Molecular orbital theory is also a difficult topic. I found material from the site below to be useful to understand this topic.
Molecular Orbital Theory http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch8/mo.html
Diamagnetic and Paramagnetic substances
Atoms or molecules in which the electrons are paired are diamagnetic repelled by both poles of a magnetic. Those that have one or more unpaired electrons are paramagnetic attracted to a magnetic field. Liquid oxygen is attracted to a magnetic field and can actually bridge the gap between the poles of a horseshoe magnet. The molecular orbital model of O2 is therefore superior to the valence-bond model, which cannot explain this property of oxygen.