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What molecular shapes are always polar?
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Martin Turner
What molecular shapes are always polar?
Polarity and Symmetry go in hand in hand. Symmetry is the apt tool to decide whether a molecular shape is polar or not. In general presence of spherical symmetry in a molecule renders it non-polar and any sort of deviation from the perfect geometry makes a molecule polar. Furthermore, this can be elaborated with the sense that a molecule possessing a point of symmetry or a plane of symmetry in its molecular shape are non polar. The spherical symmetry does not mean mere presence of a perfect shape, the connectivity also matters. For example in case of CH4 the symmetry is spherical hence is non polar but if one of the connectivity is replaced by Cl the molecule becomes polar.
Any shape with different connectivity of atoms or a shape with absence of POS or COS is polar.
Polarity and Symmetry go in hand in hand. Symmetry is the apt tool to decide whether a molecular shape is polar or not. In general presence of spherical symmetry in a molecule renders it non-polar and any sort of deviation from the perfect geometry makes a molecule polar. Furthermore, this can be elaborated with the sense that a molecule possessing a point of symmetry or a plane of symmetry in its molecular shape are non polar. The spherical symmetry does not mean mere presence of a perfect shape, the connectivity also matters. For example in case of CH4 the symmetry is spherical hence is non polar but if one of the connectivity is replaced by Cl the molecule becomes polar.
Any shape with different connectivity of atoms or a shape with absence of POS or COS is polar.
Polar bonds; created when two dissimilar atoms are bonded together one atom has a higher electronegativity than the other so it pulls the bonding pair of electrons closer to itself than the other atom it’s bonded to.
Assymetry. If the polar bonds are all aligned in a way that cancels out their dipole moments then despite the polarity of the bonds the molecule will have no net dipole.
Both of these conditions need to be there. An asymmetric molecule that has non-polar bonds will not be polar (or at least it will be only slightly polar.
The geometric shapes of the molecules that have the correct symmetry to cancel out any bond dipoles are the 5 basic geometries from VSEPR theory.
Polar bonds; created when two dissimilar atoms are bonded together one atom has a higher electronegativity than the other so it pulls the bonding pair of electrons closer to itself than the other atom it’s bonded to.
Assymetry. If the polar bonds are all aligned in a way that cancels out their dipole moments then despite the polarity of the bonds the molecule will have no net dipole.
Both of these conditions need to be there. An asymmetric molecule that has non-polar bonds will not be polar (or at least it will be only slightly polar.
The geometric shapes of the molecules that have the correct symmetry to cancel out any bond dipoles are the 5 basic geometries from VSEPR theory.
No one should expect the simple textbook answers from me. The real world is a lot more complicated than your textbook tells you, unless maybe your textbook is Born & Wolf.
Yes, light is always polarized. Each individual photon is considered to be some combination of two polarization states which can be linear or circular states.
What do people mean when they say that light is unpolarized?
To be called unpolarized, light must contain (close enough to) equal amounts of s and p (or any two orthogonal axes, as long as both are mutually orthogonal to the propagation direction) polarization components and the phase relationship between the two axes must be random. “Close enough” depends on the sensitivity of your sensor.
Typically white light is given as an example of unpolarized light, but this is usually only approximately correct. The blackbody radiation from an object typically is not completely randomly polarized because the light does not just come from a surface. Typically the light may come from a few wavelengths deep within the object and then the light refracts at the surface of the object. Refraction (unless it occurs at the surface normal) has polarization variation in transmission and reflection according to the Fresnel reflection and transmission coefficients. But the polarization may be subtle. It probably is not enough to notice by putting on Rayban sunglasses. It may take high quality linear polarizers and sensitive detectors to resolve the polarization of a blackbody source.
Some lasers will emit unpolarized light if there are no non-normal optical components in the resonator so that all of the components are polarization insensitive. I built a couple of lasers like that back in 1974–1976.
If you look at most light sources with polarization resolving equipment, you can see polarization variations across the light source where your view of part of the light source is not normal to the source’s surface. But if you do not image the source and just take the emitted light at some distance away and analyze the polarization, the average across the entire source will appear to be what we call “unpolarized.” But, there is polarized light coming from that source: it just depends on how you average it, or not.
Starlight, before it encounters the atmosphere, might be considered unpolarized. Since the star is unresolved, all the the polarization from different parts of the star is averaged. and therefore what we call unpolarized. But as we are now starting to resolve stars, even this will end up being untrue.
I want to correct another myth. People say that (1) two crossed polarized beams do not interfere or that (2) two different wavelengths do not interfere or that (3) incoherent light sources do not interfere. In fact, they do interfere in all of these cases. It is just that you won’t see this interference by eye. You need a polarization sensitive detector in case one, an extremely fast detector in case two and an equal path interferometer in case 3.
In fact, I may use terms like incoherent or unpolarized very loosely in casual conversation, but when I make a formal presentation I have to be very specific. All light is partially polarized and partially coherent. There is no such thing as pure monochromatic light or purely polarized light. I have been building and using lasers for almost 50 years, so don’t go quoting the freshman physics nonsense about laser light.
No one should expect the simple textbook answers from me. The real world is a lot more complicated than your textbook tells you, unless maybe your textbook is Born & Wolf.
Yes, light is always polarized. Each individual photon is considered to be some combination of two polarization states which can be linear or circular states.
What do people mean when they say that light is unpolarized?
To be called unpolarized, light must contain (close enough to) equal amounts of s and p (or any two orthogonal axes, as long as both are mutually orthogonal to the propagation direction) polarization components and the phase relationship between the two axes must be random. “Close enough” depends on the sensitivity of your sensor.
Typically white light is given as an example of unpolarized light, but this is usually only approximately correct. The blackbody radiation from an object typically is not completely randomly polarized because the light does not just come from a surface. Typically the light may come from a few wavelengths deep within the object and then the light refracts at the surface of the object. Refraction (unless it occurs at the surface normal) has polarization variation in transmission and reflection according to the Fresnel reflection and transmission coefficients. But the polarization may be subtle. It probably is not enough to notice by putting on Rayban sunglasses. It may take high quality linear polarizers and sensitive detectors to resolve the polarization of a blackbody source.
Some lasers will emit unpolarized light if there are no non-normal optical components in the resonator so that all of the components are polarization insensitive. I built a couple of lasers like that back in 1974–1976.
If you look at most light sources with polarization resolving equipment, you can see polarization variations across the light source where your view of part of the light source is not normal to the source’s surface. But if you do not image the source and just take the emitted light at some distance away and analyze the polarization, the average across the entire source will appear to be what we call “unpolarized.” But, there is polarized light coming from that source: it just depends on how you average it, or not.
Starlight, before it encounters the atmosphere, might be considered unpolarized. Since the star is unresolved, all the the polarization from different parts of the star is averaged. and therefore what we call unpolarized. But as we are now starting to resolve stars, even this will end up being untrue.
I want to correct another myth. People say that (1) two crossed polarized beams do not interfere or that (2) two different wavelengths do not interfere or that (3) incoherent light sources do not interfere. In fact, they do interfere in all of these cases. It is just that you won’t see this interference by eye. You need a polarization sensitive detector in case one, an extremely fast detector in case two and an equal path interferometer in case 3.
In fact, I may use terms like incoherent or unpolarized very loosely in casual conversation, but when I make a formal presentation I have to be very specific. All light is partially polarized and partially coherent. There is no such thing as pure monochromatic light or purely polarized light. I have been building and using lasers for almost 50 years, so don’t go quoting the freshman physics nonsense about laser light.
Polarity and Symmetry go in hand in hand. Symmetry is the apt tool to decide whether a molecular shape is polar or not. In general presence of spherical symmetry in a molecule renders it non-polar and any sort of deviation from the perfect geometry makes a molecule polar. Furthermore, this can be elaborated with the sense that a molecule possessing a point of symmetry or a plane of symmetry in its molecular shape are non polar. The spherical symmetry does not mean mere presence of a perfect shape, the connectivity also matters. For example in case of CH4 the symmetry is spherical hence is non polar but if one of the connectivity is replaced by Cl the molecule becomes polar.
Any shape with different connectivity of atoms or a shape with absence of POS or COS is polar.
Polarity and Symmetry go in hand in hand. Symmetry is the apt tool to decide whether a molecular shape is polar or not. In general presence of spherical symmetry in a molecule renders it non-polar and any sort of deviation from the perfect geometry makes a molecule polar. Furthermore, this can be elaborated with the sense that a molecule possessing a point of symmetry or a plane of symmetry in its molecular shape are non polar. The spherical symmetry does not mean mere presence of a perfect shape, the connectivity also matters. For example in case of CH4 the symmetry is spherical hence is non polar but if one of the connectivity is replaced by Cl the molecule becomes polar.
Any shape with different connectivity of atoms or a shape with absence of POS or COS is polar.
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For a molecule to be polar it needs two things:
Both of these conditions need to be there. An asymmetric molecule that has non-polar bonds will not be polar (or at least it will be only slightly polar.
The geometric shapes of the molecules that have the correct symmetry to cancel out any bond dipoles are the 5 basic geometries from VSEPR theory.
For a molecule to be polar it needs two things:
Both of these conditions need to be there. An asymmetric molecule that has non-polar bonds will not be polar (or at least it will be only slightly polar.
The geometric shapes of the molecules that have the correct symmetry to cancel out any bond dipoles are the 5 basic geometries from VSEPR theory.
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No one should expect the simple textbook answers from me. The real world is a lot more complicated than your textbook tells you, unless maybe your textbook is Born & Wolf.
Yes, light is always polarized. Each individual photon is considered to be some combination of two polarization states which can be linear or circular states.
What do people mean when they say that light is unpolarized?
To be called unpolarized, light must contain (close enough to) equal amounts of s and p (or any two orthogonal axes, as long as both are mutually orthogonal to the propagation direction) polarization components and the phase relationship between the two axes must be random. “Close enough” depends on the sensitivity of your sensor.
Typically white light is given as an example of unpolarized light, but this is usually only approximately correct. The blackbody radiation from an object typically is not completely randomly polarized because the light does not just come from a surface. Typically the light may come from a few wavelengths deep within the object and then the light refracts at the surface of the object. Refraction (unless it occurs at the surface normal) has polarization variation in transmission and reflection according to the Fresnel reflection and transmission coefficients. But the polarization may be subtle. It probably is not enough to notice by putting on Rayban sunglasses. It may take high quality linear polarizers and sensitive detectors to resolve the polarization of a blackbody source.
Some lasers will emit unpolarized light if there are no non-normal optical components in the resonator so that all of the components are polarization insensitive. I built a couple of lasers like that back in 1974–1976.
If you look at most light sources with polarization resolving equipment, you can see polarization variations across the light source where your view of part of the light source is not normal to the source’s surface. But if you do not image the source and just take the emitted light at some distance away and analyze the polarization, the average across the entire source will appear to be what we call “unpolarized.” But, there is polarized light coming from that source: it just depends on how you average it, or not.
Starlight, before it encounters the atmosphere, might be considered unpolarized. Since the star is unresolved, all the the polarization from different parts of the star is averaged. and therefore what we call unpolarized. But as we are now starting to resolve stars, even this will end up being untrue.
I want to correct another myth. People say that (1) two crossed polarized beams do not interfere or that (2) two different wavelengths do not interfere or that (3) incoherent light sources do not interfere. In fact, they do interfere in all of these cases. It is just that you won’t see this interference by eye. You need a polarization sensitive detector in case one, an extremely fast detector in case two and an equal path interferometer in case 3.
In fact, I may use terms like incoherent or unpolarized very loosely in casual conversation, but when I make a formal presentation I have to be very specific. All light is partially polarized and partially coherent. There is no such thing as pure monochromatic light or purely polarized light. I have been building and using lasers for almost 50 years, so don’t go quoting the freshman physics nonsense about laser light.
See also: Bill Otto's answer to What’s the difference between normal light and laser? Are there infrared laser, ultraviolet laser, radio laser, X-ray laser, and gamma ray laser?
Bill Otto's answer to Can any of our best telescopes show a star (other than the sun) as more than a point of light?
Is there such thing as a white laser?
No one should expect the simple textbook answers from me. The real world is a lot more complicated than your textbook tells you, unless maybe your textbook is Born & Wolf.
Yes, light is always polarized. Each individual photon is considered to be some combination of two polarization states which can be linear or circular states.
What do people mean when they say that light is unpolarized?
To be called unpolarized, light must contain (close enough to) equal amounts of s and p (or any two orthogonal axes, as long as both are mutually orthogonal to the propagation direction) polarization components and the phase relationship between the two axes must be random. “Close enough” depends on the sensitivity of your sensor.
Typically white light is given as an example of unpolarized light, but this is usually only approximately correct. The blackbody radiation from an object typically is not completely randomly polarized because the light does not just come from a surface. Typically the light may come from a few wavelengths deep within the object and then the light refracts at the surface of the object. Refraction (unless it occurs at the surface normal) has polarization variation in transmission and reflection according to the Fresnel reflection and transmission coefficients. But the polarization may be subtle. It probably is not enough to notice by putting on Rayban sunglasses. It may take high quality linear polarizers and sensitive detectors to resolve the polarization of a blackbody source.
Some lasers will emit unpolarized light if there are no non-normal optical components in the resonator so that all of the components are polarization insensitive. I built a couple of lasers like that back in 1974–1976.
If you look at most light sources with polarization resolving equipment, you can see polarization variations across the light source where your view of part of the light source is not normal to the source’s surface. But if you do not image the source and just take the emitted light at some distance away and analyze the polarization, the average across the entire source will appear to be what we call “unpolarized.” But, there is polarized light coming from that source: it just depends on how you average it, or not.
Starlight, before it encounters the atmosphere, might be considered unpolarized. Since the star is unresolved, all the the polarization from different parts of the star is averaged. and therefore what we call unpolarized. But as we are now starting to resolve stars, even this will end up being untrue.
I want to correct another myth. People say that (1) two crossed polarized beams do not interfere or that (2) two different wavelengths do not interfere or that (3) incoherent light sources do not interfere. In fact, they do interfere in all of these cases. It is just that you won’t see this interference by eye. You need a polarization sensitive detector in case one, an extremely fast detector in case two and an equal path interferometer in case 3.
In fact, I may use terms like incoherent or unpolarized very loosely in casual conversation, but when I make a formal presentation I have to be very specific. All light is partially polarized and partially coherent. There is no such thing as pure monochromatic light or purely polarized light. I have been building and using lasers for almost 50 years, so don’t go quoting the freshman physics nonsense about laser light.
See also: Bill Otto's answer to What’s the difference between normal light and laser? Are there infrared laser, ultraviolet laser, radio laser, X-ray laser, and gamma ray laser?
Bill Otto's answer to Can any of our best telescopes show a star (other than the sun) as more than a point of light?
Is there such thing as a white laser?
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