what exactly is atomic spin?

Answer 1

Before we go to Atomic spin, some background knowledge is necessary. All sub-atomic particles have a property called Spin. The property called Spin is considered most quantum mechanical of all properties associated with particles. Because it has no equivalent in classical theory.
It deals with Angular Momentum of sub-atomic (and atomic) particles and hence, roughly comparable to classical angular momentum of a body. But this is not really correct. Spin of a sub-atomic particle cannot be treated the same way we treat a spinning top. Let me give u a few facts as to why it is different.
First of all, unlike classical spin, spin of sub-atomic particles is quantised. That means they can only take certain discrete values. But the values are always the same for all same type of particles (like 1/2 for electrons). In the case of matter particles (sub-atomic), they can only be integral multiples of 1/2. These follow certain rules described by Fermi-Dirac statistics and hence are called Fermions (in honour of Fermi). And for mass-less force carrying particles (like photons, gravitons etc) it can be integral multiples of 1 and follow Bose-Einstein statistics and are called Bosons (in honour of Bose).
Now, the amazing thing is this. If u were to consider an electron as a spinning top, then u would end up with a situation in which even after rotating it a full 360 degrees, u still dont end up where u started from. U have to rotate it twice ! That is 720 degrees. Can u imagine that? None of us can ! Yet its the only way experimental data (from Stern-Gerlach experiment) can be modeled and an appropriate theory constructed to explain the Atomic Model employing scientific method. In fact scientists were forced to include a bizarre concept such as spin because they needed to explain certain features in electromagnetic spectrum give out be Atoms and that could not be done by previous theories. And the theory has been very very successful. Today ur able to use Internet because these theories work and we were able to employ them !
Now there are other bizarre issues. Like electrons and other matter particles exhibit wave nature as well as behave like particles. If regarded purely as particles, then we have to accept that they can be at two places at once! I don't want to get into all these issues. But what i am saying is that we cant simply regard electrons and other sub-atomic particles purely as particles. Hence the spin of electrons and protons cant be regarded in a classical manner. Strictly speaking, we cant picture quantum spin. But its a mathematical model and a scientific concept we have to come to terms with.
Now, for the Atomic Spin. In an Atom, we have Protons and Neutrons orbiting inside a nucleus in different shells(yes orbiting). Each Proton and Neutron has an orbital angular momentum ( which is also quantised) in addition to spin. We have to add all the orbital and spin angular momentum of all the protons and neutrons in the nucleus to obtain nuclear spin. Then, we take the electrons orbiting the nucleus and add all the orbital and spin angular momentum values of all the electrons ( there are different ways of doing this, but lets not get into that). The net nuclear spin plus the net spin due to all the electrons give us the Atomic Spin, which may even be Zero. How? Because particles having the same magnitude of spin but in opposite directions (like spin 1/2 and - 1/2) cancel out each other.
Now, for example, if the net nuclear spin is an integer. And we have odd number of electrons. Then the net Atomic Spin will be due to the one odd electron. So its value will be 1/2.
Any particle including Atoms which have Spin value as integral multiples of 1/2 follow Fermi-Dirac theory and that explains ordinary matter. But amazingly, some atoms like helium, with spin 1, follow Bose-Einstein rules, which is shocking. Because it means two Atoms can occupy the same place at the same time! This explains super fluidity. Atomic spin theory was also used to develop MRI technology. So u see, as abstract and bizarre as it may be, it has practical uses and it works fine !
We can even add all the Atomic Spins to get molecular spin. Can we then add all molecular spins and then get, say the spin of a ball? Obviously a ball does not behave according to Quantum Physics rules.......yet its made up of particles obeying those rules ! So isn't there a contradiction? These questions have not yet been resolved. All that can be said as of now is that Quantum Physics rules can only be employed in situations where the difference in different energy levels of the system is very low. They just dont work at the macro scopic level which we are familiar with in our everyday existance.

Answer 2

Check out the Wiki explanation:

"There are a set of quantum numbers associated with the energy states of the atom. The four quantum numbers n, l, m, and s specify the complete and unique quantum state of a single electron in an atom called its wavefunction or orbital. The wavefunction of the Schrödinger wave equation reduces to the three equations that when solved lead to the first three quantum numbers. However, line emission spectra of some atoms when measured in an external magnetic field turned out to be more complicated than predicted by the first three quantum numbers. There needed to be a fourth quantum number that could properly predict spectra that matched the complexity found in nature so that this new quantum number had to behave as if it were also derived from the algebra of angular momentum vectors. A solution to this problem was suggested in early 1925 by George Uhlenbeck and Samuel Goudsmit, students of Paul Ehrenfest (who rejected the idea), and independently by Ralph Kronig, one of Landé's assistants, by introducing the idea of the self-rotation of the electron which would naturally be an angular momentum vector." [See source.]

Key in the above discussion is the sentence "However, line emission spectra of some atoms when measured in an external magnetic field turned out to be more complicated than predicted by the first three quantum numbers." This indicates that not all atoms have a need for the fourth quantum number (spin). So, contrary to what you may have heard elsewhere, not all atoms have spin.

Answer 3

Atomic nuclei have a "spin" associated w/ them, which is analogous to angular momentum for a macroscopic object. It is as if the nucleus is precessing about an axis.

All of this derives from the relativistic Schrodinger equation which Dirac derived.

In short, the spin means that in the presence of a magnetic field (or an electric field) the alignment of the nucleus with that field affects the energy of the nucleus. Of course it's quantum mechanics, so you can only have quantized states and the number of states you can have is 2s+1, where s is your spin.

This is how MRI/NMR works. In the presence of a strong magnetic field, the energy levels of the different atom spin states are far enough apart that they absorb RF energy. This is monitored and gives you the signal.

Answer 4

Electron Spin Definition

Answer 5

Electrons orbiting the atom act effectively create a tiny electric powered modern-day. in accordance to Ampère's regulation, an electric powered modern-day produces a magnetic field. The electrons fairly have a quantum components reported as spin which could be in contact in producing a magnetic field. From Wikipedia: The foundation of the magnetic moments that create the magnetisation could be the two microscopic electric powered currents equivalent to the action of electrons in atoms, or the spin of the electrons.

Answer 6

It is a lame attempt, by physicist who don't understand the nucleus anymore, to try to reconcile the energy/momentum anomolies found when crashing particles together. Listen to others try to explain it - its comical.

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