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Question Restated (was Re: Homework question

Question Restated (was Re: Homework question - Chemistry Forum

Question Restated (was Re: Homework question - Chemistry Forum. Discuss chemical reactions, chemistry.


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  #1  
Old 09-25-2003, 08:41 PM
TC
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Default Question Restated (was Re: Homework question



Thanks for your answers, but I think I've been unclear in what
the problem is. I'm happy that I understand the basic notion
of electronegativity.

My question is fundamentally a paedegogical one, and not really
about chemistry per se. Unlike some of the nonsense subjects
our kids are being taught in high school, I regard chemistry
as one of the remaining "hard" respectable subjects. As such,
it can withstand, and should demand, rigour.

So, let me ask again, via analogy:

I'm saying that defining "electronegativity" in the following way:

"Electronegativity is a measure of the attraction which an atom involved
in a covalent bond has for the bonding electrons"

is wrong, in the same way that defining "height" in the following way:

"Height is the number of inches from the head to the toes of a tall person"

is wrong.

The issue is the inclusion of the word "covalent" in the first, and
"tall" in the second.

As I understand it, "covalent" refers to a specific subset of bonds that arise
when the difference in electronegativity between the bonding atoms falls within
a specific range.

Similarly, "tall" refers to a specific subset of persons when the number of
inches from head to toe falls within a specific range.

So the offending words should be removed. Unless they are removed,
one is tempted to think that electronegativity does not apply to atoms
in an ionic bond, and that short or medium people don't have heights.

Am I being a pedant? Absolutely not. This is chemistry we're talking
about here, not media studies or, god forbid, home economics.
It's as much about teaching our kids rigour, as about electrons.

tc
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  #2  
Old 09-25-2003, 08:58 PM
Marvin Margoshes
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Default Question Restated (was Re: Homework question


"TC" <[Only registered users see links. ].uk> wrote in message
news:4ed53853.0309251241.7cdf417e@posting.google.c om...
involved
person"
arise
within
of

My hunch is that the problem is a teacher with poor knowledge of chemistry.
It is hard to find good high school science teachers. Too often, a teacher
of some other subject is pressed into service. How do I know this? One way
is postings in sci.chem in the past, from teachers in that situation who are
desparate for help.


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  #3  
Old 09-26-2003, 01:14 AM
Josh Halpern
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Default Question Restated (was Re: Homework question



Marvin Margoshes wrote:

The problem is that one cannot draw a sharp line in the sand
and say that this side of the line is covalent, and that ionic.
You can pretty well describe extremes as covalent (tall
as in 2 meters) and ionic (short as in 1.3 meters) but
that is about it.


That's wrong. Actual science involves a fair amount of judgement,
and hard and fast rules are few and hard to come by. While one
should simplify for beginners, providing clear cases as examples,
as practicioners one must also know the limits of such classification.
As MM points out, it is rare to find a HS chemistry teacher with
such expertise, and they are thus overfond of strict rules and often
react badly to being questioned.

josh halpern

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  #4  
Old 09-26-2003, 07:59 AM
TC
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Default Question Restated (was Re: Homework question

Josh Halpern <[Only registered users see links. ]> wrote in message news:<[Only registered users see links. ]>...


In which case a definition of electronegativity which applies
only to "covalent" is wrong, isn't it? Look at the following
two definitions of EN:

"Electronegativity is a measure of the attraction which an atom involved
in a covalent bond has for the bonding electrons"

or

"Electronegativity is a measure of the attraction which an atom involved
in a bond has for the bonding electrons"

Which is better?

I think the former is inferior because it suggests that electronegativity
does not apply to ionic bonds. Of course the teacher may be
re-defining "covalent" to mean "any kind of shared electron bonding",
and considers "ionic" as a synonym for "very very polar covalent".

But that's not how the rest of the planet uses the words. For most people,
part of the meaning of the word "covalent" is, "not ionic", in the same
way that "tall" includes the notion of "not short". "Covalent", "polar
covalent", "ionic" *just are* coarse measures of electronegativity,
in the same way that "short", "medium" and "tall" *just are* coarse
measures of height.

And remember what is going on here - the teacher is trying to *define*
the word in order to explain the concept. An unnecessarily imprecise
definition is unnecessarily confusing to students.

tc
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  #5  
Old 09-26-2003, 01:32 PM
Dr. George O. Bizzigotti
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Default Question Restated (was Re: Homework question

On Fri, 26 Sep 2003 01:14:19 GMT, Josh Halpern
<[Only registered users see links. ]> wrote:







I would expand on Josh Halpern's description of the problem and his
response to the original poster. Much (Most?) of our "advanced"
education in chemistry consists of unlearning simple models, and then
learning more complex models to replace them. In the case of bonding,
our most advanced understanding is very complex. However, most high
school students are not yet ready to start solving Hamiltonians, so we
use simplifications to explain things in ways they can understand.*
Covalent vs. ionic is just such a case; it is more or less based on a
view of bonds as electrons between 2 atoms. It is an immensely useful
concept, as many cases behave as if they were close to that situation.
The concept is useful in even more cases if one doesn't get too hung
up on the rigor of things.** However, strictly speaking, bonding is a
phenomenon that involves all the electrons and all the atoms in the
immediate vicinity of whatever species one is discussing. This means
that there are some instances where one gets inaccurate understanding
by considering atoms a pair at a time, and the "rigorous" application
of simple concepts such as electronegativity, ionic, and covalent
begin to break down.

*If one considers the chemical bond and goes back and read Pauling's
book, it strikes me that what we typically teach freshmen chemists is
actually a simplification of Pauling's simplification.

**I'm thinking, for example, of some of the explanations of
substituent effects on reactivity in aromatic systems. Organic
chemists casually draw a host of canonical resonance structures that
would make a theoretician blush, but if you look at the theoretician's
explanation in all it's rigorous glory (sometimes a bit of squinting
is required), there is a rough correspondence between the
explanations. (Note that I'm speaking here of a subset of what has
become the general practice; the original work, e.g., Hammett's, tends
to be a lot more rigorous than many of us sometimes realize)


I think it's less a question of expertise than of temperament. Josh
has described a classic case of what the Magliozzi brothers call Male
Answer Syndrome (perhaps an authoritarian variant). Some of the best
science teachers may not know much more chemistry, but they are not
afraid to say "I don't know."

The bottom line, is that using rigor and strict rules with what is
inherently a non-rigorous simplification of our best understanding of
a phenomenon is not necessarily a productive exercise.

Regards,

George
************************************************** ********************
Dr. George O. Bizzigotti Telephone: (703) 610-2115
Mitretek Systems, Inc. Fax: (703) 610-1558
3150 Fairview Park Drive South E-Mail: [Only registered users see links. ]
Falls Church, Virginia, 22042-4519
************************************************** ********************


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  #6  
Old 09-27-2003, 09:25 AM
Frank X
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Default Question Restated (was Re: Homework question


"TC" <[Only registered users see links. ].uk> wrote in message
news:4ed53853.0309252359.21b6cd78@posting.google.c om...
people,

Why don't you ask the teacher for the definiton of covalent bond?


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  #7  
Old 12-14-2003, 11:03 PM
Fuller
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Default Question Restated (was Re: Homework question

Holy cow! What a lot of wrangling over a simple matter.
Electronegativity is simply "the relative power of a bonded atom to
attract electrons to itself". The bonds can be anything from covalent
to ionic, even metallic -- the nature of the bond doesn't affect the
definition of electronegativity.
M. Fuller

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