So electron’s have spins of either plus or minus one-half, but can they change spin from plus one-half to minus one-half?
If you have a hydrogen atom with one electron with a plus one-half spin, and you transfer another electron with plus one-half spin, they cannot both occupy the 1s orbital. So will one of them change to a minus one-half spin, or will it just occupy the 2s orbital? If it does occupy the 2s orbital, is that fraction of the hydrides much more reactive due to it’s radical nature?
Yes, they absolutely can and do change spin, provided that the angular momentum of something else in the system changes accordingly, to keep the total constant. That’s how we know that spin is a form of angular momentum.
Spin flipping of electrons in hydrogen atoms produces what’s known as the 21-centimeter line in radio astronomy. It’s an extremely useful emission line since it allows detection of neutral hydrogen, a very plentiful substance in the universe.
Since we currently believe electrons are point particles without physical size, “spin” is more of a quantum mechanical state than physical rotation about an axis. This video summarizes the history of how spin was discovered and how viewing it as physical rotation is misleading:
This is incorrect. The spin changes, full stop. Spin is a vector quantity, and a change in the magnitude or direction constitutes a change in spin. Quantum mechanically these two pieces are often dealt with separately (e.g., through spin quantum numbers like S[sup]2[/sup] and S[sub]z[/sub]), but that technical detail isn’t correctly handled by saying “spin never changes”.
To be specific, in QM angular momentum is a vector quantity, but there is an uncertainly relationship (like the position-momentum one) between the three components of the vector.
I.e. if the classical angular momentum vector is (l[sub]x[/sub], l[sub]y[/sub], l[sub]z[/sub]), then in QM, pinning down one of these components exactly, makes the other two components maximally uncertain. Just like how in the classical Hiesenberg position/momentum relationship, pinning down position makes momentum uncertain, and vice versa.
But unlike the position/momentum relationship, there is one more thing you can fix about the angular momentum vector – its length: sqrt(l[sub]x[/sub][sup]2[/sup] + l[sub]y[/sub][sup]2[/sup] + l[sub]z[/sub][sup]2[/sup]). So out of this three dimensional vector, there ends up being only these two numbers you can figure out.
For the spin angular momentum of an electron, the “length of the vector” number is fixed. It is just a property of being an electron that your spin vector has that certain length – if that changed, you would no longer have an electron. Which is what joema means by “spin can’t change”
The “single component you can fix” of the vector can be changed, but it is constrained by the fact that no single component of the vector can be bigger than the vector’s length. For electrons that fact (plus the fact that it is quantized) means that that component can be one of two states. So that component can change, but only in a very restricted way.
The famous example of this, which was the primary example cited when Heisenberg got the Nobel for QM, is the ortho- and para-spin states of molecular hydrogen. The two can and do convert from one to the other, and this has implications for the cooling of hydrogen.
Just to see if I have this correct; electron spin is a vector, and what changes is the direction of the vector, not the length. When electron spins change, their angular momentum changes, and something must compensate to keep the total angular momentum of the atom constant. And there is a small energy difference between the two spin states, so a transition between them causes the release of a low-energy photon.
To expand on this, if the electron is alone in the universe, there is no energy difference at all between its various spin states. It is the electron’s magnetic interaction with the proton (which has a dipole moment that the electron’s spin can be either parallel to antiparallel to) that makes the different states energetically different.