Bond Polarity 8.4 Polar Bonds and Molecules > 8 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

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1 Describing Polar Covalent Bonds Electron density of a hydrochloric acid molecule Describing Polar Covalent Bonds Polarity may also be represented by an arrow pointing to the more electronegative atom. H Cl 7 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 8 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Polar Covalent Bonds The O H bonds in a water molecule polar. Electronegativity difference range Electronegativity Differences and Bond Types Most probable type of bond Example Nonpolar covalent H H (0.0) Moderately polar covalent Very polar covalent δ+ δ H Cl (0.9) δ+ δ H F (1.9) >2.0 Ionic Na + Cl (2.1) 9 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 10 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 1

2 Describing Polar Covalent Bonds There is no sharp boundary between ionic and covalent bonds. As the electronegativity difference between two atoms increases, the polarity of the bond increases. If the difference is > 2.0, the electrons will likely be pulled away completely by one of the atoms (ionic bond). Identifying Bond Type Sample Problem 8.3 Which type of bond (nonpolar covalent, moderately polar covalent, very polar covalent, or ionic) will form between each of the following pairs of atoms? a. N and H b. F and F c. Ca and Cl d. Al and Cl 11 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 12 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 8.3 Sample Problem 8.3 Identify the electronegativities of each atom using Table 6.2. a. N(3.0), H(2.1) b. F(4.0), F(4.0) c. Ca(1.0), Cl(3.0) d. Al(1.5), Cl(3.0) Calculate the electronegativity difference between the two atoms. a. N(3.0), H(2.1); 0.9; moderately polar covalent b. F(4.0), F(4.0); 0.0; nonpolar covalent c. Ca(1.0), Cl(3.0); 2.0; ionic d. Al(1.5), Cl(3.0); 1.5; very polar covalent 13 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 14 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 2

3 Describing Polar Covalent The presence of a polar bond in a molecule often makes the entire molecule polar. In a polar molecule, one end of the molecule is slightly negative, and the other end is slightly positive. Describing Polar Covalent δ+ δ H Cl A molecule that has two poles is called a dipolar molecule, or dipole. 15 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 16 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Polar Covalent Describing Polar Covalent A carbon dioxide molecule has two polar bonds and is linear. O C O Polar bonds Nonpolar Molecule Polar bonds in the same plane pointing in opposing directions 17 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 18 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 3

4 Describing Polar Covalent The water molecule also has two polar bonds. But, the bond polarities do not cancel So the molecule is polar. Predicting Polarity Is there a high electronegative element (N, O, F, Cl) bonded to a low electronegative element (C, H, P)? Do the bond polarities all point toward one end of the molecule? Polar Bonds, Polar Molecule Does the central atom have lone pair(s)? 19 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 20 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Polar Covalent Polar or Nonpolar? F 2 (or any of the diatomics, for that matter) Carbon tetrachloride Phosphorus pentachloride Boron trifluoride Ammonia (NH 3 ) How do the strengths of intermolecular attractions compare with ionic and covalent bonds? 21 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 22 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 4

5 Intermolecular (interparticle) attractions are weaker than either ionic or covalent bonds. Inter = between can be attracted to each other by a variety of different forces. Two types of attractions between molecules dipole interactions (strongest of the weak) dispersion forces (weakest of the weak) 23 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 24 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Dipole interactions Dipole interactions occur between polar molecules electrical attraction between the oppositely charged regions of polar molecules (dipoles). 25 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 26 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 5

6 Dispersion forces the weakest of all molecular interactions caused by the motion of electrons. between nonpolar molecules. Dispersion forces E- momentarily on one side of a molecule Electric force temporarily repels e- in a neighboring molecule. The strength of dispersion forces generally increases as the number of electrons in a molecule increases. 27 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 28 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Fluorine and chlorine have relatively few electrons and are gases at ordinary room temperature and pressure because of their especially weak dispersion forces. Bromine molecules therefore attract each other sufficiently to make bromine a liquid under ordinary room temperature and pressure. Iodine, with a still larger number of electrons, is a solid at ordinary room temperature and pressure. Hydrogen Bonds 29 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 30 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 6

7 Hydrogen Bonds Hydrogen bonds Special case of dipole interactions Hydrogen in one molecule N, O, or F in a neighboring molecule Hydrogen Bonds 95 % weaker than the average covalent bond. Strongest of the intermolecular forces. Extremely important in determining the properties of water and biological molecules. 31 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 32 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. CHEMISTRY & YOU A snowflake s shape is determined by the interactions of hydrogen bonds during its formation. and Molecular Properties Why are the properties of covalent compounds so diverse? 33 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 34 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 7

8 The physical properties of a compound depend on the type of bonding it displays in particular, on whether it is ionic or covalent. state at room temperature melting point, boiling point The diversity of physical properties among covalent compounds is mainly because of widely varying intermolecular attractions. The melting and boiling points of most molecular compounds are low (compared with those of ionic compounds). Intermolecular forces are much weaker than ionic and covalent bonds 35 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 36 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. A few solids that consist of molecules do not melt until the temperature reaches 1000 C or higher. Most of these very stable substances are network solids (or network crystals) Diamond is an example of a network solid. Each carbon atom in a diamond is covalently bonded to four other carbons, interconnecting carbon atoms throughout the diamond. Diamond does not melt; rather, it vaporizes to a gas at 3500 C and above. 37 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 38 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 8

9 Characteristics of Ionic and Molecular Compounds Characteristic Ionic Compound Molecular Compound Representative unit Formula unit Molecule Bond formation Transfer of one or more electrons between atoms Type of elements Metallic and nonmetallic Nonmetallic Sharing of electron parts between atoms Physical state Solid Solid, liquid, or gas Melting point High (usually above 300 C) High (usually below 300 C) Solubility in water Usually high Usually low Electrical conductivity of aqueous solution Good conductor Poor to non-conducting Why do network solids take so much more heat to melt than most covalent compounds? Melting a network solid requires breaking covalent bonds throughout the solid. Melting most covalent compounds only requires breaking the weak attractions between molecules. 39 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 40 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 9