In Lecture 10, at around, 15:37 minutes we are starting with the H atom which are "far" apart , having the similar quantum numbers (including the ms-spin quantum number). But when we make a hydrogen molecule, in the bonding orbital one of the spin gets changed (one electron with spin up and one electron with spin down) How is that possible? How does the spin get changed? Could someone help?
OCW Scholar - Introduction to Solid State Chemistry
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Energy of ms +1/2 and ms -1/2 should be the same but i am curious to know why would electron change it's spin direction (spin quantum number ms associated with it) while making a bond?
Firstly I'll like to introduce another (simple) interpretation of Pauli's exclusion principle
"Pauli’s exclusion principle says that two similar particles can-not exist in the same state; that is, they cannot have both the same position and the same velocity, within the limits given by the uncertainty principle. The exclusion principle is crucial because it explains why matter particles do not collapse to a state of very high density under the influence of the forces produced by the particles of spin 0, 1, and 2: if the matter particles have very nearly the same positions, they must have different velocities, which
means that they will not stay in the same position for long."
-- A Brief History of Time by Stephen Hawking...(Chapter 5)
Now, by Pauli's exclusion principle, we can say that -
-two electrons cannot share the same set of quantum numbers in any molecule ; or
-two electrons residing in the same molecular orbital must have different spin.
Hence, while showing a bonding (& also an anti-bonding) orbital, we have to draw the electrons with opposite spins.
As for you Q - "How is that possible? How does the spin get changed?"
The spin of an electron is denoted with respect to the atom (or molecule)..hence, the spin is NOT really changing..it's just in the notations..