CA_Lesson_3_Molecular++++Structures.pptx

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1. Molecular Structures Lesson 3

2. Focus Question How are electrons shared in covalent molecules?

3. New Vocabulary structural formula resonance coordinate covalent bond

4. Review Vocabulary ionic bond: the electrostatic force that holds oppositely charged particles together in an ionic compound

5. Structural Formulas A structural formula uses letter symbols and bonds to show relative positions of atoms.

6. Structural Formulas Drawing Lewis structures Predict the location of atoms. The atom with the least attraction for shared electrons will be the central atom. (It’s usually closest to the left side of the periodic table). Others become terminal atoms. Hydrogen is always a terminal atom. Determine the number of electrons available for bonding. This is the number of valence electrons. Determine the number of bonding pairs. Divide the number of electrons available by two. Place bonding pairs. Place a single bond between the central atoms and each terminal atom.

7. Structural Formulas 5. Determine the number of electron pairs remaining. Subtract the number of pairs in Step 4 from the number of bonding pairs in Step 3. Place lone pairs around terminal atoms (except H atoms) to satisfy the octet rule. Remaining pairs will be assigned to the central atom. 6. Determine whether the central atom satisfies the octet rule. If not, convert one or two of the lone pairs on the terminal atoms into a double or a triple bond between the terminal atom and the central atom. Carbon, nitrogen, oxygen, and sulfur often form double and triple bonds.

8. Lewis Structure for a Covalent Compound with Single Bonds Use with Example Problem 3. Problem Ammonia is a raw material used in the manufacture of many products, including fertilizers, cleaning products, and explosives. Draw the Lewis structure for ammonia (NH 3 ). Response ANALYZE THE PROBLEM Ammonia molecules consist of one nitrogen atom and three hydrogen atoms. Because hydrogen must be a terminal atom, nitrogen is the central atom. SOLVE FOR THE UNKNOWN Find the total number of valence electrons available for bonding. There are 8 valence electrons available for bonding. Determine the total number of bonding pairs. To do this, divide the number of available electrons by two. = 4 pairs Four pairs of electrons are available for bonding.  

9. Lewis Structure for a Covalent Compound with Single Bonds EVALUATE THE ANSWER Each hydrogen atom shares one pair of electrons, as required, and the central nitrogen atom shares three pairs of electrons and has one lone pair, providing a stable octet. SOLVE FOR THE UNKNOWN Place a bonding pair (a single bond) between the central nitrogen atom and each terminal hydrogen atom. Determine the number of bonding pairs remaining. Subtract the number of pairs used in these bonds from the total number of pairs of electrons available. 4 pairs total − 3 pairs used = 1 pair available The remaining pair—a lone pair—must be added to either the terminal atoms or the central atom. Because hydrogen atoms can have only one bond, they have no lone pairs. Place the remaining lone pair on the central nitrogen atom.

10. Lewis Structure for a Covalent Compound with Multiple Bonds Use with Example Problem 4. Problem Carbon dioxide is a product of all cellular respiration. Draw the Lewis structure for carbon dioxide (CO 2 ). Response ANALYZE THE PROBLEM The carbon dioxide molecule consists of one carbon atom and two oxygen atoms. Because carbon has less attraction for shared electrons, carbon is the central atom, and the two oxygen atoms are terminal. SOLVE FOR THE UNKNOWN Find the total number of valence electrons available for bonding. There are 16 valence electrons available for bonding. Determine the total number of bonding pairs by dividing the number of available electrons by two. = 8 pairs Eight pairs of electrons are available for bonding.  

11. Lewis Structure for a Covalent Compound with Multiple Bonds EVALUATE THE ANSWER Both carbon and oxygen now have an octet, which satisfies the octet rule. SOLVE FOR THE UNKNOWN Place a bonding pair (a single bond) between the central carbon atom and each terminal oxygen atom. Determine the number of electron pairs remaining. Subtract the number of pairs used in these bonds from the total number of pairs of electrons available. Subtract the number of pairs used in these bonds from the total number of pairs of electrons available. 8 pairs total − 2 pairs used = 6 pairs available Add three lone pairs to each terminal oxygen atom. Determine the number of electron pairs remaining. Subtract the lone pairs from the pairs available. 6 pairs available − 6 pairs used = 0 pairs available Examine the incomplete structure above (showing the placement of the lone pairs). Note that the carbon atom does not have an octet and that there are no more electron pairs available. To give the carbon atom an octet, the molecule must form double bonds. Use a lone pair from each O atom to form a double bond with the C atom.

12. Structural Formulas Lewis Structures for Polyatomic Ions To find the total number of electrons available for bonding in a polyatomic ion, first find the number available in the atoms present in the ion. Add the ion charge if the ion is negative, or subtract the charge if the ion is positive.

13. Lewis Structure for a Polyatomic Ion Use with Example Problem 5. Problem Draw the correct Lewis structure for the polyatomic ion phosphate (PO 4 3- ). Response ANALYZE THE PROBLEM You are given that the phosphate ion consists of one phosphorus atom and four oxygen atoms and has a charge of 3-. Because phosphorus has less attraction for shared electrons than oxygen, phosphorus is the central atom and the four oxygen atoms are terminal atoms. SOLVE FOR THE UNKNOWN Find the total number of valence electrons available for bonding. Determine the total number of bonding pairs. = 16 pairs Draw single bonds from each terminal oxygen atom to the central phosphorus atom.  

14. EVALUATE THE ANSWER All of the atoms have an octet, and the group has a net charge of 3-. SOLVE FOR THE UNKNOWN Subtract the number of pairs used from the total number of pairs of electrons available. 16 pairs total − 4 pairs used = 12 pairs available Add three lone pairs to each terminal oxygen atom. 12 pairs available − 12 lone pairs used = 0. Subtracting the lone pairs used from the pairs available verifies that there are no electron pairs available for the phosphorus atom. The Lewis structure for the phosphate ion is shown. Lewis Structure for a Polyatomic Ion

15. Resonance Structures Resonance is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion. Two or more correct Lewis structures that represent a single ion or molecule are resonance structures. The molecule behaves as though it has only one structure. The bond lengths are identical to each other and intermediate between single and double covalent bonds .

16. Resonance Structures This figure shows three correct ways to draw the structure for (NO 3 ) -1 .

17. Exceptions to the Octet Rule Odd Number of Valence Electrons A small group of molecules might have an odd number of valence electrons and be unable to form an octet around each atom.

18. Exceptions to the Octet Rule Suboctets and Coordinate Covalent Bonds A few compounds form suboctets—stable configurations with fewer than 8 electrons around an atom. A coordinate covalent bond forms when one atom donates both of the electrons to be shared with an atom or ion that needs two electrons. An example is shown on the next slide.

19. Exceptions to the Octet Rule

20. Lewis Structure: Exception to the Octet Rule Use with Example Problem 6. Problem Xenon is a noble gas that will form a few compounds with nonmetals that strongly attract electrons. Draw the correct Lewis structure for xenon tetrafluoride (XeF 4 ). Response ANALYZE THE PROBLEM You are given that a molecule of xenon tetrafluoride consists of one xenon atom and four fluorine atoms. Xenon has less attraction for electrons, so it is the central atom. SOLVE FOR THE UNKNOWN First, find the total number of valence electrons. Determine the total number of bonding pairs. Use four bonding pairs to bond the four F atoms to the central Xe atom.  

21. Lewis Structure: Exception to the Octet Rule EVALUATE THE ANSWER This structure gives xenon 12 total electrons, an expanded octet. Xenon compounds, such as the XeF 4 shown here, are toxic because they are highly reactive. SOLVE FOR THE UNKNOWN Determine the number of remaining pairs. 18 pairs available - 4 pairs used = 14 pairs available Add three pairs to each F atom to obtain an octet. Determine how many pairs remain. Place the two remaining pairs on the central Xe atom.  

22. Quiz Hydrogen often forms double bonds. D Hydrogen is always a terminal atom. C The central atom is usually the one closest to the right side of the periodic table. B The atom with the most attraction to shared electrons will be the central atom in the molecule. A Which of the following is true about drawing Lewis structures? 1. CORRECT

23. Quiz Bond lengths are identical to each other and intermediate between single and double covalent bonds. D C Resonance structures are two or more correct Lewis structures for a single ion or molecule. B Resonance occurs when more than one valid Lewis structure can be written for a molecule or ion. A What is not true about resonance and resonance structures? 2. CORRECT The molecule behaves as though it has multiple structures.

24. Quiz stable configurations with less than 8 electrons around the atom D molecules with double-bonded carbon atoms C coordinate covalent bonds B molecules with odd numbers of valence electrons A Which of the following is not an exception to the octet rule? 3. CORRECT

25. Quiz a bond in which one atom donates both electrons to be shared D a double bond C a bond in which both atoms donate electrons to be shared B a bond that has more than one Lewis structure A Which of the following best describes a coordinate covalent bond? 4. CORRECT