Structural formulas show the relative positions of atoms within a molecule. Section 3: Molecular Structures K What I Know W What I Want to Find Out L What I Learned

7(C) Construct electron dot formulas to illustrate ionic and covalent bonds. 7(B) Write the chemical formulas of common polyatomic ions, ionic compounds containing main group or transition metals, covalent compounds, acids, and bases. Molecular Structures Copyright © McGraw-Hill Education

Essential Questions What are the basic steps used to draw Lewis structures? Why does resonance occur, and what are some resonance structures? Which molecules are exceptions to the octet rule, and why do these exceptions occur? Molecular Structures Copyright © McGraw-Hill Education

Review ionic bond Molecular Structures Copyright © McGraw-Hill Education Vocabulary New structural formula resonance coordinate covalent bond

Structural Formulas A structural formula uses letter symbols and bonds to show relative positions of atoms. Molecular Structures Copyright © McGraw-Hill Education

Structural Formulas Drawing Lewis Structures Predict the location of certain atoms. The atom that has the least attraction for shared electrons will be the central atom in the molecule (usually, the one closer to the left side of the periodic table). All other atoms become terminal atoms. Note: Hydrogen is always a terminal atom. Determine the number of electrons available for bonding. The number of valence electrons. Determine the number of bonding pairs. Divide the number of electrons available for bonding by two. Molecular Structures Copyright © McGraw-Hill Education

Structural Formulas Place the bonding pairs. Place a single bond between the central atoms and each of the terminal atoms. Determine the number of bonding pairs remaining. Subtract the number of bonding pairs in step 4 from the number of bonding pairs in step 3. Place lone pairs around terminal atoms, except hydrogen, to satisfy the octet rule. Any remaining pairs will be assigned to the central atom. 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 bond or a triple bond between the terminal atom and the central atom. Remember: carbon, nitrogen, oxygen and sulfur often form double and triple bonds. Molecular Structures Copyright © McGraw-Hill Education

Structural Formulas Atoms within a polyatomic ion are covalently bonded. The procedure for drawing Lewis structures is similar to drawing them for covalent compounds. The difference is, you need to determine the number of electrons available for bonding. Find the number of electrons available in the atoms present and then subtract the ion charge if the ion is positive or add the ion charge if the ion is negative. Molecular Structures Copyright © McGraw-Hill Education

Molecular Structures Copyright © McGraw-Hill Education 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.

Molecular Structures Copyright © McGraw-Hill Education 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.

Molecular Structures Copyright © McGraw-Hill Education 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.

Molecular Structures Copyright © McGraw-Hill Education 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.

Molecular Structures Copyright © McGraw-Hill Education 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.

Molecular Structures Copyright © McGraw-Hill Education 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

Resonance Structures Resonance is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion. This figure shows three correct ways to draw the structure for (NO 3 ) -1. Molecular Structures Copyright © McGraw-Hill Education

Resonance Structures 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. Molecular Structures Copyright © McGraw-Hill Education

Exceptions to the Octet Rule Some molecules do not obey the octet rule. A small group of molecules might have an odd number of valence electrons. NO 2 has five valence electrons from nitrogen and 12 from oxygen and cannot form an exact number of electron pairs. Molecular Structures Copyright © McGraw-Hill Education

Exceptions to the Octet Rule A few compounds form stable configurations with less than 8 electrons around the atom—a suboctet. 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. Molecular Structures Copyright © McGraw-Hill Education

Exceptions to the Octet Rule A third group of compounds has central atoms with more than eight valence electrons, called an expanded octet. Elements in period 3 or higher have a d-orbital and can form more than four covalent bonds. Molecular Structures Copyright © McGraw-Hill Education

Molecular Structures Copyright © McGraw-Hill Education 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.

Molecular Structures Copyright © McGraw-Hill Education 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.

Molecular Structures Copyright © McGraw-Hill Education Review Essential Questions What are the basic steps used to draw Lewis structures? Why does resonance occur, and what are some resonance structures? Which molecules are exceptions to the octet rule, and why do these exceptions occur? Vocabulary structural formulacoordinate covalent bond resonance