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Nitrousoxidedissolvesinfats.Thegasisaddedunderpressure lo .rt, of whipped topping' When the valve is opened' the gas

""p."at, ttrus

"*prnding (whipping) the topping and forcing it out

ofthe can.

(a) How many valence electrons are present in a mol- 3'81 ecule of N2O?

(b) Write two equivalent contributing structures for

this molecule. The connectivity in nitrous oxide is

N-N-O. (c) Exptain why the following is not an acceptable

contributing structure:

tN-N:0

Section 3.10 How Do We Predict Bond Angles in Covalent Molecules?

3.79 Answer true or false' (a) The letters VSEPR stand for valence-shell

electron-Pair rePulsion'

(b) In predicting bond angles about a central atom in a covalent molecule, the VSEPR model considers

only shared electron pairs (electron pairs involved

in forming covalent bonds)' (c) The VSEPR model treats the two electron pairs

r ofa double bond as one region ofelectron density I und the three electron pairs ofa triple bond as I one region ofelectron densitY'

(d) In carbon dioxide, O:C:O, carbon is sur- rounded by four pairs ofelectrons and the

VSEPR model predicts 109'5" for the O-C-O bond angle.

(e) For a central atom surrounded by three regions of

electron density, the VSEPR model predicts bond

angles of l-20'. (f) The geometry about a carbon atom surrounded by

three regions ofelectron density is described as

trigonal Planar' (g) For a central atom surrounded by four regions of

electron density, the VSEPR model predicts bond

angles of360"/4 : 90''

(h) For the ammonia molecule, NH3, theVSEPR model predicts H-N-H bond angles of 109'5''

(i) For the ammonium ion, NHa+, theVSEPR model predicts H-N-H bond angles of l'09'5''

0) The VSEPR model applies equally wellto cova- lent compounds ofcarbon, nitrogen, and oxygen'

(k) In water, H-O-H, the oxygen atom forms cova- lent bonds to two other atoms, and therefore' the

VSEPR model predicts an H-O-H bond angle of 180".

(l) Ifyou fail to consider unshared pairs ofvalence electrons when you use the VSEPR model' you

will arrive at an incorrect prediction' (m) Given the assumptions of the VSEPR model' the

onlybond angles it predicts for compounds ofcarbon'

nitrogen, urd o*yg".t are 109'5', 120', and 180''

State the shape of a molecule whose central atom is

surrounded bY: (a) T$o regions ofelectron density

(b) Three regions ofelectron density

Hydrogen and oxygen combine in different ratios to

for* rirO (water) and IIro, (hydrogen peroxide)' (a) How many valence electrons are found in H2O? In

HzO2?

(b) Draw Lewis structures for each molecule in part (a). Be certain to show all valence electrons'

about the oxygen atom in water and about each

oxygen atom in hYdrogen Peroxide'

Hydrogen and nitrogen combine in different ratios

toform three compounds: NH3 (ammonia), NzHa (hydrazine), and N2H2 (diimide)'

(a) How many valence electrons must the Lewis

structure of each molecule show?

(b) Draw a Lewis structure for each molecule'

atom(s) in each molecule'

Predict the shape of each molecule'

(a) CHa G) PHB (c) CHFg (d) SO2 (e) SOs (0 CCIzF2 G) Ntt, ft) PCls Predict the shaPe ofeach ion.

(a) NOz- &) NHa* (c) Cos'-

Section 3.11 How Do We Determine lf a Molecule ls Polar? 3.85 Answer true or false.

(a) To predict whether a covalent molecule is polar or

nonpolar, you must know both the polarity of each

bond and the geometry (shape) of the molecule'

(b) A molecule may have two or more polar bonds

and still be nonPolar' (c) A1I molecules with polar bonds are polar'

(d) If water were a linear molecule with an H-O-H bond angle of 180", water would be a nonpolar

molecule.

104 ChaPter 3 Chemical Bonds

8.78 Nitrous oxide, N2O, Iaughing gas, is a colorless' nontoxic, tasteless, and odorless gas' It is used as an inhalation anesthetic in dental and other surger-

ies. Because nitrous oxide is soluble in vegetable

oils (fats), it is used commercially as a propellant in whiPPed toPPings.

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