Chapter 7: Covalent Bonding and Lewis Formula’s Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Chapter 7: Covalent Bonding and Lewis Formula’s By Doba Jackson, Ph.D. Copyright © 2010 Pearson Prentice Hall, Inc.

Electronegativity concept Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electronegativity concept Electronegativity: The ability of an atom in a molecule to attract the shared electrons in a covalent bond. Copyright © 2010 Pearson Prentice Hall, Inc.

Definitions of Ionic and Covalent compounds Ionic bonds are a result of a combination of a metal (electropositive element) and a non-metal (electronegative element). In ionic bonds, atoms are attracted to each other by opposite charges. Covalent bonds are a result of the combination of two non-metals (two electronegative elements). In covalent bonds, atoms are attracted to each other by a shared pair of electrons.

Types & Strengths of Covalent Bonds (Dissociation energies) Chapter 5: Covalent Bonds and Molecular Structure Types & Strengths of Covalent Bonds (Dissociation energies) 11/11/2018 78 Copyright © 2010 Pearson Prentice Hall, Inc.

A Comparison of Ionic and Covalent Bonds Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 A Comparison of Ionic and Covalent Bonds Physical properties are properties we can directly see and measure. Ionic compounds and covalent compounds are different in their physical properties (ex. NaCl and HCl) Copyright © 2010 Pearson Prentice Hall, Inc.

Points to consider: Ionic verses Covalent compounds Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Points to consider: Ionic verses Covalent compounds Point 1: Ionic compounds are usually solids (when pure) Point 2: Ionic compounds have very high boiling and melting points Point 3: Covalent compounds can be either solids, liquids or gases Point 4: Covalent compounds have relatively lower boiling and melting points Copyright © 2010 Pearson Prentice Hall, Inc.

Why does NaCl have such high boiling and melting points? Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Why does NaCl have such high boiling and melting points? Copyright © 2010 Pearson Prentice Hall, Inc.

Types & Strengths of Covalent Bonds Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Types & Strengths of Covalent Bonds 78 Copyright © 2010 Pearson Prentice Hall, Inc.

A chemical representation of a covalent compound (ethanol) Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 A chemical representation of a covalent compound (ethanol) Copyright © 2010 Pearson Prentice Hall, Inc.

Drawing Structural Formula’s Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Drawing Structural Formula’s EXAMPLE 5.1 Propane, C3H8, has a structure in which the three carbon atoms are bonded in a row, each end carbon is bonded to three hydrogens, and the middle carbon is bonded to two hydrogens. Draw the structural formula, using lines between atoms to represent covalent bonds. Copyright © 2010 Pearson Prentice Hall, Inc.

Naming Covalent Compounds: Must know these prefixes Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Naming Covalent Compounds: Must know these prefixes Copyright © 2010 Pearson Prentice Hall, Inc.

Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Choose which compound is more electronegative, then change the suffix to -ide Write the name: electropositive - electronegative Add the suffix “ide” to the end of the electronegative atom. - Use the prefixes to indicate the multiplicity of the both atoms. Copyright © 2010 Pearson Prentice Hall, Inc.

Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Example: naming N2O4 N2O4 The first element (Nitrogen) is more electropositive and takes the name of the element. The second element (Oxygen) is more electronegative and takes the name of the element with an “ide” modification to the ending. The prefix is added to the front of each to indicate the number of each atom. dinitrogen tetraoxide Copyright © 2010 Pearson Prentice Hall, Inc.

Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 WORKED EXAMPLE 5.2 Give systematic names for the following compounds Solution PCl3 (b) N2O3 (c) P4O7 (d) BrF3 Phosphorus trichloride Dinitrogen trioxide Tetraphosphorus heptoxide Bromine trifluoride Copyright © 2010 Pearson Prentice Hall, Inc.

Chemical Representation: Electron-Dot Structures Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Chemical Representation: Electron-Dot Structures Molecular model’s: shows the 3D connectivity. Electron-Dot Structures (Lewis Structures): A representation of an atom’s valence electrons by using dots. Copyright © 2010 Pearson Prentice Hall, Inc.

Lewis Electron-Dot Structures Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Lewis Electron-Dot Structures Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures Single bonds. Copyright © 2010 Pearson Prentice Hall, Inc.

Lewis Electron-Dot Structures Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Lewis Electron-Dot Structures Copyright © 2010 Pearson Prentice Hall, Inc.

Chapter 5: Covalent Bonds and Molecular Structure WORKED EXAMPLE 5.3 Drawing an Electron-Dot Structure 11/11/2018 Draw an electron-dot structure for phosphine, PH3. Solution Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures of Polyatomic Molecules Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures of Polyatomic Molecules Step 1: Valence Electrons Count the total number of valence electrons for all atoms in the molecule. Add one additional electron for each negative charge in an anion or subtract one for each positive charge in a cation. Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures of Polyatomic Molecules Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures of Polyatomic Molecules Step 2: Connect Atoms Draw lines to represent bonds between atoms. For hydrogen and second row atoms, use the number of bonds listed below. For third row and greater atoms, they may have more bonds than predicted by the octet rule. Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures of Polyatomic Molecules Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures of Polyatomic Molecules Step 3: Assign Electrons to the Terminal Atoms Subtract the number of electrons used for bonding in the previous step from the total number determined in step 1. Complete each terminal atom’s octet (except for hydrogen). Step 4: Assign Electrons to the Central Atom If unassigned electrons remain after step 3, place them on the central atom. Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures of Polyatomic Molecules Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures of Polyatomic Molecules Step 5: Multiple Bonds If no unassigned electrons remain after step 3 but the central atom does not yet have an octet, use one or more lone pairs of electrons from a neighboring atom to form a multiple bond (either a double or a triple). Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures of Polyatomic Molecules Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures of Polyatomic Molecules Draw an electron-dot structure for H2O. Step 1: 2(1) + 6 = 8 valence electrons Step 4: bonding pair of electrons lone pair of electrons H O H O Step 2: Step 2: The positioning of the terminal atoms about the central is not critical as long as they simply surround the central atom. Step 3: The terminal atom is hydrogen. Step 5: The central atom has an octet so no multiple bonding. Step 3: 8 – 2(2) = 4 nonbonding electrons Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures of Polyatomic Molecules Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures of Polyatomic Molecules Draw an electron-dot structure for CCl4. Step 1: 4 + 4(7) = 32 valence electrons Cl C Cl C Step 2: Step 4: Step 4: The central atom already has an octet. Step 5: The central atom has an octet so no multiple bonding. Step 3: 32 – 4(2) = 24 nonbonding electrons Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures of Polyatomic Molecules Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures of Polyatomic Molecules Draw an electron-dot structure for H3O1+. Step 1: 3(1) + 6 - 1 = 8 valence electrons 1+ H O H O Step 2: Step 4: Step 1: Subtract 1 from the total number of valence because of the 1+ charge. Step 3: The terminal atoms are hydrogen. Step 5: The central atom has an octet so no multiple bonding. Don’t forget to show the charge on the ion! Step 3: 8 – 3(2) = 2 nonbonding electrons Copyright © 2010 Pearson Prentice Hall, Inc.

Draw electron-Dot formula’s for the following compounds Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Draw electron-Dot formula’s for the following compounds AlCl3 BH3 NH3 CH4 C2H6 H2O2 C2H4 Cl2CO CH2O C2H4O2 Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures of Polyatomic Molecules Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures of Polyatomic Molecules Draw an electron-dot structure for CH2O. Step 1: 4 + 2(1) + 6 = 12 valence electrons H C O H C O Step 2: Step 5: Step 2: The least electronegative atom is the central one (hydrogen can’t be the central atom). Step 3: Two of the terminal atoms are hydrogen. Step 4: There are no remaining electrons to place on the central atom. Step 5: “Borrow” a pair from oxygen to complete the central atom’s octet. H C O H C O Step 3: Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures of Polyatomic Molecules Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures of Polyatomic Molecules Draw an electron-dot structure for SF6. Step 1: 6 + 4(7) = 34 valence electrons F S F S Step 2: Step 3: Step 2: The central atom is in row 3 so there can be more than 8 electrons. Step 4: There are no remaining electrons. Step 5: The central atom has at least 8 electrons so no multiple bonding. Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures of Polyatomic Molecules Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures of Polyatomic Molecules Draw an electron-dot structure for ICl3. Step 1: 7 + 3(7) = 28 valence electrons Cl I Cl Cl Cl Step 2: Step 4: I Step 4: The remaining electrons go on the central atom. Step 5: The central atom has at least 8 electrons so no multiple bonding. Cl I Step 3: Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures and Formal Charges Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures and Formal Charges Draw an electron-dot structure for O3. Step 1: 3(6) = 18 valence electrons O Step 2: O Step 4: Step 4: There is only 1 more pair of electrons. Thus, the central oxygen only has 3 pairs of electrons (less than an octet). Step 5: There is a choice to be made. If all that is desired is the electron-dot structure, then either of the 2 terminal oxygen atoms could have been chosen. O O Step 3: Step 5: Copyright © 2010 Pearson Prentice Hall, Inc.

Formal Charges are charges isolated to specific atoms in an electron-dot structure Formal Charge of an atom is a value that shows whether an atom has gained or lost electrons through bonding The sum of all formal charges of every atom should equal the net charge of the molecule. Formal Charge # of valence e- # of bonding e- # of nonbonding e- - = -

Example of a molecule with a formal charge Formal Charge = Valence electrons – Bonding electrons/2 – nonbonding electrons

Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Formal Charges Formal Charge # of valence e- in free atom - 2 1 nonbonding e- bonding = Calculate the formal charge on each atom in O3. O The sum of the formal charges will be equal to the overall charge of the molecule or ion. 1 2 1 2 1 2 6 - (4) - 4 = 0 6 - (6) - 2 = +1 6 - (2) - 6 = -1 Copyright © 2010 Pearson Prentice Hall, Inc.

Electron-Dot Structures and Formal Charges Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures and Formal Charges Draw an electron-dot structure for OF2. Step 1: 2(7) + 6 = 20 valence electrons F O Step 2: F O Step 4: 20 – 4 = 16 electrons F O Step 3: 16 – 12 = 4 electrons Copyright © 2010 Pearson Prentice Hall, Inc.

An alternative structure of OF2 Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 An alternative structure of OF2 Draw an electron-dot structure for OF2. Step 1: 2(7) + 6 = 20 valence electrons O F Step 2: O F Step 4: 20 – 4 = 16 electrons O F Step 3: 16 – 12 = 4 electrons Copyright © 2010 Pearson Prentice Hall, Inc.

Formal Charges in two structures of OF2 Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Formal Charges in two structures of OF2 F O 1 2 1 2 1 2 7 - (2) - 6 = 0 6 - (4) - 4 = 0 7 - (2) - 6 = 0 + - O F + 1 2 1 2 1 2 7 - (2) - 6 = 0 7 - (4) - 4 = +1 6 - (2) - 6 = -1 Copyright © 2010 Pearson Prentice Hall, Inc.

Problem 2. 7- Nitromethane has the structure indicated Problem 2.7- Nitromethane has the structure indicated. Explain why it must have a formal charge on N and O. Formal Charge = Valence electrons – Bonding electrons/2 – nonbonding electrons

Problem 2.8- Calculate the formal charges for the nonhydrogen atoms in the following molecules +1 -1 +1 -1 +1 -1 Formal Charge = Valence electrons – Bonding electrons/2 – nonbonding electrons

Electron-Dot Structures and Resonance Chapter 5: Covalent Bonds and Molecular Structure 11/11/2018 Electron-Dot Structures and Resonance Move a lone pair from this oxygen? Step 4: O Or, move a lone pair from this oxygen? Either electron-dot structure suggests that ozone has a double and a single bond. A bond analysis actually shows one type of bond and it’s neither a single nor a double bond. A resonance hybrid attempts to overcome this shortcoming of electron-dot structures. At this stage, we usually just take this simplified look at resonance structures. Organic chemistry, for example, takes a much deeper look at resonance. O O Resonance Copyright © 2010 Pearson Prentice Hall, Inc.

Points to consider for Resonance forms Rule 1: Individual resonance forms are imaginary. Rule 2: Resonance forms differ only in their placement of their nonbonding or π electrons. Rule 3: Different Resonance forms of a substance don’t have to be equivalent. Rule 4: Resonance obeys normal rules of valency.

Only difference is where you place the π and non-bonding electrons Resonance forms Only difference is where you place the π and non-bonding electrons