<< Chapter < Page Chapter >> Page >

However, it is very important to realize that these two structures are identical in the Lewis model, because both show that the oxygen atom has a complete octet of valence electrons,forms two single bonds with hydrogen atoms, and has two pairs of unshared electrons in its valence shell. In the same way, the twostructures for Freon 114 shown here are also identical .

These two drawings do not represent different structures or arrangements of the atoms in the bonds.

Finally, we must keep in mind that we have drawn Lewis structures strictly as a convenient tool for ourunderstanding of chemical bonding and molecular stability. It is based on commonly observed trends in valence, bonding, and bondstrengths. These structures must not be mistaken as observations themselves, however. As we encounter additional experimentalobservations, we must be prepared to adapt our Lewis structure model to fit these observations, but we must never adapt ourobservations to fit the Lewis model.

Extensions of the lewis structure model

With these thoughts in mind, we turn to a set of molecules which challenge the limits of the Lewis model indescribing molecular structures. First, we note that there are a variety of molecules for which atoms clearly must bond in such away as to have more than eight valence electrons. A conspicuous example is S F 6 , where the sulfur atom is bonded to six F atoms. As such, the S atommust have 12 valence shell electrons to form 6 covalent bonds. Similarly, the phosphorous atom in P Cl 5 has 10 valence electrons in 5 covalent bonds, the Cl atom in Cl F 3 has 10 valence electrons in 3 covalent bonds and two lone pairs. We also observe the interesting compounds of the noble gas atoms, e.g. Xe O 3 , where noble gas atom begins with eight valence electrons evenbefore forming any bonds. In each of these cases, we note that the valence of the atoms S, P, Cl, and Xe are normally 2, 3, 1, and 0,yet more bonds than this are formed. In such cases, it is not possible to draw Lewis structures in which S, P, Cl, and Xe obeythe octet rule. We refer to these molecules as "expanded valence" molecules, meaning that the valence of the centralatom has expanded beyond the expected octet.

There are also a variety of molecules for which there are too few electrons to provide an octet for every atom. Most notably, Boron and Aluminum, from Group III, displaybonding behavior somewhat different than we have seen and thus less predictable from the model we have developed so far. These atomshave three valence shell electrons, so we might predict a valence of 5 on the basis of the octet rule. However, compounds in whichboron or aluminum atoms form five bonds are never observed, so we must conclude that simple predictions based on the octet rule arenot reliable for Group III.

Consider first boron trifluoride, B F 3 . The bonding here is relatively simple to model with a Lewis structure if we allow each valenceshell electron in the boron atom to be shared in a covalent bond with each fluorine atom.

Note that, in this structure, the boron atom has only six valence shell electrons, but the octet rule is obeyedby the fluorine atoms.

We might conclude from this one example that boron atoms obey a sextet rule. However, boron will form a stableion with hydrogen, B H 4 - , in which the boron atom does have a complete octet. In addition, B F 3 will react with ammonia N H 3 for form a stable compound, N H 3 B F 3 , for which a Lewis structure can be drawn in which boron has acomplete octet, shown here .

Compounds of aluminum follow similar trends. Aluminum trichloride, Al Cl 3 , aluminum hydride, Al H 3 , and aluminum hydroxide, Al ( O H ) 3 , all indicate a valence of 3 for aluminum, with six valenceelectrons in the bonded molecule. However, the stability of aluminum hydride ions, Al H 4 - , indicates that Al can also support an octet of valence shellelectrons as well.

We conclude that, although the octet rule can still be of some utility in understanding the chemistry of Boronand Aluminum, the compounds of these elements are less predictable from the octet rule. This should not be disconcerting, however. Theoctet rule was developed in on the basis of the observation that, for elements in Groups IV through VIII, the number of valenceelectrons plus the most common valence is equal to eight. Elements in Groups I, II, and III do not follow this observation mostcommonly.

Resonance structures

Another interesting challenge for the Lewis model we have developed is the set of molecules for which it ispossible to draw more than one structure in agreement with the octet rule. A notable example is the nitric acid molecule, H N O 3 , where all three oxygens are bonded to the nitrogen. Two structurescan be drawn for nitric acid with nitrogen and all three oxygens obeying the octet rule.

In each structure, of the oxygens not bonded to hydrogen, one shares a single bond with nitrogen while the othershares a double bond with nitrogen. These two structures are not identical, unlike the two freon structures in , because the atoms are bonded differently in the two structures.

Review and discussion questions

Compounds with formulae of the form C n H 2 n + 2 are often referred to as "saturated" hydrocarbons. Using Lewis structures, explain how and in what sense thesemolecules are "saturated."

Got questions? Get instant answers now!

Molecules with formulae of the form C n H 2 n + 1 ( e.g. C H 3 , C 2 H 5 ) are called "radicals" and are extremely reactive. UsingLewis structures, explain the reactivity of these molecules.

Got questions? Get instant answers now!

State and explain the experimental evidence and reasoning which shows that multiple bonds are stronger andshorter than single bonds.

Got questions? Get instant answers now!

Compare N 2 to H 4 N 2 . Predict which bond is stronger and explain why.

Got questions? Get instant answers now!

Explain why the two Lewis structures for Freon 114, shown in Figure 21 , are identical. Draw a Lewis structures for an isomer of Freon 114, that is,another molecule with the same molecular formula as Freon 114 but a different structural formula.

Got questions? Get instant answers now!

Questions & Answers

how do they get the third part x = (32)5/4
kinnecy Reply
can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
okay, so you have 6 raised to the power of 2. what is that part of your answer
I don't understand what the A with approx sign and the boxed x mean
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
I'm not sure why it wrote it the other way
I got X =-6
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
oops. ignore that.
so you not have an equal sign anywhere in the original equation?
Commplementary angles
Idrissa Reply
im all ears I need to learn
right! what he said ⤴⤴⤴
what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
Kevin Reply
a perfect square v²+2v+_
Dearan Reply
kkk nice
Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Embra Reply
if |A| not equal to 0 and order of A is n prove that adj (adj A = |A|
Nancy Reply
rolling four fair dice and getting an even number an all four dice
ramon Reply
Kristine 2*2*2=8
Bridget Reply
Differences Between Laspeyres and Paasche Indices
Emedobi Reply
No. 7x -4y is simplified from 4x + (3y + 3x) -7y
Mary Reply
is it 3×y ?
Joan Reply
J, combine like terms 7x-4y
Bridget Reply
how do you translate this in Algebraic Expressions
linda Reply
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Got questions? Join the online conversation and get instant answers!
QuizOver.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, General chemistry i. OpenStax CNX. Jul 18, 2007 Download for free at http://cnx.org/content/col10263/1.3
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'General chemistry i' conversation and receive update notifications?