Lesson 37 Covalent Compounds Objectives: - The student will define covalent bond and molecular compound. - The student will classify bonds as ionic, polar covalent, or non-polar covalent. - The student will list and explain the characteristics of the three types of intermolecular forces. PA Science and Technology Standards: A; A; D

I. Molecular compounds have covalent bonds. a. Comparing water and sodium chloride i. Melting and boiling points – NaCl High, H 2 O Low 1. Melting Points  Water – 0 o C, NaCl – 801 o C 2. Boiling Points  Water – 100 o C, NaCl – 1413 o C 3. This tells us that there must not be as strong an attraction between water molecules as is between salt ions. ii. Conductivity – NaCl high, H 2 O Low 1. Distilled or deionized water conducts essentially no current. 2. Liquid sodium chloride (not salt dissolved in water) will conduct a current well. 3. This tells us that there must not be ions to carry the current in water, as there are in salt.

b. These properties can be explained by a new kind of bond. i. Covalent bond – a bond formed when atoms share pairs of electrons. ii. In a covalent bond, individual atoms are bonded together, rather than a matrix of atoms bonded together. iii. Molecular compound – a substance consisting of atoms that are covalently bonded together.

II. Bond properties are related to electronegativity differences. a. Few compounds are completely ionic or covalent in nature. b. These terms refer to the ends of a spectrum of bonding. c. The bonds in many compounds share some of the characteristics of each type of bond.

d. Differences in electronegativity values will determine the characteristics of the bond and the compound. i. The greater the difference in electronegativity between the atoms in the bond, the more ionic the bond will be. ii. The lower the difference in electronegativity, the more covalent the bond will be. e. Classification of differences i. 4.0 – considered completely ionic – not one you will run in to. ii. >2.1 – 3.9 – considered ionic iii. 0.4 – 2.0 – considered polar covalent iv. <0.4 – considered non-polar covalent

f. When bonds form between atoms which have the same electronegativity, or the same attraction for a pair of electrons, it is considered a nonpolar covalent bond.

III.Covalent bonds with uneven electron sharing are polar. a. When atoms of different elements bond, they do not share electrons evenly. b. The electrons spend more time around one atom than around the other. c. This creates what is known as a polar covalent bond.

d. This type of bond causes the atoms to have a slight or partial charge. i. The atom with the greater electronegativity 1. Has a stronger attraction for the shared pair 2. Retains the shared pair longer than the other atom 3. Has a partial negative charge. ii. The atom with the lesser electronegativity 1. Has a weaker attraction for the shared pair 2. Has the shared pair around it less time than the other atom 3. Has a partial positive charge.

e. Examples of Polar Covalent Bonds Electronegativity: virtual labs 1 video

IV.Weak attractions also form between molecules. a. Intermolecular forces, or attractions resulting from forces between molecules, are much weaker than the attraction of bonds. b. They do not involve the sharing, giving, or taking of electrons, unlike bonding.

c. Three kinds of intermolecular forces i. London Forces 1. Attractions formed between non-polar molecules. 2. The weakest of the IM forces. 3. Their strength is based on the number of electrons in the compound. More = stronger forces. 4. Produce low boiling points in substances.

ii. Dipole Forces 1. Attractions formed between polar molecules 2. Second strongest of the IM forces. 3. Their strength is based on how close together the atoms in the separate molecules can get. 4. Produce high boiling points in substances.

iii. Hydrogen bonding 1. Water molecules have a much stronger attraction for each other than the above forces would allow, and more than similar compounds such as H 2 S. 2. Hydrogen bond – attraction occurring when a hydrogen atom bonded to a strongly electronegative atoms (fluorine, nitrogen, or oxygen) is also attracted to another electronegative atom, often from a different molecule. 3. Note – three requirements from above definition: a. N, O, or F b. Must be bonded directly to a hydrogen. c. That hydrogen must be attracted to another N, O, or F.

4. These occur because of the strong attraction that fluorine, nitrogen, or oxygen have for a shared pair of electrons, and because hydrogen is so small it can get very close to other atoms. 5. Hydrogen bonds are not true bonds – they are much weaker than a true bond, but stronger than the other types of IM forces. 6. They are responsible for many characteristics of water that we take for granted, as well as some other things you may not even know about. a. Surface tension in water b. Capillary action c. Water has a larger volume when frozen. d. Bonding between DNA strands e. Bonding between enzymes and substrates.

Questions: 1. Define covalent bond and molecular compound. 2. Explain the major difference between covalent bonds and ionic bonds. 3. List and explain the characteristics of each of the three kinds of intermolecular forces. (major characteristics only) 4. Determine whether each of the following bonds is an ionic, polar covalent, or non-polar covalent bond. a. Br-Br b. H-Br c. O-O d. C-O e. H-I f. Ca-O g. Mg-F h. Si-O i. Pb-I