Lewis Dot Diagrams Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

Bonding Theories Ionic Model Skeleton Diagrams Identifies how atoms are linked together Fails to describe the bonding in ions and certain molecules (e.g., CO, NH4+, CN–) Fails to explain the geometry of molecules Lewis Dot Diagrams 4/17/2017

Lewis Dot Diagrams 1s2 F H H F H F H F H F H F H F H F F H Each dot represents an electron in the outer (valence) shell in one of the atoms. 1s2 • F H • H F • H F • H F • H F Each atom attempts to attain a stable outer shell electronic configuration • H F • H F • H F by sharing electrons with other atoms. • F H (typically a noble gas electronic configuration) 4/17/2017

Didn’t your mother always tell you to share? 4/17/2017

Lewis Dot Diagrams F H Each atom sharing an electron can count that electron towards attaining its stable electronic configuration. isoelectronic with helium (DUET) isoelectronic with neon (OCTET) • F H Each pair of electrons which are shared forms a bond between those atoms. 4/17/2017

Lewis Dot Diagrams Bonding electrons may come from both atoms or they may only come from one atom. The most stable bonding results when both atoms donate electrons to the bond. 4/17/2017

Carbon Monoxide Diagram C 4 valence e– eight electrons { O 6 valence e– • O C • • • • total 10 valence e– { eight electrons 10 8 2 6 4 Three pairs of electrons is a triple bond. Both atoms are isoelectronic with neon. C º O • 4/17/2017

Carbon Dioxide Diagram eight electrons { { eight electrons C 4 valence e– two O 12 valence e– • • • • C O total 16 valence e– • • { eight electrons 14 16 10 12 6 8 2 4 Carbon dioxide has two double bonds. All three atoms are isoelectronic with neon. C O • Draw the skeleton diagram for carbon dioxide. 4/17/2017 Determine the total number of valence e- available.

{ { Ammonia Diagram eight electrons N 5 valence e– • N H three H 3 valence e– total 8 valence e– { two electrons Nitrogen is isoelectronic with neon. Each hydrogen is isoelectronic with helium. 4/17/2017

Carbon Tetrachloride Diagram eight electrons { C 4 valence e– total 32 valence e– four Cl 28 valence e– • Cl C eight electrons Carbon is isoelectronic with neon. Each chlorine is isolectronic with argon. 4/17/2017

Draw the Skeleton Diagram and Lewis Dot Diagram for Each of the Following methane dioxygen ozone water hydrogen peroxide ammonium ion 4/17/2017

Types of Bonds The degree with which electrons are shared determines the type of bond that forms. Covalent Bonds Ionic Bonds 4/17/2017

Covalent Bonding Results when each atom equally shares with the other atoms the electrons in its bonds. One pair of shared electrons equals one bond. Occurs with atoms of about the same electronegativity. C H 4/17/2017

Ionic Bonding Na Cl Results when "bonding" electrons are transferred completely to one of the atoms. Occurs with atoms of greatly different electronegativity. - + • Cl atom with low electronegativity loses control of the electron Na • • • • atom with high electronegativity gains complete control of the electron isoelectronic with argon isoelectronic with neon Lewis Dot structures are more appropriate for covalent bonding. 4/17/2017

Methanol methyl alcohol, wood alcohol, CH4O Skeleton diagram Lewis Dot diagram C 4 valence e– H C O H 4 valence e– • O 6 valence e– Lines or pairs of dots can be used to represent a bond. total 14 valence e– How many electrons are shown? 7 e– pairs All valence electrons are represented as bonds and nonbonding electrons 4/17/2017

Methanol methyl alcohol, wood alcohol, CH4O Lewis Dot diagram oxygen needs 8 e– to complete its outer shell • O H C carbon needs 8 electrons to complete its outer shell of electrons (1s2 2s2 2p6) each hydrogen only needs 2 e– to complete its outer shell (1s2) 4/17/2017 Next

Pair Activity Lewis Dot Structures Develop a procedure to draw a Lewis Dot diagram. Initially, one person should be responsible for writing all the information. The other person should be responsible for saying what should be written. Hint: Draw a valid skeleton diagram, and use that information to help draw the dot diagram. 4/17/2017

Lewis Dot Structures On a card or a piece of paper write each sequential step necessary to draw a Lewis Dot diagram. number each step. amend and update the list. Either person may do this. draw the Lewis Dot diagram of acetic acid, CH3COOH. 4/17/2017

Acetic Acid C • H O 4/17/2017

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Formal Charge Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

Formal Charge H C O Assume bonding electrons belong 1/2 to each bonding atom. H C O • The formal charge is the net charge on each atom. 4/17/2017

Formal Charge e– shared formal unshared = e– donated - - charge 2 formal charge unshared electrons = e– donated - - H C O • formal charge of oxygen 6 valence electrons 4/2 shared electrons 4 unshared electrons = - - 4/17/2017

Formal Charges of Atoms Methanol 4 - 8/2 - 0 = 0 • • C O H • 1 - 2/2 - 0 = 0 6 - 4/2 - 4 = 0 • Atoms with low formal charges generally indicate the correct bonding pattern of chemically stable molecules. Formal charge = valence electrons - electrons shared/2 - electrons not shared C: 4 - 8/2 - 0 = 0 H: 1 - 2/2 - 0 = 0 O: 6 - 4/2 - 4 = 0 4/17/2017

Incorrect Diagrams for Methanol What are the formal charges for each atom in the following structures? • O H C H C O • C: 4 - 6/2 - 2 = -1 H: 1 - 2/2 - 0 = 0 O: 6 - 6/2 - 2 = +1 C: 4 - 4/2 - 4 = -2 H: 1 - 2/2 - 0 = 0 O: 6 - 8/2 - 0 = +2 Validity Check The sum of the formal charges always equals the charge on the molecule or ion. Validity Check The sum of the formal charges always equals the charge on the molecule or ion. 4/17/2017

Incorrect Diagrams for Methanol If either of these structures was stable, which one would more likely be stable? • O H C • O H C C: 4 - 6/2 - 2 = -1 H: 1 - 2/2 - 0 = 0 O: 6 - 6/2 - 2 = +1 C: 4 - 4/2 - 4 = -2 H: 1 - 2/2 - 0 = 0 O: 6 - 8/2 - 0 = +2 4/17/2017

Cyanide Ion Lewis Dot diagram Skeleton diagram C 4 valence e– • Lewis Dot diagram Skeleton diagram C 4 valence e– • • • C N • • N 5 valence e– charge 1 e– total 10 valence e– C: 4 - 6/2 - 2 = -1 Convert to a Lewis Dot diagram N: 5 - 6/2 - 2 = 0 5 e– pairs total charge = -1 What are the formal charges for each atom in this ion? 4/17/2017

Cyanide Ion C N C N • • Which “end” of cyanide ion is negative? total charge = -1 Cyanide ion reacts with iron(III) ion to make the hexacyanoferrate(III) ion. Does the iron(III) ion bond to carbon or nitrogen? 4/17/2017

Hexacyanoferrate(III) ion • 3- Carbon is the negative end of the cyanide ion. 4/17/2017

In an ion, some atom must have a non-zero formal charge. Ammonium Ion Lewis Dot diagram Skeleton diagram • N H + N H + • N 5 valence e– total 8 valence e– H 4 valence e– charge -1 e– H: 1 - 2/2 - 0 = 0 N: 5 - 8/2 - 0 = +1 total charge = +1 Nitrogen usually forms 3 bonds 4 e– pairs In an ion, some atom must have a non-zero formal charge. Why? Which atoms have a non-zero formal charge? 4/17/2017

Formal Charges What are the formal charges on the atoms in hydroxide ion hydronium ion (oxonium ion) 4/17/2017

Chlorate Ion – Cl-O-O-O – Cl O Skeleton diagram usually the unique atom is in the center two possibilities – O Cl Cl-O-O-O – chain structures are uncommon correct structure 4/17/2017

Chlorate Ion Lewis Dot diagram Skeleton diagram Cl 7 valence e– • O Cl O Cl O 18 valence e– charge 1 e– total 26 valence e– Cl: 7 - 6/2 - 2 = +2 O: 6 - 2/2 - 6 = -1 total charge = -1 Only 6 electrons shown (3 pairs) 13 e– pairs Complete the Lewis Dot diagram Why is chlorate ion a good oxidizing agent? 4/17/2017

Perchlorate Ion Lewis Dot diagram Skeleton diagram O 24 valence e– total 32 valence e– charge 1 e– Cl 7 valence e– • O Cl O O O Cl O Cl: 7 - 8/2 - 0 = +3 O: 6 - 2/2 - 6 = -1 total charge = -1 Only 8 electrons shown (4 pairs) Why are organic perchlorate salts potentially explosive? 16 e– pairs 4/17/2017

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