Unit 6

Read Sections 4.6, 4.7, 4.8, 4.9 and 4.10 before viewing the slide show.

Unit 15Molecular CompoundsLewis Structures for Molecular Compounds and IonsPolyatomic IonsNaming Molecular Compounds

Lewis Structures for Molecular Compounds and Ions (4.7, 4.10)Nonmetals those to the right of the stair-step line can react with each other by sharing electronsThe driving force is similar to that for ionic compounds the attainment of an octet of electrons as summarized in the Octet RuleIn molecular compounds, though, the atoms do not form individual ions, they share electrons throughout their structureIn this unit, we will look at drawing Lewis structures representing these molecular compounds as well as other aspects of these shared electron systems

Key Aspects of Lewis Structures (4.10)Key elements of a Lewis structure for a molecular compound include:The number of valence electrons drawn equals the total number of valence electrons in the compound or ion.Electrons are shared in such a way that each atom is effectively surrounded by eight electrons (two for hydrogen, other exceptions exist).All or some of the electrons around an atom may be shared with other atoms in the structure

An Approach to Drawing Lewis Structures (4.10)Several approaches exist for drawing Lewis structures this is just one you might consider:Draw skeletal structure for the atoms. Things that might be helpful are that hydrogen can only take one bond so it has to be on the outside; carbon typically has four bonds, nitrogen has three bonds, and oxygen has two bonds; polyatomic molecules and ions typically have a central atom surrounded by other atoms.Connect each atom to its neighbor(s) with a single bond which consists of two electrons. The bond may be characterized by two electrons (:) or a single line () attaching the two atoms.Place electrons around the outside of the atoms so each has eight total electrons including all of those bonded to it as well as the nonbonded (or lone pair) electrons.

(Continued on Next Slide)

An Approach to Drawing Lewis Structures Continued (4.10)Count the number of valence electrons the structure should have. This is the sum of the valence electrons on all atoms. For ions, the number of valence electrons is increased by the charge and for negative ions and reduced by the charge for positive ions.Count the number of valence electrons in your structure.If the valence electron counts match from steps 4 and 5 you are done. If there are too many valence electrons in your structure, take away a nonbonded pair and slide a nonbonded pair from an adjacent atom to make a double chemical bond. Continue this process until the number of valence electrons in the structure matches the number that should be present.

Examples of Drawing Lewis Structures (4.10)

Examples of Drawing Lewis Structures (4.10)

Examples of Drawing Lewis Structures (4.10)

Polyatomic Ions (4.9)An ion formed from two or more bonded atoms is called a polyatomic ion.These ions will often react as units and typically do not change during the course of a chemical reaction.There is not a wonderfully logical way of determining the names of polyatomic ions, though there are some patterns.Table 4.4 (on page 106 or the next slide) gives a short list of common polyatomic ions. Dont panic you dont have to know all of these.For our purposes, the focus will be on five polyatomic ions:ammonium NH4+hydroxide OH- nitrate NO3-sulfate SO42- phosphate PO43-On the proctored midterm, I will not give you the information above you will be expected to know these five. If you need others, I will give you this information for them.

Writing Formulas for and Naming Compounds with Polyatomic Ions (4.9)Writing formulas using polyatomic ions works just like it did with binary ionic compounds think of the polyatomic ion as one big ion.In naming the compounds, the polyatomic ion retains its name no changing of the ending or anything like that. Examples:

Na+ and SO42- make Na2SO4 called sodium sulfate

NH4+ and S2- make (NH4)2S called ammonium sulfide

Ba2+ and PO43- make Ba3(PO4)2 called barium phosphate

Naming Binary Molecular Compounds (4.7-4.8)Writing formulas for binary molecular compounds is a little trickier than for ionic compounds since the atoms dont really have a charge (they are sharing electrons) and many different compounds can be formed from the same two elements.The naming is fairly direct. Prefixes are used (see the table to the right) to indicate the number of atoms of each type in the molecule. The mono- prefix is only used in reference to the second atom in the compound.Examples: CO carbon monoxide CO2 carbon dioxide N2O dinitrogen monoxide (also known as nitrous oxide or laughing gas) NO2 nitrogen dioxide N2O4 nitrogen tetroxide N2O5 dinitrogen pentoxide# of atomsPrefix1mono-2di-3tri-4tetra-5penta-6hexa-7hepta-8octa-9nona-10deca-

How many different elements (not atoms elements) are in the compound?What types of elements are they?Metal keeps its name. For Groups 1 and 2, Al, Zn, and Ag no modification is necessary. For other metals, use a Roman numeral in parentheses to indicate the charge on each ion of the metal.Polyatomic ions retain their names. (Ionic compound)Metal keeps its name. For Groups 1 and 2, Al, Zn, and Ag no modification is necessary. For other metals, use a Roman numeral in parentheses to indicate the charge on each ion of the metal.Nonmetal changes its ending to ide.(Molecular or covalent compound)First element retains its name.Second element switches to ide ending. Each name is preceded by a prefix indicating the number of its atoms (di-, tri-, tetra-, etc.). Mono- is only used for the second element.Exactly twoMore than twoMetal-nonmetalNonmetal-nonmetalGSB Modified 2/1/2010Summary of Inorganic Nomenclature

Draw the Lewis structure for CCl4

StepProgress

1. Draw the skeletal structureCl

Cl C Cl

Cl

2. Connect atoms by a single bondClCl C ClCl

3. Add electrons to each atom to total eight electrons notice the there are two electrons in each bond..:Cl: .. .. :Cl C Cl: :Cl:

4. Count up the valence electrons you should have 4 for C and 7 each for 4 Cl equals 32 valence electrons

5. Your structure has 32 valence electrons it is done

Draw the Lewis structure for NH3

StepProgress

1. Draw the skeletal structureH

H N H

2. Connect atoms by a single bondHH N H

3. Add electrons to each atom to total eight electrons notice the there are two electrons in each bondHH N H

4. Count up the valence electrons you should have = 5 for N and 1 each for 3 H equals 8 valence electrons

5. Your structure has 8 valence electrons it is done

Draw the Lewis structure for SO2

StepProgress

1. Draw the skeletal structureO S O

2. Connect atoms by a single bondO S O

3. Add electrons to each atom to total eight electrons notice there are two electrons in each bond .. .. ..: O S O:

4. Count up the valence electrons you should have = 6 for sulfur and 6 for each of 2 oxygens = 6 + 2 6 = 18 electrons

5. Your structure has 20 valence electrons it needs to lose two electrons to get down to the 18 from Step 4. Remove any two nonbonded electrons and, to maintain the octet, slide two from nonbonded electrons from an adjacent atom into the bond. ..: O S O:

6. Now there is the correct number of valence electrons and each atom is surrounded by 8. Bonds count as two each and nonbonded electrons count one each.

Some Common Ions

PositiveNegative

1+Li+LithiumCu+Copper (I) or

Cuprous1-F-FluorideClO-Hypochlorite

Na+SodiumHg22+Mercury (I) or

MercurousCl-ChlorideClO2-Chlorite

K+PotassiumAg+SilverBr-BromideClO3-Chlorate

Rb+RubidiumNH4+AmmoniumI-IodideClO4-Perchlorate

Cs+CesiumNO3-NitrateIO4-Periodate

H+HydrogenNO2-NitriteOH-Hydroxide

C2H3O2-AcetateSCN-Thiocyanate

MnO4-PermanganateHCO3-Hydrogen Carbonate or

Bicarbonate

CN-CyanideO22-Peroxide

H-Hydride

2+Mg2+MagnesiumCu2+Copper (II) or

Cupric2-O2-OxideS2-Sulfide

Ca2+CalciumHg2+Mercury (II) or

MercuricSO42-SulfateSO32-Sulfite

Sr2+StrontiumSn2+Tin (II) or

StannousCO32-CarbonateC2O42-Oxalate

Ba2+BariumZn2+ZincCrO42-ChromateCr2O72-Dichromate

Fe2+Iron (II) or

FerrousPb2+Lead (II) or

Plumbous

Cd2+CadmiumMn2+Manganese (II) or

ManganousS2O32-Thiosulfate

Cr2+Chromium (II) or

ChromousCo2+Cobalt (II) or

Cobaltous

3+Al3+AluminumAs3+Arsenic3-N3-NitrideP3-Phosphide

Cr3+Chromium (III) or

ChromicCo3+Cobalt (III) or

CobalticPO43-PhosphatePO33-Phosphite

Fe3+Iron (III) or

FerricAsO43-ArsenateAsO33-Arsenite

BO33-Borate

See also Tables 2.4 and 2.5 in 11th Edition of Brown, LeMay, Bursten, Murphy GSB Last Modified 9/3/2008