Isotopes (1 Viewer)

[tazzyboys]

Are the stability of isotopes governed by repulsive and nuclear forces?
If so is this required knowledge for the HSC?

 

[davidbarnes]

Isn't the stability based upon neutron/proton (n/p) ratio. Elements less than atomic number 20 need a ratio od 1:1 , elements greater than atomic number 20 need a ratio of 1/1.5 I think, then from elements 83 onwards the ratio exceeds 1:1.5 and they are radioactive (transuranic). So if the ratio is greater than the element allows, then it will become stable. I.e if element 78 isotope has a ratio of 1:1.6 it would be unstable.

 

[ssglain]

I'm quite glad you raised this question - most chemistry students are satisfied with the explanation that "the stability based upon the neutron/proton (n/p) ratio". This is what all text books stop at and I was never content with that answer because "why is the stability based upon the neutron/proton ratio?" becomes the next question.

The stability of an atom refers to the stability of its nucleus, which is maintained by the attractive short range nuclear forces between nucleons (the particles within a nulceus - so neutrons and protons). The neutron/proton ratio within the nucleus affects the action of nuclear forces because Coulomb repulsion between protons (which are positively charged) increases as the atom becomes larger and the number of particles within the nucleus increasons. This unbalances the short range nuclear force which glues the nucleus together and makes the atom unstable.

 

[xiao1985]

Yes, and the force are referred to as strong nuclear force (IIRC).

One question which may follow from this point on is that: then why don't nuclei just have alot of neutrons, if electromagnetic repulsion is the only force governing the nuclei?

 

[tazzyboys]

Thanks for the quick replies.
But with regards to this info, is it required in the HSC? or will the simple p:n ratio be accepted if a question was raised about the stability of a nucleus.

 

[Steth0scope]

For chem it won't be required.

If you do the Quanta to Quarks option in Physics, you may need it.

 

[xiao1985]

Also remember that atomic number beyond 83 (Bi) are all radioactive.

With atomic number below 83, there are 2 elements (Tc, 43; and pm, 61) has no stable isotopes.