Probing an atomic nucleus

[Mr Skeptic]

How exactly do we know the structure of a nucleus? How certain are we that it is the right description?

[toastywombel]

I got this from http://www.windows.ucar.edu/tour/link=/kids_space/atom.html

 

"There is no simple answer to your questions. There is no "atomic microscope" which would allow one to look inside an atom and say, "Aha! There's 7 blue protons, 6 green neutrons, and 7 red electrons." The way the structure of the atom was devised was through a long series of experiments."

 

http://cnx.org/content/m12433/latest/

 

This is a link to some experiments that are and were used to discover the structure of the atom. It explains how we know that the positive and mass particles are in the nucleus and why the electrons are on the outside, it seems to have something to do with how atoms bond together to make molecules.

 

I am guessing we cannot be totally certain it is the correct structure and that is why it is still a theory. But, I would imagine our assumptions about how the atom is structured are derived from how the atom behaves under different conditions and with other elements. From what I know we can predict how elements will react to other elements pretty accurately, so I would imagine that the theorised structure of an atom is correct.

 

I did some more reading on the structure of the nucleus and it seems we know that tracking particles that are shot through a piece of gold foil. This can be used to determine that the nucleus of the atom is heavy, positively charged, and very small in comparison to the size of the atom.

 

This is from the above listed website.

 

"The result of the experiment is initially

counter-intuitive. Most of the α particles pass

through the gold foil undeflected, as if there had been nothing in

their path! A smaller number of the particles are deflected sharply

as they pass through the foil, and a very small fraction of the

α particles are reflected backwards off of the gold

foil. How can we simultaneously account for the lack of any

deflection for most of the α particles and for the

deflection through large angles of a very small number of

particles?

 

First, since the majority of the positively

charged α particles pass through the gold foil

undeflected, we can conclude that most of the volume of each gold

atom is empty space, containing nothing which might deflect an

α particle. Second, since a few of the positively

charged α particles are deflected very sharply, then

they must encounter a positively charged massive particle inside

the atom. We therefore conclude that all of the positive charge and

most of the mass of an atom is contained in a

nucleus. The nucleus must be very small, very massive,

and positively charged if it is to account for the sharp

deflections. A detailed calculation based assuming this model

reveals that the nucleus must be about 100,000 times smaller than

the size of the atom itself. The electrons, already known to be

contained in the atom, must be outside of the nucleus, since the

nucleus is positively charged. They must move in the remaining

space of the much larger volume of the atom. Moreover, in total,

the electrons comprise less than 0.05% of the total mass of an

atom."

 

Further observations that are used to determine the structure of atoms are X-Ray Emissions and Ionisation Energies

Edited December 10, 2009 by toastywombel

[Mr Skeptic]

Thanks. Unfortunately, most of that I already knew.

 

To clarify, I am asking about the structure of the nucleus only, not the whole atom. For example, how do we know that a proton is made of quarks, and how did we measure the masses, charges, and magnetic dipole fields of them? Were any of these just guessed?

[toastywombel]

I read a book called Quantum World by Kenneth Ford it helped explain a lot of this to me, you might want to check it out. It is a heavy read though.

 

When we collide certain sub-atomic particles such as an electron into a proton at extremely high velocities, the electron does not act as if it is bouncing off of a single object. The results from repeating this experiment vary, suggesting that the proton is made up of multiple objects.

 

Furthermore the fundamental particles that we have attempted to discover originate from the periodic table of particle physics (if thats what you want to call it). It was designed by GellMann and Neemann. Since predicting that these particles (such as quarks) would exist scientists have devised experiments to find out if they do.

 

Also we cannot individually observe quarks because the force that holds them together gets stronger and stronger as they are further apart. Despite that, however, the six flavours of quarks have all been observed though through testable methods.

 

This is from http://www.physlink.com/Education/AskExperts/ae134.cfm?CFID=23513914&CFTOKEN=5b6ea376c0d760ee-7B3A6DB4-15C5-EE01-B9552391B841B929

 

"When protons and neutrons are struck with particles that truly are fundamental (like electrons, neutrinos, photons, etc.) the protons and neutrons reveal their structure in the way the colliding particle rebounds. This is analogous to the way Rutherford discovered the nucleus within the atom by bombarding gold with radiation. The results of these experiments show that the proton, for example, is composed of three fundamental objects with just the right properties to be the postulated 'quarks' of Gell-Mann. Furthermore, the theory that describes the interactions of quarks with each other also predicts the properties of the composite objects they form. These predictions have been proven to be correct, allowing us to develop a 'periodic table' of the known baryons and mesons -- another spectacular success of the quark theory."

 

I would suspect we measure the poles and electric charges of quarks by measuring the results from particle collisions with protons or neutrons and seeing what changes when we change the charge of the particles we are colliding with the protons or neutrons. Other than that I do not know exactly how all the properties of the quarks are recorded.

 

Hope this helps.

[Cap'n Refsmmat]

Thanks. Unfortunately, most of that I already knew.

 

To clarify, I am asking about the structure of the nucleus only, not the whole atom. For example, how do we know that a proton is made of quarks, and how did we measure the masses, charges, and magnetic dipole fields of them? Were any of these just guessed?

 

I am guessing that particle accelerators have a large role in figuring things like this.

[toastywombel]

I am guessing that particle accelerators have a large role in figuring things like this.

 

Yes the do, look at above post!