2. CHEMICAL BONDING Chemical bonding can involve two extremes: (1) the equal sharing of electrons between two like atoms to form a covalent bond, and (2) the complete transfer of an electron from one atom to another to form an ionic bond. Between these extremes is a range of bonds in which the electron pair is on the average closer to one atom than the other. An example of unequal sharing of electrons is found in hydrogen fluoride.

3. The bonding electrons are on the average closer to the fluorine than to the hydrogen atom. The movement of the negatively charged electrons away from hydrogen toward fluorine, due to a difference in electronegativity, builds up a partial negative charge on the fluorine and a partial positive charge on the hydrogen.

4. This is not a complete transfer of an electron from hydrogen to fluorine; it is merely a drifting of electrons toward fluorine. H F ---- ---- ---- ---- ---- ----

5. H F ---- When a charge separation of this type is present, the molecule possesses an electric dipole, and the bond is called a polar covalent bond, or simply a polar bond. The direction of the electric dipole of a bond may be predicted from the electronegativity of the atoms.

6. Describe the bonding which exists between chlorine and silicon. Look up, on the periodic table, the electronegativities (En) of chlorine and silicon. Chlorine - 3.16 Silicon -1.90 Find the difference in electronegativity (  En) 3.16 - 1.90= 1.26 Use the % ionic character table on the periodic table

7. Obviously 1.26 is in between 1.20 and 1.30 so interpolation is necessary If the %IC scale (30% - 34%) is divided into 10 equal parts, each part is 4% /10 = 0.4%, so the table becomes  En of 1.26 has a %IC of 32.4

8. 1.201.211.221.231.241.251.261.271.281.291.30 Difference in Electronegativity % Ionic Character 30.430.831.231.6 32 32.432.833.233.6 3034  En of 1.26 = 32.4% IC

9. If the %IC is 0 the bond is covalent. If 0 <%IC< 50 the bond is polar covalent If %IC > 50 the bond can be described as ionic. The bond between silicon and chlorine is polar covalent. This means when they share an electron pair the electrons spend more time closer to the chlorine since it has a greater electronegativity, making the chlorine end more negative and the silicon end more positive. This can be illustrated by:

10. Cl Si %IC of 32.4

11. Describe the bonding which exists between bromine and phosphorus. Look up, on the periodic table, the electronegativities (En) of bromine and phosphorus. bromine - 2.96 phosphorus -2.19 Find the difference in electronegativity (  En) 2.96 - 2.19= 0.77 Use the % ionic character table on the periodic table

12. 0.700.710.720.730.740.750.760.770.780.790.80 Difference in Electronegativity % Ionic Character 12.312.612.913.2 13.5 13.814.114.414.7 1215  En of 0.77 = 14.1% IC

13. If the %IC is 0 the bond is covalent. If 0 <%IC< 50 the bond is polar covalent If %IC > 50 the bond can be described as ionic. The bond between phosphorus and bromine is polar covalent. This means when they share an electron pair the electrons spend more time closer to the bromine since it has a greater electronegativity, making the bromine end more negative and the phosphorus end more positive. This can be illustrated by:

14. Br P %IC of 14.1

15. Sample Problems 1. Using the chart on the periodic table determine the % Ionic Character for each of these bonds? (a) B--Cl (c) C--I (e) N--S (b) Ca--F (d) O--Br

16. LEWIS DIAGRAMS OF MOLECULES Lewis based his idea of covalent bonding on Bohr's planetary model of the atom in which electrons revolve in shells around the nucleus. The electrons were assigned to various shells so that the number of electrons in the outermost shell of the atoms in the Group A families of the periodic table equals their group number.

17. According to the accepted theory only outer electrons are involved in electron transfer to form ions. So Lewis reasoned that only electrons in the outer shell took part in covalent bonding. He represented the valence electrons by electron dot formulas, or diagrams. Lewis also assumed that atoms tend to acquire a noble gas electron arrangement when they form covalent molecules.

18. Lewis's ideas can be used to understand covalent bonding in molecules. Consider the hydrogen fluoride molecule HF. The hydrogen atom has one electron and the fluorine atom has seven in its outermost shell. The hydrogen and fluorine atoms may approach one another and each contribute its unpaired electron to form an electron pair which is shared between them.

19. H + F -----> H F The dot diagram for water is: 2H + O ------- O H H Sample Problems Draw both electron dot and structural diagrams for Cl 2, NH 3, ICl, CF 4, SCl 2, CH 2 Cl 2.