Chemistry Question is this the correct answer?

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Sarah_g_24
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Q: Explain the trend in melting points of the following:
Al - 660 oC Si - 1410 oC P - 44 oC

My answer:
Silicon has the highest melt point because it is a metalloid and therefore has giant covalent bonding between 4 Silicon atoms. Phosphorus and Aluminium are simple molecules and therefore have weaker bonds with only van der Waals forces (induced dipole interactions) is this right?
Phosphorus has the lowest melt point becuase it has less electrons than Aluminium and therefore weaker dipole interactions. Less energy is needed to overcome and break the bonds than Aluminium.
Aluminium has induced dipole interactions like Phosphorus but they are stronger because more electrons are involved.

Is this correct or can anything else be added? Also, when talking about van der Waals forces, do these cover London dispersion (induced dipole) forces AND permanent dipole interactions? Whats the best way to refer to these intermolecular interactions?
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gd99
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You’re correct about Silicon - you could also mention that it has a macromolecular structure to perhaps gain a mark for that keyword.
Aluminium is not a simple molecule. It is a metal, and therefore has metallic bonding throughout the substance giving it a higher melting point than Phosphorus as metallic bonding is quite strong. Phosphorus is a simple molecule, with each separate molecule only held to the other surroundings molecules by weak van der Waals forces, which require the least amount of energy to overcome them.
To clarify, van der Waals forces are the same thing as London dispersion forces, but they are not permanent dipole interactions which require a polar bond to be present in the molecule. Whether you call them London forces or van der Waals forces depends on your exam board, I think. I usually just refer to the interactions by name. Also, the amount of electrons don’t have anything to do with the trend in these melting points, as each element exhibits a different typed bonding which is a far larger determining factor in their melting points.
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Sarah_g_24
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(Original post by gd99)
You’re correct about Silicon - you could also mention that it has a macromolecular structure to perhaps gain a mark for that keyword.
Aluminium is not a simple molecule. It is a metal, and therefore has metallic bonding throughout the substance giving it a higher melting point than Phosphorus as metallic bonding is quite strong. Phosphorus is a simple molecule, with each separate molecule only held to the other surroundings molecules by weak van der Waals forces, which require the least amount of energy to overcome them.
To clarify, van der Waals forces are the same thing as London dispersion forces, but they are not permanent dipole interactions which require a polar bond to be present in the molecule. Whether you call them London forces or van der Waals forces depends on your exam board, I think. I usually just refer to the interactions by name. Also, the amount of electrons don’t have anything to do with the trend in these melting points, as each element exhibits a different typed bonding which is a far larger determining factor in their melting points.
Wowza thank you so much!! :biggrin: That makes so much more sense now. Gave a rep :thumbsup:
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gd99
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(Original post by Sarah_g_24)
Wowza thank you so much!! :biggrin: That makes so much more sense now. Gave a rep :thumbsup:
Glad it was helpful. Good luck with your exams!
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Sarah_g_24
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(Original post by gd99)
Glad it was helpful. Good luck with your exams!
Ahhh! I'm so sorry, back with another question :s: I've just looked at what my OCR book says, why is this so confusing...

"The term van der Waals forces has sometimes been used to describe induced dipole - dipole interactions. The International Union of Pure and Applied Chemistry (IUPAC), which publishes guidelines for chemical terminology, recommends that van der Waals forces be used for both permenant and induced dipole - dipole interactions. So the term van der Waals forces is ambiguous.
London forces is the correct term to use when describing induced dipole - dipole interactions. In other sources, you may see the term London dispersion forces. Whoever said the language of science is straightforward?" - pg 78 Oxford book A-level Chemistry for OCR

I will just be calling them by their names from now on (induced / permenant interactions and ionic, covalent metallic and hydrogen bonds) - have I forgotten any?
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Pigster
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(Original post by Sarah_g_24)
I will just be calling them by their names from now on (induced / permenant interactions and ionic, covalent metallic and hydrogen bonds) - have I forgotten any?
Call them induced dipole dipole and permanent dipole dipole.
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Sarah_g_24
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(Original post by Pigster)
Call them induced dipole dipole and permanent dipole dipole.
Ah sorry I thought I'd put this, thank you.
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gd99
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(Original post by Sarah_g_24)
Ahhh! I'm so sorry, back with another question :s: I've just looked at what my OCR book says, why is this so confusing...

"The term van der Waals forces has sometimes been used to describe induced dipole - dipole interactions. The International Union of Pure and Applied Chemistry (IUPAC), which publishes guidelines for chemical terminology, recommends that van der Waals forces be used for both permenant and induced dipole - dipole interactions. So the term van der Waals forces is ambiguous.
London forces is the correct term to use when describing induced dipole - dipole interactions. In other sources, you may see the term London dispersion forces. Whoever said the language of science is straightforward?" - pg 78 Oxford book A-level Chemistry for OCR

I will just be calling them by their names from now on (induced / permenant interactions and ionic, covalent metallic and hydrogen bonds) - have I forgotten any?
Not sure I can help here, I’m afraid. I do AQA, and they definitely just want us to call them van der Waals forces. I guess that OCR want you to be more specific and call them London dispersion forces.
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