Answers Questions 16.2 Molecular polarity 1. Write a dot diagram for the HCl molecule. Find the difference in electronegativity between the hydrogen and chlorine atoms Difference in electronegativity = 1.06 Is the bond nonpolar covalent, polar covalent, or ionic? Polar covalent What is the percentage ionic character in the bond? 22% ionic character Which atom is slightly negative and which is slightly positive? Why? Chlorine is slightly negative because it has a higher attraction for the bond electrons than the hydrogen. About 22% of the time the electron bond pair is around the chlorine atom only. The element with the higher electronegativity will always be slightly negative. Hydrogen is slightly positive because the bond electrons are completely absent part of the time. 2. Why does increasing the number of pi bonds add to charge density? Sigma bonds are formed from hybridized orbitals and will have some s orbital and some p orbital character. Single bonds are always sigma bonds. Pi bonds are formed from an overlap of unhybridized p orbitals. The pi bonds will overlap the sigma bonds, creating a volume with a larger charge density because you have more electron bond pairs in the volume between the atoms bonded together. Single bond Sigma bond only
Double Bond 1 Sigma bond and 1 Pi bond The sigma bonds are arranged at 120 degrees in a plane. The pi bond has two parts, one above the plane and one below the plane. The pi bond overlaps the sigma to form the double bond.
Triple Bond 1 Sigma bond and 2 Pi bonds The triple bond is formed from one sigma bond and two pi bonds. The sigma bond forms a linear molecule (180 degrees) while the two pi bonds overlap the sigma to form a "pig in a blanket" tunnel of pi electrons.
3. Build models and draw sketches of CH 4, CH 3 Cl, CH 2 Cl 2, CHCl 3, and CCl 4 All models have 4 single bonds, sp3 hybridized, with a bond angle of 109.5 degrees. No pi bonds are present. Is the carbon-hydrogen bond polar? The C-H bond in nonpolar (difference in electronegativity of 0.45) Is the carbon-chlorine bond polar? The C-Cl bond is polar (difference in electronegativity of 0.61) Which atom is negative and which is positive? Chlorine is slightly negative since it has the higher electronegativity Which of the CH 3 Cl, CH 2 Cl 2, CHCl 3, and CCl 4 molecules has the biggest distance between the center of positive and negative charges? CH 3 Cl has the largest distance between the center of positive and negative charges. The dipole moment is a vector from the center of positive charge to the center of negative charge of the molecule. The larger the dipole moment the more polar the molecule. Why is the center of negative and positive charges the same in CCl 4? Because the slightly negative chlorines are arranged symmetrically the centers of positive and negative charges are both in the center of the molecule.
4. Draw a sketch of BBr 3 Is the boron-bromine bond polar or nonpolar? Difference of electronegativity = 0.92 Polar covalent bond with bromine negative and boron positive. Locate the center of positive and negative charge in the BBr 3 molecule. Why is the molecule nonpolar? The slightly negative charges are distributed symmetrically on the molecule In general molecular polarity can be summed up as follows: Negative areas of a molecule are caused by more the electronegative atom of polar bonds, with the strength of the negative charge increasing with higher percentage ionic character. Lone pairs and pi bonds produce weaker areas of negative charge. If a molecule has no polar bonds, lone pairs, or pi bonds it cannot be polar. If a molecule has negative charges distributes symmetrically it is not polar. If polar bonds are on the opposite end of a molecule from nonpolar bonds the molecules will be polar because the centers of positive and negative charge will be far apart.
Finding the shapes of molecules from the hybridization 5. For each molecule below sketch the molecule, tell if each bond is polar or nonpolar, put S- by each negative part of the molecule and S+ by each positive part of the molecule, and tell if the molecule is polar or nonpolar. Na 2 S Sulfur has 2 bond pair and 2 lone pair (sp3 hybridized) (distorted tetrahedral) Difference in electronegativity is 1.65. Polar covalent bond with sulfur negative and sodium positive
AlF 3 Aluminum has 3 bond pair no lone pair (sp 2 hybridized) (Trigonal planar) Difference in electronegativity is 2.37. Ionic Bond SiI 4 Silicon has 4 bond pair, no lone pair (sp 3 hybridized) (Tetrahedal) Difference in electronegativity is 0.76 Polar covalent bond Iodine is negative, Silicon is positive NH 3 Nitrogen has 3 bond pair, 1 lone pair. (sp 3 hybridized) (distorted tetrahedral) Difference in electronegativity = 0.94 Polar Covalent bond Nitrogen is slightly negative, Hydrogen is slightly positive
H 2 O Oxygen has 2 bond pairs and 2 lone pairs (sp 3 hybridization) (distorted tetrahedral) Difference in electronegativity = 1.34 Polar covalent bond Oxygen is slightly negative, Hydrogen is slightly positive PCl 3 Phosphorus has 3 bond pair and 1 lone pair (sp 3 hybridized) (distorted tetrahedral) Difference in electronegativity is 0.97 Polar covalent bond Chlorine is slightly negative, phosphorus is slightly positive
AsBr 5 Arsenic must have 5 bond pairs and no lone pairs. (sp 3 d hybridized) (trigonal bipyramidal) Difference in electronegativity = 0.78 Polar covalent bond Bromine is slightly negative, Arsenic is slightly positive SCl 6 Sulfur has 6 bond pairs, no lone pairs (sp 3 d 2 hybridized) (octahedral) Difference in electronegativity = 0.58 Polar covalent bond Chlorine slightly negative, sulfur slightly positive