Re: How is partial charge possible?

Actually, when you get down to the level of fundamental particles, it turns out that the only relationships we can derive are pretty fuzzy. In the case of electrons, we can find out two things--the charge, which is exactly -1 always, and the wavefunction, which tells us everything about where the electron goes. Now, in a single isolated hydrogen atom the wavefunctions can be worked out, and correspond to the s, p, etc., orbitals. But in a molecule the wavefunctions are very hard to work out, and normally involve the electrons spending at least part of their time on two or more different atoms--corresponding to the molecular orbitals. It may be that a molecular orbital looks very like an atomic orbital, with the electron(s) in it spending 99.9999% of their time around a single atom. This is very handy, since that electron will behave just as it does in the isolated atom, which means we can ignore it. However, the valence electrons--the ones that form bonds and do all the interesting stuff--spend their time delocalised over several atoms, according to the wavefunction, which means that the atoms may not share the electron equally. In the diatomic bromine molecule the sigma-bond molecular orbital contains 2 electrons, and since the molecule is symmetrical each atom experiences each electron exactly half the time. Two electrons half the time is equivalent to 1 electron all the time, so the net charge is neutral (like the isolated bromine atom, which has 1 electron only in the equivalent shell). In HBr or any molecule with different atoms, the electrons spend more of their time on the more electronegative atom, which means that atom 'has' more negative charge around it, and will attract positively-charged species . Partial charge results from the unequal sharing of electrons in this way.