covalent bond between two atoms: X–Y • In a covalent bond, electrons are shared; sharing is uneven, unless X and Y are the same. • The Electronegativity of an element is a measure of how much the element attracts the shared electrons in a chemical bond. • Electronegativity increases with increasing Zeff. It follows a similar trend to EA (both are involved with how badly an element wants to accept electrons), and has an inverse relationship to atomic size. • So electronegativity increases as you go up a group or left to right across a period. Figure 9.16 The Pauling electronegativity (EN) scale. Figure 9.17 Electronegativity and atomic size. Electronegativity (cont.) • The greater the electronegativity difference (DEN) in a bond, the more uneven the electron sharing is, and the bond has greater ionic character (remember we said bonding models represent extremes; in fact there is a continuum of bonding types) • A bond in which electrons are shared unevenly is called a polar covalent bond. Polar bonds have both covalent and ionic character. • e.g. H–H non polar ∂– O polar – more electron density on O, partial negative ∂+ H H charge (∂–), dipole moment. Na+Cl– – Extreme case of polar bond – Electron completely transferred from Na to Cl 3.0 Figure 9.18 2.0 DEN Boundary ranges for classifying ionic character of chemical bonds. 0.0 SAMPLE PROBLEM 9.3 Determining Bond Polarity from EN Values PROBLEM: (a) Use a polar arrow to indicate the polarity of each bond: N-H, F-N, I-Cl. (b) Rank the following bonds in order of increasing polarity: H-N, H-O, H-C. PLAN: (a) Use Figure 9.16(button at right) to find EN values; the arrow should point toward the negative end. (b) Polarity increases across a period. SOLUTION: (a) The EN of N = 3.0, H = 2.1; F = 4.0; I = 2.5, Cl = 3.0 N-H F-N I - Cl (b) The order of increasing EN is C < N < O; all have an EN larger than that of H. H-C < H-N < H-O Compound Semiconductors • The Group IVA (14) elements Si and Ge are semiconductors; can also make compound semiconductors, usually isoelectronic to Group IVA. e.g. GaAs, isoelectronic to Ge, has Zinc Blende structure, same as diamond structure of Ge, but Eg is different Substance DEN Band Gap Germanium 0 0.66 eV GaAs 0.4 1.42 eV ZnSe 0.8 2.70 eV CuBr 0.9 2.90 eV • Band gap correlates with DEN. As DEN increases, bonding becomes more ionic, making the electrons harder to move, so band gap increases.
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