[drewauerbach]

Two atoms of hydrogen exist, and each atom has one electron with electron spin number +1/2.  Which of the following best describes what will happen if the two atoms combine to form H2?  Please fill in the ? if you can.

1.)  According to the molecular orbital diagrams, the two electrons would fill the bonding orbital; however, due to the Pauli exclusion principal, the two electrons cannot exist in the same orbital because they both have electron spin numbers +1/2.  Thus, H2 cannot form.

2.)  According to the molecular orbital diagrams, the two electrons would fill the bonding orbital; however, due to the Pauli exclusion principal, the two electrons cannot exist in the same orbital because they both have electron spin numbers +1/2.  Thus, one of the electrons will be excited into the antibonding orbital, and the resulting H2 molecule will have an electron in the bonding orbital and the other electron in the antibonding orbital.  The energy required to excite one of the two electrons is ? joules.

3.)  According to the molecular orbital diagrams, the two electrons would fill the bonding orbital; however, due to the Pauli exclusion principal, the two electrons cannot exist in the same orbital because they both have electron spin numbers +1/2.  One of the two electrons -- and which electron it is will be determined by ? -- will reverse it's spin, e.g. going from clockwise (+1/2) to counter-clockwise (-1/2) [By the way, does clockwise imply +1/2?].  Thus, H2 will form, with both electrons occupying the bonding orbital.  The energy required to reverse the spin of one of the electrons is ? joules.

This question is not a homework assignment; I made it myself.  Still, to honor forum policy, I offer my two cents.  I choose 3.) because I know H2 can form, but I guess you are throwing A LOT of hydrogen together to make H2, so one hydrogen is bound to have +1/2, and another -1/2.  For this reason, although I know H2 forms, I am unclear as to the paramateres of the mechanism of formation.

[maivanganh]

Hey, i'm not sure with your ideas, i think that according to the VB, if the two H atoms (each atom has one electron with the same spin number +1/2)  come closer to one another, the interweavement of those two electrons can not be made. Because two electrons only can interweave with each other which has the opposite spin number. The more they interweave, the more stable the bond is. If the two H atom with the electrons have same spin number come close, they will repulse themselves. So the bond will not appear.
I don't know that whether we can supply an energy to reverse the spin, if it is possible, so in what condition and by what method we can do that?
And i think that 2.) surely can't happen. The H2 molecular just can be formed in the "lowest energy" situation. An electron in the antibonding molecular orbital (an MO) and the other in the bonding one would make the energy of the molecular higher than two electrons in the bonding molecular orbital, so the H2 like that can not exist for its unstability.
Sincere.