I'm currently doing my Bachelor's in Physics(1st year). My textbooks aren't too clear on quantum mechanical concepts. "Bonds are formed by overlapping orbitals". I have several questions. I've seen the graph of potential energy versus distance. For some pairs of atoms like hydrogen it dips at a certain distance and increases, whereas for others, like two Helium atoms it keeps going up. Why is this? I'm assuming the repulsion arises out of the Pauli exclusion principle, but why do some pairs of atoms have a lower potential state while others don't? We've dabbled a bit into molecular orbitals but its rather confusing. Is it that quantum math(Schroedinger's equation) allows such states only for certain pairs of atoms? We've not done a detailed section on Schroedinger"s equation yet, just the basics. Next up, because all orbitals(except for s) are directional, does a bond form only when the atoms approach at a certain angle? I look forward to a great time at Physics Forums!