im missed a couple of lessons at school and im slightly confused by the following:
Does electronegativity only affect covalent bonds?
If not, What effect (if any) does electronegativity have on an ionic bonding pair).
What is shielding? How does it change across periods?
Does the electonegativity of an atom have anything to do with its nuclear charge?
What is a dipole and does it have anything to do with electronegativity?
Electronegativity can be defined as the power of an atom to draw electron density to itself in a covalent bond....so yes, electronegativity is a concept only connected with covalency.
What is shielding? How does it change across periods?
The trend in atomic radius and ionisation energy (apart from the little jumps)depends on the idea that as you move from one atom to another from left to right, there is an increase in the number of protons in the nucleus which is matched by an increase in the number of electrons in the outside shell.
The increase in the number of protons causes the attraction between the nucleus and the electrons to increase. If the repulsion between the electrons in the outside shell were also increased by as much there would be no change in atomic radius etc but the trouble is that the repulsion between the electrons is only slight;y increased and nowhere near enough to match the increased attraction to the nucleus.
If we ignore distance, electrostatic attraction size is basically the multiplication of the two charges attracting each other.
So a three-plus positive charge attracting a one-minus electron is three times as strong a force as two one-minus electrons repelling each other.
Consequently, as the nucleus charge increases, the electrons are attracted more strongly to it.
This leads to a decrease in atomic radius and an increase in ionisation energy.
Does the electonegativity of an atom have anything to do with its nuclear charge?
Yes. If you think about all the ideas about ionisation energy and atomic radius but apply them to an electron in the covalent bond betwen one atom and another, you will see that an atom at the right hand side of the periodic table has a stronger pull on the electron than one from the left hand side. If you build in the idea of size as well, small atoms are more attractive to the electrons in a covalent bond (higher electronegativity) than larger ones. Consequently, fluorine is the most electronegative.
What is a dipole and does it have anything to do with electronegativity?
A dipole is a separation of charge over a distance. It is a bit like a couple in mechanics. If a neutral molecule (such as HCl) has an electron-rich chlorine end and an electron-deficient hydrogen end, it will have a dipole. A bigger charge that is separated (ie the bigger the electronegativity difference) and a bigger distance over which it is separated (bigger molecule)means that the dipole is larger. Big dipole-dipole attraction is one thing that leads to high melting and boiling points.
Could you tell me what a dipole moment is?
Are all dipole moments of the same value (for example, is it a case of a molecule having a dipole or not)?
My textbook says that some molecules have dipoles that cancel. Does that depend on how the atoms are arranged in the molecule? Do only certain atoms have the ability to create a dipole (for example, can a hydrogen atom bonded to a carbon atom create a dipole)?
Do dipoles affect bond angles, and if so, how and by how much?
Could you tell me what a dipole moment is?
Are all dipole moments of the same value (for example, is it a case of a molecule having a dipole or not)?
No. The dipole moment is the size of the charge separation multiplied by the distance of the separation. It is very much like a lever in physics where a small force a long way from the fulcrum can have a bigger effect than a large force very near to it. A small charge separation over a large distance can be just as big or even bigger a dipole moment than a large charge separation over a small distance.
My textbook says that some molecules have dipoles that cancel. Does that depend on how the atoms are arranged in the molecule?
If there is symmetry in the molecule, the dipole moments can cancel out. It is a bit like having four people pulling on ropes attached to a load in the middle. If they all pull equally, there is no overall force on the load. Even a very strong pull will have no effect if all the forces balance out.
Do only certain atoms have the ability to create a dipole (for example, can a hydrogen atom bonded to a carbon atom create a dipole)?
Any non-identical pair of atoms will have a dipole moment of some size, even if it is very small. Whether the dipole mooment has an overall effect depends on the symmetry of the molecule. Even if the molecule is asymmetrical, the resultant dipole moment might be too small to be significant.
Do dipoles affect bond angles, and if so, how and by how much?
thanks alot. I think i've got my head around dipoles thanks to you.
The next topic is van der Waals forces. My text book says that v.d.w. forces explain why the boiling point of noble gases increase as the atomic numbers of the noble gases increase, and that they also explain the boiling points of hydrocarbons as the carbon chain length increases, however i dont see why or how.
Also (this is a bit off topic), if the electron clouds in atoms found a position in which they "balanced", what (if anything) would happen?
also, to what extent do we need to know about (AQA specification) "understanding the importance of hydrogen bonding in determining the boiling points of compounds and structures of some solids (e.g. ice)?
Look at the boiling points of group six hydrides (water, H2S, H2Se, H2Te). There is a gradual increase down the group except for water which is ridiculously high.
The general increase is because of increased VdW between molecules because of their increased surface area. This means that the instantaneous dipoles set up in the molecules are larger because of the electrons being less tightly bound and able to spread over a larger distance.
Water is anomalous because although it has relatively small VdW, it has H-bonding between molecules. This is an example of the special effect of H-bonding. Another would be that ice floats on water. The H-bonding holds the water molecules in a much more open structure than you would expect for molecules without the possibility of H-bonding. Since the same mass is spread out over a larger volume, the density (D=M/V) is less and so the ice floats.
The boiling point of noble gases increase as the atomic numbers of the noble gases increase
Although the instataneous dipole in a single atom (as we have with the noble gases) is only very small (the electron distortion can only be over a very small distance since we don't have the length of the molecule to play with), the VdW is small.
the boiling points of hydrocarbons as the carbon chain length increases.
Again, the longer the chain, the more the distortion can take place. If only the same amount of charge can be displaced instantly, it will have more effect if it can be displaced over a longer distance. Think of it a bit like levers, a partiucular force can have a much greater effect if it is applied over a long distanc ewith a long seesaw.
Also (this is a bit off topic), if the electron clouds in atoms found a position in which they "balanced", what (if anything) would happen?
I am sorry, I don't really know what you mean by this question. Try to rephrase it or perhaps say what you think will happen and then I can comment.
I've got my chemistry mock tomorrow and there were just a few last things I'd like to check with you:
What are simple molecules and ions (AQA spec: bond angles)?
Which atoms can hold more than 8 electrons in their outer shell?
Which atoms can hold less than 8 electrons in their outer shell?
On a resource sheet i was given, it shows an arrow going from P to Cl in a PCl5 molecule. Does that mean that its a co-ordinate bond? If so, Why?
Do we need to know anything about stereoisomerism?
Another resource sheet i was given shows
H....H
l....l
H-C-O-C-H
l....l
H....H
it says that this is methoxymethane. is this a shorter way of writing methyloxymethane or do i have the wrong end of the stick?
Thanks so much for your help these last 2 weeks. (i will no doubt be in need of it again soon)
What are simple molecules and ions (AQA spec: bond angles)?
There are some (see specification) that you must know (eg water, ammonia etc) and then there are some that you need to be able to work out. For AQA they will always be a central atom with a number of satellite atoms. You will have done some examples in class.
Which atoms can hold more than 8 electrons in their outer shell?
"Expanding the octet" is a trick for larger atoms. There is not a fundamental reason why it doesn't happen for smaller ones but it starts in period three. The reason is that the disadvantage of putting electrons in a higher orbital is paid back by being able to form more bonds. If the promotion energy is only small (because the energy levels are quite close together) then it can be advantageous.
Which atoms can hold less than 8 electrons in their outer shell?
Typically, Be, B, Al
On a resource sheet i was given, it shows an arrow going from P to Cl in a PCl5 molecule. Does that mean that its a co-ordinate bond? If so, Why?
An arrow normally indicates a coordinate bond but PCl5 is best explained by expansion of octet. Five electrons around P, add the five coming in form the chlorine atoms as they make bonds gives ten electrons (and so five pairs) around the P. Hence trigonal bipyramid shape.
Do we need to know anything about stereoisomerism?
No
Another resource sheet i was given shows
H....H
l....l
H-C-O-C-H
l....l
H....H
it says that this is methoxymethane. is this a shorter way of writing methyloxymethane or do i have the wrong end of the stick?
There may be a detailed rule in the IUPAC scheme for deciding if it is really methoxymethane or methyloxymethane. Personally I would go for methoxymethane. It doesn't really matter.
Thanks so much for your help these last 2 weeks. (i will no doubt be in need of it again soon)
Glad it has helped. If it works, tell your friends. One good reason for telling friends is that the advertisers on the site help to keep it as a free resource for people like you. The more internet hits they record, the happier they are and the more they areb likely to continue their valuable support.
Do we need to know about:
co-oridnatly bonded ions,CO
triple bonded substances e.g C2H2
Shapes of anions e.g. CO3, S04, NO3
Also, would you be able to tell me why the bond angle for diamond is 109.5 and why the van der Waals forces in Graphtie have no effect whatsoever on the bond angles.
Shapes of anions e.g. CO3, S04, NO3 Not these with double bonds in but you do need to be able to deal with single bonded ones like NH2- (2 is sub, - is superscript)
Also, would you be able to tell me why the bond angle for diamond is 109.5 and why the van der Waals forces in Graphtie have no effect whatsoever on the bond angles.
VdW forces don't affect bond angles anyway. The three pairs of electrons around eaxch of the graphite carbons will form a trigonal planar arrangement with the extra electron from each carbon atom contributing to a delacalised sea of electrons (a bit like a poor version of a metal).
im missed a couple of lessons at school and im slightly confused by the following:
Does electronegativity only affect covalent bonds?
If not, What effect (if any) does electronegativity have on an ionic bonding pair).
What is shielding? How does it change across periods?
Does the electonegativity of an atom have anything to do with its nuclear charge?
What is a dipole and does it have anything to do with electronegativity?
Thanks
Electronegativity can be defined as the power of an atom to draw electron density to itself in a covalent bond....so yes, electronegativity is a concept only connected with covalency.
What is shielding? How does it change across periods?
The trend in atomic radius and ionisation energy (apart from the little jumps)depends on the idea that as you move from one atom to another from left to right, there is an increase in the number of protons in the nucleus which is matched by an increase in the number of electrons in the outside shell.
The increase in the number of protons causes the attraction between the nucleus and the electrons to increase. If the repulsion between the electrons in the outside shell were also increased by as much there would be no change in atomic radius etc but the trouble is that the repulsion between the electrons is only slight;y increased and nowhere near enough to match the increased attraction to the nucleus.
If we ignore distance, electrostatic attraction size is basically the multiplication of the two charges attracting each other.
So a three-plus positive charge attracting a one-minus electron is three times as strong a force as two one-minus electrons repelling each other.
Consequently, as the nucleus charge increases, the electrons are attracted more strongly to it.
This leads to a decrease in atomic radius and an increase in ionisation energy.
Does the electonegativity of an atom have anything to do with its nuclear charge?
Yes. If you think about all the ideas about ionisation energy and atomic radius but apply them to an electron in the covalent bond betwen one atom and another, you will see that an atom at the right hand side of the periodic table has a stronger pull on the electron than one from the left hand side. If you build in the idea of size as well, small atoms are more attractive to the electrons in a covalent bond (higher electronegativity) than larger ones. Consequently, fluorine is the most electronegative.
What is a dipole and does it have anything to do with electronegativity?
A dipole is a separation of charge over a distance. It is a bit like a couple in mechanics. If a neutral molecule (such as HCl) has an electron-rich chlorine end and an electron-deficient hydrogen end, it will have a dipole. A bigger charge that is separated (ie the bigger the electronegativity difference) and a bigger distance over which it is separated (bigger molecule)means that the dipole is larger. Big dipole-dipole attraction is one thing that leads to high melting and boiling points.
Hope this helps.
thanks for that. Continuing with dipoles:
Could you tell me what a dipole moment is?
Are all dipole moments of the same value (for example, is it a case of a molecule having a dipole or not)?
My textbook says that some molecules have dipoles that cancel. Does that depend on how the atoms are arranged in the molecule? Do only certain atoms have the ability to create a dipole (for example, can a hydrogen atom bonded to a carbon atom create a dipole)?
Do dipoles affect bond angles, and if so, how and by how much?
Could you tell me what a dipole moment is?
Are all dipole moments of the same value (for example, is it a case of a molecule having a dipole or not)?
No. The dipole moment is the size of the charge separation multiplied by the distance of the separation. It is very much like a lever in physics where a small force a long way from the fulcrum can have a bigger effect than a large force very near to it. A small charge separation over a large distance can be just as big or even bigger a dipole moment than a large charge separation over a small distance.
My textbook says that some molecules have dipoles that cancel. Does that depend on how the atoms are arranged in the molecule?
If there is symmetry in the molecule, the dipole moments can cancel out. It is a bit like having four people pulling on ropes attached to a load in the middle. If they all pull equally, there is no overall force on the load. Even a very strong pull will have no effect if all the forces balance out.
Do only certain atoms have the ability to create a dipole (for example, can a hydrogen atom bonded to a carbon atom create a dipole)?
Any non-identical pair of atoms will have a dipole moment of some size, even if it is very small. Whether the dipole mooment has an overall effect depends on the symmetry of the molecule. Even if the molecule is asymmetrical, the resultant dipole moment might be too small to be significant.
Do dipoles affect bond angles, and if so, how and by how much?
thanks alot. I think i've got my head around dipoles thanks to you.
The next topic is van der Waals forces. My text book says that v.d.w. forces explain why the boiling point of noble gases increase as the atomic numbers of the noble gases increase, and that they also explain the boiling points of hydrocarbons as the carbon chain length increases, however i dont see why or how.
Also (this is a bit off topic), if the electron clouds in atoms found a position in which they "balanced", what (if anything) would happen?
also, to what extent do we need to know about (AQA specification) "understanding the importance of hydrogen bonding in determining the boiling points of compounds and structures of some solids (e.g. ice)?
thanks
Hydrogen bonding:
Look at the boiling points of group six hydrides (water, H2S, H2Se, H2Te). There is a gradual increase down the group except for water which is ridiculously high.
The general increase is because of increased VdW between molecules because of their increased surface area. This means that the instantaneous dipoles set up in the molecules are larger because of the electrons being less tightly bound and able to spread over a larger distance.
Water is anomalous because although it has relatively small VdW, it has H-bonding between molecules. This is an example of the special effect of H-bonding. Another would be that ice floats on water. The H-bonding holds the water molecules in a much more open structure than you would expect for molecules without the possibility of H-bonding. Since the same mass is spread out over a larger volume, the density (D=M/V) is less and so the ice floats.
The boiling point of noble gases increase as the atomic numbers of the noble gases increase
Although the instataneous dipole in a single atom (as we have with the noble gases) is only very small (the electron distortion can only be over a very small distance since we don't have the length of the molecule to play with), the VdW is small.
the boiling points of hydrocarbons as the carbon chain length increases.
Again, the longer the chain, the more the distortion can take place. If only the same amount of charge can be displaced instantly, it will have more effect if it can be displaced over a longer distance. Think of it a bit like levers, a partiucular force can have a much greater effect if it is applied over a long distanc ewith a long seesaw.
Also (this is a bit off topic), if the electron clouds in atoms found a position in which they "balanced", what (if anything) would happen?
I am sorry, I don't really know what you mean by this question. Try to rephrase it or perhaps say what you think will happen and then I can comment.
I've got my chemistry mock tomorrow and there were just a few last things I'd like to check with you:
What are simple molecules and ions (AQA spec: bond angles)?
Which atoms can hold more than 8 electrons in their outer shell?
Which atoms can hold less than 8 electrons in their outer shell?
On a resource sheet i was given, it shows an arrow going from P to Cl in a PCl5 molecule. Does that mean that its a co-ordinate bond? If so, Why?
Do we need to know anything about stereoisomerism?
Another resource sheet i was given shows
l....l
H-C-O-C-H
l....l
H....H
it says that this is methoxymethane. is this a shorter way of writing methyloxymethane or do i have the wrong end of the stick?
Thanks so much for your help these last 2 weeks. (i will no doubt be in need of it again soon)
What are simple molecules and ions (AQA spec: bond angles)?
There are some (see specification) that you must know (eg water, ammonia etc) and then there are some that you need to be able to work out. For AQA they will always be a central atom with a number of satellite atoms. You will have done some examples in class.
Which atoms can hold more than 8 electrons in their outer shell?
"Expanding the octet" is a trick for larger atoms. There is not a fundamental reason why it doesn't happen for smaller ones but it starts in period three. The reason is that the disadvantage of putting electrons in a higher orbital is paid back by being able to form more bonds. If the promotion energy is only small (because the energy levels are quite close together) then it can be advantageous.
Which atoms can hold less than 8 electrons in their outer shell?
Typically, Be, B, Al
On a resource sheet i was given, it shows an arrow going from P to Cl in a PCl5 molecule. Does that mean that its a co-ordinate bond? If so, Why?
An arrow normally indicates a coordinate bond but PCl5 is best explained by expansion of octet. Five electrons around P, add the five coming in form the chlorine atoms as they make bonds gives ten electrons (and so five pairs) around the P. Hence trigonal bipyramid shape.
Do we need to know anything about stereoisomerism?
No
Another resource sheet i was given shows
H....H
l....l
H-C-O-C-H
l....l
H....H
it says that this is methoxymethane. is this a shorter way of writing methyloxymethane or do i have the wrong end of the stick?
There may be a detailed rule in the IUPAC scheme for deciding if it is really methoxymethane or methyloxymethane. Personally I would go for methoxymethane. It doesn't really matter.
Thanks so much for your help these last 2 weeks. (i will no doubt be in need of it again soon)
Glad it has helped. If it works, tell your friends. One good reason for telling friends is that the advertisers on the site help to keep it as a free resource for people like you. The more internet hits they record, the happier they are and the more they areb likely to continue their valuable support.
Do we need to know about:
co-oridnatly bonded ions,CO
triple bonded substances e.g C2H2
Shapes of anions e.g. CO3, S04, NO3
Also, would you be able to tell me why the bond angle for diamond is 109.5 and why the van der Waals forces in Graphtie have no effect whatsoever on the bond angles.
Thanks
Do we need to know about:
co-oridnatly bonded ions,CO NO
triple bonded substances e.g C2H2 NO
Shapes of anions e.g. CO3, S04, NO3 Not these with double bonds in but you do need to be able to deal with single bonded ones like NH2- (2 is sub, - is superscript)
Also, would you be able to tell me why the bond angle for diamond is 109.5 and why the van der Waals forces in Graphtie have no effect whatsoever on the bond angles.
VdW forces don't affect bond angles anyway. The three pairs of electrons around eaxch of the graphite carbons will form a trigonal planar arrangement with the extra electron from each carbon atom contributing to a delacalised sea of electrons (a bit like a poor version of a metal).
Thanks