Bonding Watch

Mutleybm1996
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A really simple question i know but could someone briefly explain how to tell if an exam says 'show the bonding in ...' How i'd tell if it was ionic, covalent, VDW, etc.

For instance in a mock exam i wrote that PH(3) is covalently bonded but apparently it's VDW forces....how would i know this? My textbook isn't very clear

Finally, what's the difference between permanent dipole-dipole, induced dipole-dipole and temporary dipole-dipole?

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KoopaTroopa
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(Original post by benwalters1996)
A really simple question i know but could someone briefly explain how to tell if an exam says 'show the bonding in ...' How i'd tell if it was ionic, covalent, VDW, etc.

For instance in a mock exam i wrote that PH(3) is covalently bonded but apparently it's VDW forces....how would i know this? My textbook isn't very clear

Finally, what's the difference between permanent dipole-dipole, induced dipole-dipole and temporary dipole-dipole?

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Ionic bonds are formed when there is a strong difference between the electronegativity of the two atoms involved.

Van der Waals is an intermolecular force but is not a bond.
Van der Waals forces are weak and insignificant compared to covalent bonds. Covalent bonds are the sharing of electrons between two atoms.

Okay, so induced dipole-dipole is another term for Van Der Waals forces. Van der Waals' forces are much weaker than all other types of bonding. A permanent dipole is due to a difference in electronegativity between the atoms involved in a covalent bond.

Hope this helps

Edit:

temporary dipoles are when the electron clouds' density around a nucleus shifts in density, forming temporary dipoles.*

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Borek
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(Original post by benwalters1996)
i wrote that PH(3) is covalently bonded but apparently it's VDW forces...[/SIZE]
As KT already signaled - this is wrong. VDW forces are intermolecular, PH3 is kept in one piece by intramolecular bonds.
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Mutleybm1996
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(Original post by KoopaTroopa)
Ionic bonds are formed when there is a strong difference between the electronegativity of the two atoms involved.

Van der Waals is an intermolecular force but is not a bond.
Van der Waals forces are weak and insignificant compared to covalent bonds. Covalent bonds are the sharing of electrons between two atoms.

Okay, so induced dipole-dipole is another term for Van Der Waals forces. Van der Waals' forces are much weaker than all other types of bonding. A permanent dipole is due to a difference in electronegativity between the atoms involved in a covalent bond.

Hope this helps

Edit:

temporary dipoles are when the electron clouds' density around a nucleus shifts in density, forming temporary dipoles.*

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So how would one know that PH(3) is bonded via dipole-dipole in the exam? Is there a way to work it out?


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Borek
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PH3 is NOT bonded by dipole-dipole interactions.

Unless you ask about intermolecular forces, from your question it is not at all clear if you see the difference.
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Mutleybm1996
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(Original post by Borek)
PH3 is NOT bonded by dipole-dipole interactions.

Unless you ask about intermolecular forces, from your question it is not at all clear if you see the difference.
Sorry, 'main intermolecular force'


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username913907
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You don't get intramolecular VDW in the kind of small molecules you'll be dealing with. It is mostly going to be an Inter-molecular force as far as you are concerned.
As to ionic/covalent..... It's a spectrum with the bonds between the metal and non-metal in a salt at one end and a C-H bond at the other end of the scale. You need to place it towards one end based upon how polar you think the bond may be. You can estimate this from the differences in electronegativity.
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Georgiecat
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(Original post by benwalters1996)
A really simple question i know but could someone briefly explain how to tell if an exam says 'show the bonding in ...' How i'd tell if it was ionic, covalent, VDW, etc.

For instance in a mock exam i wrote that PH(3) is covalently bonded but apparently it's VDW forces....how would i know this? My textbook isn't very clear

Finally, what's the difference between permanent dipole-dipole, induced dipole-dipole and temporary dipole-dipole?

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Though the P-H bonds in PH3 are covalent, molecules of PH3 are attracted to one another by VDW forces, if that makes sense? I'm pretty sure that's what your textbook meant.

Permanent dipole-dipole is when one of the atoms/ions within a molecule has greater electronegativity (its nucleus has a greater positive charge, and so it is better at attracting electrons, so there will be a greater electron density around it), so there are generally more electrons at one end of the molecule, and so - as electrons are negatively charged - that end of the molecule will take on a slightly negative charge, and so the other end of the molecule will take a on a slightly positive charge due to the lack of electrons, creating a permanent dipole. This only happens in polar molecules - where one of the species within the molecule has greater electronegativity than the other. This permanent dipole within molecules means that there will be weak forces of attraction between the positive end of one polar molecule and the negative end of another polar molecule.

Temporary dipole-dipoles are actually present in all molecules. If you consider a molecule with electrons moving randomly around their orbitals, it's very unlikely that all the electrons are always going to be equally spaced within the molecule - it's more likely that there will be more electrons at one end of the molecule, creating a slightly negative charge at that end of the molecule and a slightly positive charge at the other end of the molecule (i.e. a dipole), but of course (because electrons are constantly moving around) the precise location of the slightly negative charge will change constantly, so the dipole will fluctuate. So this does create forces of attraction between molecules, but to understand why you need to understand induced dipoles...

Imagine, if you will, that a molecule with all the electrons shifted to the left-hand side (so the left hand side is slightly negative, and the right-hand side is slightly positive - also, let's call this molecule A) meets a molecule where all the electrons are equally spaced throughout the molecule - molecule B. The electrons in molecule B will be attracted to the slightly positive end of molecule A, creating polarity in molecule B - the left hand side of molecule B will become slightly negative, and the right-hand side slightly positive. As the electrons in molecule A move, say to the right-hand side of the molecule (making the right-hand side of molecule A slightly negative and the left-hand side slightly positive) the slight negative charge on the right hand side of molecule A will repel the electrons in molecule B to the other side of molecule B, reversing the polarity of B as the left-hand side of molecule B becomes slightly positive and the right hand side becomes slightly negative. This could actually happen between many, many molecules, if you think about it, and it is responsible for weak forces of attraction between molecules. These are VDW forces.

It is these intermolecular forces that hold covalently-bonded molecules, like PH3, to one another, but they are quite weak and easy to overcome, hence simple covalent substances have low boiling points, even though the covalent bonds within the molecules themselves are strong.

If any of this didn't make sense, please tell me!
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cuckoo99
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(Original post by benwalters1996)
So how would one know that PH(3) is bonded via dipole-dipole in the exam? Is there a way to work it out?


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All molecules/atoms will have a degree of van der waals interaction between one and other which was quite nicely explained by KT and probably by others in this thread;p
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Mutleybm1996
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(Original post by Georgiecat)
Though the P-H bonds in PH3 are covalent, molecules of PH3 are attracted to one another by VDW forces, if that makes sense? I'm pretty sure that's what your textbook meant.

Permanent dipole-dipole is when one of the atoms/ions within a molecule has greater electronegativity (its nucleus has a greater positive charge, and so it is better at attracting electrons, so there will be a greater electron density around it), so there are generally more electrons at one end of the molecule, and so - as electrons are negatively charged - that end of the molecule will take on a slightly negative charge, and so the other end of the molecule will take a on a slightly positive charge due to the lack of electrons, creating a permanent dipole. This only happens in polar molecules - where one of the species within the molecule has greater electronegativity than the other. This permanent dipole within molecules means that there will be weak forces of attraction between the positive end of one polar molecule and the negative end of another polar molecule.

Temporary dipole-dipoles are actually present in all molecules. If you consider a molecule with electrons moving randomly around their orbitals, it's very unlikely that all the electrons are always going to be equally spaced within the molecule - it's more likely that there will be more electrons at one end of the molecule, creating a slightly negative charge at that end of the molecule and a slightly positive charge at the other end of the molecule (i.e. a dipole), but of course (because electrons are constantly moving around) the precise location of the slightly negative charge will change constantly, so the dipole will fluctuate. So this does create forces of attraction between molecules, but to understand why you need to understand induced dipoles...

Imagine, if you will, that a molecule with all the electrons shifted to the left-hand side (so the left hand side is slightly negative, and the right-hand side is slightly positive - also, let's call this molecule A) meets a molecule where all the electrons are equally spaced throughout the molecule - molecule B. The electrons in molecule B will be attracted to the slightly positive end of molecule A, creating polarity in molecule B - the left hand side of molecule B will become slightly negative, and the right-hand side slightly positive. As the electrons in molecule A move, say to the right-hand side of the molecule (making the right-hand side of molecule A slightly negative and the left-hand side slightly positive) the slight negative charge on the right hand side of molecule A will repel the electrons in molecule B to the other side of molecule B, reversing the polarity of B as the left-hand side of molecule B becomes slightly positive and the right hand side becomes slightly negative. This could actually happen between many, many molecules, if you think about it, and it is responsible for weak forces of attraction between molecules. These are VDW forces.

It is these intermolecular forces that hold covalently-bonded molecules, like PH3, to one another, but they are quite weak and easy to overcome, hence simple covalent substances have low boiling points, even though the covalent bonds within the molecules themselves are strong.

If any of this didn't make sense, please tell me!
Very helpful, thanks!

This says that PH3 is dipole-dipole though :/


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Georgiecat
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(Original post by benwalters1996)
Very helpful, thanks!

This says that PH3 is dipole-dipole though :/


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Glad to be of help
Yes - the intermolecular forces between molecules of PH3 are dipole-dipole and dispersion forces. PH3 has a lone pair, and three hydrogen atoms covalently bonded to a phosporous molecule. The lone pair repels the hydrogens to one end of the molecule, creating a "phosphorous end" and a "hydrogen end" to the molecule, if that makes sense (if you can't quite visualise it, see this picture). Phosphorous has greater electronegativity than hydrogen, so the electrons are attracted by the phosphorous nucleus to the "phosphorous end" of the molecule, so the "phosphorous end" of the molecule is slightly negative while the "hydrogen end" of the molecule is slightly positive - this creates a dipole, so PH3 molecules are polar. This means the slightly positive hydrogen end of one PH3 molecule and the slightly negative phosphorous end of another PH3 molecule will be attracted, creating dipole-dipole intermolecular forces. And then dispersion forces are present in all molecules.
VDW forces is a general term for any dipole-related forces between molecules.
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Mutleybm1996
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(Original post by Georgiecat)
Glad to be of help
Yes - the intermolecular forces between molecules of PH3 are dipole-dipole and dispersion forces. PH3 has a lone pair, and three hydrogen atoms covalently bonded to a phosporous molecule. The lone pair repels the hydrogens to one end of the molecule, creating a "phosphorous end" and a "hydrogen end" to the molecule, if that makes sense (if you can't quite visualise it, see this picture). Phosphorous has greater electronegativity than hydrogen, so the electrons are attracted by the phosphorous nucleus to the "phosphorous end" of the molecule, so the "phosphorous end" of the molecule is slightly negative while the "hydrogen end" of the molecule is slightly positive - this creates a dipole, so PH3 molecules are polar. This means the slightly positive hydrogen end of one PH3 molecule and the slightly negative phosphorous end of another PH3 molecule will be attracted, creating dipole-dipole intermolecular forces. And then dispersion forces are present in all molecules.
VDW forces is a general term for any dipole-related forces between molecules.
Thanks! Is there a way of double checking which molecules have lone pairs/a formula for working out bond angles? (A-level)
I'm ok when it's things i've revised but it's hard to apply it if they randomly ask for what the intermolecular forces are in some random molecule
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Georgiecat
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(Original post by benwalters1996)
Thanks! Is there a way of double checking which molecules have lone pairs/a formula for working out bond angles? (A-level)
I'm ok when it's things i've revised but it's hard to apply it if they randomly ask for what the intermolecular forces are in some random molecule

Well, for lone pairs, look at how many bonds the "central" atom has formed and compare that to how many electrons it had in its outer shell (which you can find out by looking at its group in the period table). For example, phosphorous has five electrons in its outer shell, and it's bonded to three hydrogens. Each P-H covalent bond requires one electron from the phosphorous atom to be shared between the phosphorous atom and the hydrogen atom (because to get a full outer shell hydrogen requires one extra electron). Three hydrogen atoms are bonded to the central phosphorous atom, but that leaves two "unused" electrons in the outer shell of phosphorous, which must be a lone pair.
For working out bond angles, they're only realistically going to ask you questions on very simple molecules - what you do is count the number of electrons in the outer shell of the "central" atom (the one which the other atoms are all attached to), add the total number of shared electrons from the atoms bonded to it (but IGNORE OXYGEN), then take into account lost or gained electrons (this step only applies to ions - if the overall charge on the molecule is +1, you take away 1 electron, and if the overall charge on the molecule is -1 you add an electron, but of course molecules which aren't ions are neutral), and then divide this number by two. This gives you the number of "bonding axes" - basically, a number which corresponds to a shape (2: linear, 3: trigonal planar etc etc etc) which you've probably looked at in class, and your teacher most likely gave you a handout with a list of numbers alongside names for different structures, and then bond angles. It's also probably in the CGP guide somewhere, if you have access to one. If not, I can dig one out of my notes, but there's no point giving you something you already have! You'll probably need to memorise that table, but that will depend on your exam board.
If that was a bit hard to follow, I'll do a worked example with PH3:
-Phosphorous is the central atom, because all the hydrogens are bonded to it, and it has 5 electrons in its outer shell
-The hydrogens each contribute 1 electron to the covalent bond they're in, so you add (3*1), which makes 8, and there is no oxygen to ignore
-PH3 is neutral (we know that because it's not an ion, so the overall charge on the molecule is 0) so we don't add or take away any electrons in this step
-8/2 is 4
-4 bonding axes with a lone pair gives you a trigonal pyramidal structure, and each bond angle is 107 degrees
Be careful with your phrasing - the intermolecular forces aren't IN molecules, they refer to the forces of attraction BETWEEN molecules, which are caused by dipoles. It is hard, but just work through it logically - dispersion forces are present in all molecules, and then look at the electronegativity of all the atoms within the molecule, and the overall shape of the molecule (i.e. is there a slightly positive and a slightly negative end, or is the central atom completely surrounded by atoms?) and work out if it's a polar molecule from there.
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