Science a GoGo's Home Page Forums General Discussion General Science Discussion Forum Intermolecular forces - halogens

why do the intermolecular forces increase as you descend group 7?
does it have anything to do with the size of the molecules

Nope, with lack of electrons, which is compensated by orbital sharing.

what do you mean by lack of electrons

I emailed your question to my daughter who a chemistry major, but who is in China right now. Following is her response:

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yes, they get bigger. all periodic trends are based on these two things

1. shielding effect
2. successive energy levels

both of these things affect the size of hte molecule. the more energy levels, the larger it is. the more shielding effect between electrons in the orbitals, the more they repel (like two magnets and you put the south ends together) and the larger hte atom will be. when atoms are fatter, they get in each others way more. kinetic molecular theory states that atoms or molecules hitting each other with the right energy in teh right spot will react. intermolecular forces is basically a specific case of this. except, they don't react to form new compounds, so they don't have to hit in a certain location. they just get in each others way. imagine a school where everyone is exactly the same, physically. if everyone's skinny and there's 2k of them, then they won't get in each others way too much. so there are less intermolecular forces. where as if you have say 2k morbidly obese kids in the same school, it would be really hard for any one kid to get around. thus, there are stronger IMF forces.

The tendency to fill up valence electron octet to the noble gas configuration. Sodium should lose one electron and to obtain seven electrons to get complete electron octet, so for sodium is much easier to lose excessive electron, then to catch some, so it's electronegative atom.

Halogens are required to obtain just a single electron to fill up electron octet, so they're strongly electropositive.
Your daughter should still learn chemistry a bit..

No doubt. AWT is surely a better explanation for ANY phenomenon, but I thought it might be good to ask her just in case the fellow wanted the answer from a person immersed in the standard chemistry.

Here exists an insintric duality between most general and most exact views of reality. Currently it seems, AWT is most general one - but definitely not the best, when it goes to exact numbers. From the same reason, we don't use quantum mechanics for computation of boiling point of water under reduced pressure, but we are using a more specific extrapolations based on thermodynamics. Not because the quantum mechanics couldn't handle it in ab-initio calculations, but because such calculation would be more tedious and sensitive to introductory parameters. Due the uncertainty principle we cannot expect true "theory of everything" and every theory has it's own applicability scope, corresponding to observable part of Universe.

You probably ment intramolecular forces, because intermolecular force have no such apparent dependency toward 7 group..

your daughter states that the more shielding, the more they repel. Surley this would interrupt the IMF

http://en.wikipedia.org/wiki/Shielding_effect

the electron repulsion only applies between a single atom's nucleus and its electrons.


http://en.wikipedia.org/wiki/Van_der_Waals_force#London_dispersion_force

**especially read the last two paragraphs, but i really recommend that whole section

"London forces become stronger as the atom or molecule in question becomes larger. "

"The London forces also become stronger with larger amounts of surface contact. Greater surface area means closer interaction between different molecules."


smart daughter.

This is indeed true - the only question is, how surface area is related to ascendance from group 1 to group 7? Whether alkali metal atoms are smaller in general, then halogen atoms? Of course not, as atom size depends on atomic weight. i.e. the period - not group number in periodical table.

..educated - still silly daughter, anyway....
Or rather a typical product of contemporary educational systems, which are preferring learning over the thinking.

Being disagreeable, uninformed, and arrogant do not equate to thinking. Nor are they signs of being anything other than disagreeable, uninformed, and arrogant.

Right, so why do you disagree with me arrogantly in uninformed way?


(Hint: note the OT irony concerning AWT in your first post)

I note that you are an idiot and that every post you make is redolent with irony.

let's remind ourselves of the question asked.

question: why do the intermolecular forces increase as you descend group 7? does it have anything to do with the size of the molecules

answer: yes. IMF increases when you descend the group, and this is due to size.

not all of the atoms across a period will be affected by the same forces. you have to look at the atom/molecule in question. are you discussing pure substances (like Na lattice and Cl2)in which case you would look at intramolecular forces? when looking at molecules, the larger the molecule, the stronger the IMF.

answering the question at hand does not make one a product of learning over thinking. it means that one is capable of thinking clearly and explaining a single topic - a sign of intelligence. being able to organize ones thoughts is one of the most valuable of skills because it is so scarce, especially in the virtual world. i would go on to mention that some posts may use chemistry terms inappropriately, but i wouldn't want to educate any potential great thinkers of our time.

Let's ease off on the name-calling. A little more decorum, a little more respect if you please.