Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: RGraham9 on March 30, 2016, 07:11:33 AM
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Hi guys , I need help in understanding why Isotopes have different physical properties. My textbook says that Isotopes have different physical properties as isotopes have different number of neutrons. The difference in the number of neutrons affect the "mass" and the physics properties of a substance is affected by this "mass".
This "mass" they are referring to , is it the Nucelon Number (Amount of Protons and Neutrons / Nucleons making up the Nucleus of one atom) ?
Thanks Guys ! Appreciate It !
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Yes exactly, electrons have virtually no mass, so all the mass comes from the nucleons. Isotopes behave the same chemically, but their masses are different.
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Okay so just to reaffirm my understanding , the difference in physical properties of isotopes is a result of the difference in the nucleon masses of one atom in each of the different isotopes correct ?
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Corrections regarding my previous statement :
Nucleon Number (Mass Number ) not Nucleon Masses
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Okay so just to reaffirm my understanding , the difference in physical properties of isotopes is a result of the difference in the nucleon masses of one atom in each of the different isotopes correct ?
I think I agree but the "one atom" bit has thrown me.
The difference in physical properties of isotopes is a result of the difference in the nucleon number between the atoms of the different isotopes.
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Sorry about throwing you off with the one atom thing. The reason I said 1 atom was because :
• From what I have read / definition of Nucleon Number/Mass Number : The nucleon number / nucleon mass is the total number of protons and neutrons of an atom of an element.
• My textbook says that the physical properties of a substance is affected by "mass" , hence I was wondering if this "mass" was actually a reference to the Mass Number / Nucleon Number of the substance. (E.g The physical properties of a substance is affected by its Nucleon Number / Mass Number.)
• Thus , with reference to the definition of the Nucleon Number / Mass Number (in bullet point 1) and bullet point 2 , won't that mean that the physical properties of a substance is affected by one atom due to its mass number ?
This is very confusing to me but I appreciate the help provided by you so far.
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Let's imagine two hypothetical clumps of carbon. Clump 1 contains 10 atoms, each of which has 6 protons and 6 neutrons. Clump 2 also contains 10 atoms. 9 of those atoms are the same as those in clump 1, while the 10th has 6 protons and 7 neutrons. In this case, clump 1 and clump 2 are not isotopes, rather, the 10th atom in clump 2 (the one with 7 neutrons) is an isotope of all the other atoms. However, you are right in saying that the overall mass of clump 2 would be greater than the overall mass of clump 1. But again, that doesn't make the clumps isotopes, but is rather explained by the presence of a different isotope in clump 2.
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Chemical reaction is all about forming new bonds and breaking old bonds , How easily(fast) a bond will form and how easily(fast) you can break old bonds . How strong a bond is forming and how much old bond was weak . These are the factors which study during the chemical reaction(not in case of nuclear reaction) .
An isotope have same no. of electron (so there is no big deal ) even it has same no. of protons ( So those "same no." of electrons are bounded exactly in the same way as they were bounded in the other isotope ) , what actually changed is no. of neutron that will cause little bit change in gravitation force and reduced mass . You should know the fact Gravitation force is much weaker force (I'm not talking about black holes ;D ) than that of electrostatic force (if we're talking about atoms). So there will be no major change in the chemical properties .
Yup in case of boiling point , density - They will get change as they have to do something with mass .
P.S. Bonds are lie , and we should use the word Overlapping of orbitals .
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Sorry i didn't read your whole question , i just read "I need help in understanding why Isotopes have different physical properties" . You can ignore my above post . it was in other context .
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@AdiDex : It's okay
@thetada : So basically ,
• Isotopes are atoms of the same element with different numbers of neutrons.(Aren't elements)
• The presence of isotopes in different amounts , in the same elements , results in different physical properties between the identical elements due to the difference in the relative atomic mass of the two identical elements ?
(Sorry if I get this wrong again. Trying my best.)
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• Isotopes are atoms of the same element with different numbers of neutrons.(Aren't elements)
• The presence of isotopes in different amounts , in the same elements , results in different physical properties between the identical elements due to the difference in the relative atomic mass of the two identical elements ?
bullet 1: isotopes are the same element
bullet 2: Yes, the difference in physical properties is caused by the different numbers of neutrons (which in turn affects relative mass). For example, carbon-14 is less stable than its isotope carbon-12, and hence radioactive. This difference forms the basis of carbon dating.
If you have just one atom of each isotope, you can say that the atoms have different physical properties. In this example, carbon-14 is radioactive, whereas carbon-12 is not.
As you point out, two pieces of carbon could have different proportions of each of the carbon isotopes. So in that sense, we can say that the pieces of carbon have different physical properties, but most of those differences operate at the atomic level. For example, if one piece did contain carbon-14, but the other did not, we could say that only the carbon-14-containing piece was radioactive, but that radioactivity will operate at the atomic level, that is, individual atoms will undergo radioactive decay.
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• The presence of isotopes in different amounts , in the same elements , results in different physical properties between the identical elements due to the difference in the relative atomic mass of the two identical elements ?
I would say 'relative isotopic mass' or 'atomic mass' rather than 'relative atomic mass'. Definition of relative atomic mass:(From http://www.ausetute.com.au/atomicmass.html)
The relative atomic mass, or atomic weight, of an element is the weighted average of the masses of the isotopes in the naturally occurring element relative to the mass of an atom of the carbon-12 isotope which is taken to be exactly 12.
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Unfortunately , I am still a bit confused by Isotopes. (Argh , sorry for wasting your time , I feel really bad for doing this.)
• So Isotopes are atoms of the same elements with different neutron numbers correct ?
• Isotopes have different mass numbers.
• In my textbook it mentions that "Isotopes have different physical properties." This is basically saying that identical elements ,
made up of different atoms with different proton numbers / isotopes will result in both the identical elements being different in terms of their physical properties and that this is the result due to the difference in the mass numbers of the isotopes of the different elements correct ?
• If the above is correct , how does the difference in mass number / nucleon number of isotopes actually affect the physical properties of that element ?
• So let's say that there is a element that is entirely made out 99 atoms and that all the 99 atoms have 6 protons and 6 neutrons. If somehow (even if this is impossible scientifically) I am able to change one of the atoms to 6 protons and 6 neutrons instead , will the physical properties of the element change drastically ? (E.g boiling point , melting point or density.)
• Lets say that we have an element entirely made out of 10 atoms. like the carbon example , and all atoms have 6 protons and 6 neutrons . On the other hand we have another identical element and that identical element has 10 atoms. However 9 of the atoms have 6 protons and neutrons while the last atom has 6 protons and 7 neutrons. Hence the mass number of the 6protons and 6neutrons atoms will be 12 while the isotope will be 13. How does this isotope (mass number 13) cause a change in the physical properties of that elements ?
Really really appreciate the help. Thank you so much and sorry for stealing your time guys.
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Compare properties of H2O and D2O in wikipedia.
https://www.wikiwand.com/en/Heavy_water
Look also at the toxicity of D2O in this article. This means that stable isotopes may show different chemical reaction in living organisms.
The textbook informations are only a selection of properties.
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let's say that there is a element that is entirely made out 99 atoms
You mean there are only 99 atoms of the element? Your statement "entirely made of 99 atoms" doesn't make much sense.
atoms have 6 protons and 6 neutrons. If somehow (even if this is impossible scientifically) I am able to change one of the atoms to 6 protons and 6 neutrons instead
Changing 6/6 to 6/6 doesn't change anything.
element entirely made out of 10 atoms
Again, not clear what you mean.
we have another identical element and that identical element has 10 atoms
What do you mean by "identical element"?
I wonder if there is not some underlying misunderstanding that makes you confused.
Or perhaps sometimes you mean "another sample of an element, sample made of different isotopes"?
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Corrections:
• if somehow (even if this is impossible scientifically) I am able to change one of the atoms to 6 protons and 7 neutrons instead will the physical properties of the element change drastically.
@Borek : Yup , what I meant be identical element was another sample of the same element , made out of different isotopes. And the 99 atoms this is a reference to a previous example.
Forgive me for the mistakes.
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C-12 and C-13 show different magnetic properties (used in nuclear magnetic resonance)
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I will try my best to rephrase the questions again.
Question 1
• This is the definition of an Isotope correct ? "Isotopes are atoms of the same elements with different numbers of neutrons."
Question 2
• Due to the difference in neutrons between isotopes , different isotopes will have different mass numbers correct ?
Question 3
• Basically there is this statement / example in my guide book that goes " Isotopes have the same chemical properties but different physical properties."My question is , what this statement is trying to say or refer to is that if we have two identical elements / samples of the same element made out of different isotopes / atoms of same element with different neutron numbers , the physical properties of the two elements will be different from one another due to the difference in the nucleon number / mass number of the atoms making up the elements correct ?
Question 4
• How exactly does the mass number of an isotope in an element affect the physical properties of that element ?
Question 5
• Lets say if I have an element that consists of 50 atoms. Each atom has 6 protons and neutrons. If I were to let's say change one of the 50 atoms to the point where that one atom has 6 protons and 7 neutrons , will there be a change in the physical properties of that element ? Will the change be drastic ?
I sincerely apologise for the previous poorly written post. I was really tired and got kind of lazy and skipped some details.
Appreciate the help provided to me so far guys. Thank you. Have a great day.
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I will try my best to rephrase the questions again.
Question 1
• This is the definition of an Isotope correct ? "Isotopes are atoms of the same elements with different numbers of neutrons." Yes.
Question 2
• Due to the difference in neutrons between isotopes , different isotopes will have different mass numbers correct ? Yes.
Question 3
• Basically there is this statement / example in my guide book that goes " Isotopes have the same chemical properties but different physical properties."My question is , what this statement is trying to say or refer to is that if we have two identical elements / samples of the same element made out of different isotopes / atoms of same element with different neutron numbers , the physical properties of the two elements will be different from one another due to the difference in the nucleon number / mass number of the atoms making up the elements correct ? You're using the word "elements" incorrectly and are confusing yourself and others. I would refresh yourself on its meaning. If you have a sample made entirely of C-12 it will have different physical properties from a sample made entirely of C-13.
Question 4
• How exactly does the mass number of an isotope in an element affect the physical properties of that element ? If you start adding or removing neutrons you're changing the atom. There are many ways this affects the properties of a sample made up of those atoms, e.g. https://www.wikiwand.com/en/Deuterium#/Physical_properties
Question 5
• Lets say if I have an element that consists of 50 atoms. Each atom has 6 protons and neutrons. If I were to let's say change one of the 50 atoms to the point where that one atom has 6 protons and 7 neutrons , will there be a change in the physical properties of that element ? Will the change be drastic ? Yes. No, probably not.
I sincerely apologise for the previous poorly written post. I was really tired and got kind of lazy and skipped some details.
Appreciate the help provided to me so far guys. Thank you. Have a great day.
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Okay I got it. Thanks guys. Appreciate it. Have a great day ahead.
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Chemistry relates primarily to the physics of the interactions between positive and negative charges - predominantly electrons and protons. Therefore changing the nuclear mass with the addition/subtraction of a neutral particle (neutron) does not usually change the chemical properties of an atom or molecules containing the atom. However neutrons have other physical properties of significance, one of which (magnetism) has already been mentioned here*. So, it can be possible to distinguish magnetically between different isotopes. Heavier isotopes are also, well, heavier, which can impact other physical properties of atoms and molecules - such as the speed at which they vibrate or rotate. These differences have a lot of implications and uses in molecular spectroscopy and analytical chemistry.
You should be aware that there are exceptions to the statement that isotopic substitution does not affect the chemical properties of atoms. A heavier isotope, for example, may be radioactive, and radioactive decay can impact the chemical identify of an atom. Likewise, substitution of isotopes can impact the rates of chemical processes through something called the kinetic isotope effect. To understand why is probably a bit above your level, but I mention it to show that statements like "isotopes have the exact same chemical properties" is a generalization that is in most cases true, but not always.
*Magnetic differences can also impact chemical reactivity in some cases, e.g., reactions and photoreactions involving radicals.
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@Corribus : I see , appreciate the info , will definitely keep aware of the generalizations. Thank you.
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You should be aware that there are exceptions to the statement that isotopic substitution does not affect the chemical properties of atoms.
Corribus, I should have known that was an oversimplification!
Can you explain why heavy water has stronger OH bonds than regular water (as mentioned in the above-refed wiki page)? It seems counter intuitive to me. Assuming that the atoms vibrate relative to each other, surely the vibrations will be higher energy for the heavier isotope? If that were true, could the bonding orbital be closer in energy to the corresponding atomic orbitals? I feel like I'm talking rubbish....
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Comment to question 5.
In carbon we have approximately 1 atom of C-13 in 100 atoms of C-12. And we can see just this 1 atom C-13 in mass spectrometer (but also 100 atoms of C-12) or in nuclear magnetic resonance (only 1 atom of C-13). And we will notice practically no change, eg. in melting or boiling points).
So all answers given by Mikasaur (yes, no, probably not) are correct.
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In general, bonds between involving heavier isotopes are slightly stronger than bonds involving lighter isotopes. Consider D2 and H2 - which is a stronger bond? Because D is heavier than H, D2 vibrates more slowly than H2. In the quantum mechanical harmonic oscillator approximation, molecules vibrate even at zero temperature, a feature called the "zero point energy", which is given by 0.5 ħω. Because D2 has a lower vibrational frequency than H2, it also has a lower energy. Another way of stating this is that it is a more stable bond, because it takes more energy to dissociate. Experimental bond dissociation energies of D2, HD, and H2 at 298 K are 443.3, 439.32, and 436.0 kJ/mol (Source: http://www.nist.gov/data/nsrds/NSRDS-NBS31.pdf). Conclusion, the D2 bond is stronger than the H2 bond. This logic extends to other bonds involving heavy isotopes, which all have lower ZPE than their lighter analogs - and, by extension, intermolecular interactions. This is evident when comparing physical properties of deuterated versus nondeuterated substances. E.g., at 100 K, the enthalpy of vaporization of tetradeuterated methane CD4 exceeds that of hydrogenated methane CH4 by 65.6 J mol-1 (Source: J Phys Chem 1989, 93, 3355) - meaning it is harder to boil, and hence intermolecular forces are stronger. The effect here is pretty small, because the intermolecular forces here are weak to begin with. The effect is more pronounced with stronger intermolecular forces like H-bonds (D-bonds): for H2O and D2O, the boiling points are at 101.325 kPa are 99.974, 101.40 °C, respectively (Source: CRC 96th Ed, p 6-9).
The stronger bonds formed by heavier isotopes are also responsible for the aforementioned Kinetic Isotope Effect - bonds to deuterium are harder to break than those to hydrogen, meaning reactions involving deuterium tend to be slower.
These effects are true not only of hydrogen/deuterium but other isotope substitutions as well. However the effects are greatest in the H/D pair because the relative increase in mass between H/D is far larger than between other isotopes. E.g., the relative mass difference between H and D is huge compared to that between 12C and 13C.
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Awesome, thanks!
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In addition to the delocalization of the protium versus deuterium:
- The electron's reduced mass differs between protium and deuterium. This changes the electron's energy levels and the strength of intramolecular bonds too.
- Different hydrogen mass changes the molecule's possible rotation states for the liquid and gas, which must influence the melting point. Though, I haven't seen data about that up to now. At 20K with liquid hydrogen it changes everything. At 273K for water it must be incidental.
- If some reaction need a proton to tunnel, the difference with a deuteron must be huge.