Covalent Bonding Notes

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Notes taken in class about covalent bonding.

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Covalent Bonding Notes

  1. 1. Covalent Bonding Many substances do not have the characteristics of ionic materials. The atoms of covalent substances acquire a noble gas electron configuration by sharing electrons with other atoms. Covalent bonding occurs when two or more nonmetals share electrons, attempting to attain a stable octet of electrons. Bond Polarities and Electronegativity The electron pairs shared between two atoms are not necessarily shared equally. We can visualize two extreme cases in the degree to which electrons are shared. On the one hand, we have bonding between two identical atoms, as in Cl2 or N2 where the electrons must be shared equally. At the other extreme, we have essentially no sharing of electrons, as illustrated by NaCl.
  2. 2. The bonding occurring in most covalent substances fall somewhere between these two extremes. In practice we describe bonds as either ionic or covalent, depending on what extreme the bond more closely resembles. The concept of bond polarity is useful in describing the sharing of electrons. Nonpolar (pure covalent) - electrons are shared equally. Polar covalent- electrons are shared unequally. Ionic - difference in ability to attract electrons is very large. We use a quantity called electronegativity to estimate whether a given bond will be nonpolar, polar covalent, or ionic. Electronegativity is defined as the ability of an atom in a molecule to attract electrons to itself. The greater an atom's electronegativity, the greater it's ability to attract electrons to itself.
  3. 3. American chemist Linus Pauling first developed the concept of electronegativity. He based his scale on bond- energy relationships. We will not be concerned with how the electronegativity values are obtained, but rather with using the concept in discussing chemical bonding. Keep in mind that electronegativities are approximate measures of the relative tendencies of the atoms of elements to attract electrons to themselves in a chemical bond. We can use the difference in electronegativity between two atoms to gauge the polarity of the bonding between them. Consider the fluorine- containing compounds listed below: Compounds F2 HF LiF Electronegativity 0 1.8 3.0 Difference Type of Bond non polar polar ionic
  4. 4. In F2, the electrons are shared equally between the fluorine atom, and the bond is nonpolar. In HF, the fluorine atom has a greater electronegativity than the hydrogen atom. Consequently the sharing of electrons is unequal (the bond is polar) We represent the sharing of the electrons in the following two ways. - + H-F or δ+ H-F δ- + and - are meant to the represent partial positive and partial negative charges + - respectively. In LiF, the far greater electronegativity of fluorine as compared to Lithium leads to the complete transfer of valence electrons off Li to F. This transfer results in the formation of Li+ and F- ions, the resultant bond is therefore ionic. Remember: the greater the electronegativity difference, the more polar the bond.
  5. 5. Electronegativity Bond Type Difference 0- 0.5 Nonpolar 0.51-2.0 Polar 2.1 - above Ionic

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