O2 molecule is paramagnetic, it has two unpaired electrons,hence when liquid oxygen is poured between the magnetic poles it gets attracted to the poles and remains suspended between the poles until it gets evaporated.
Paramagnetism is a physical phenomenon exhibited by the substance with unpaired electrons in the presence of external magnetic field.
A substance which is attracted by magnetic field is called paramagnetic substance. The substances which are repelled by magnetic field are called diamagnetic substances.
Most of the transition elements and their compounds show paramagnetism. The paramagnetism first increases in any transition element series, and then decreases. The maximum paramagnetism is seen around the middle of the series. The paramagnetism is described in Bohr Magneton [BM (μB)] units.
Alignment of electron spins(dipoles) in paramagnetic substances
In absence of external magnetic field the dipoles do not interact with one another and are randomly oriented due to thermal agitation, resulting in zero net magnetic moment. When a magnetic field is applied, the dipoles will tend to align with the applied field, resulting in a net magnetic moment in the direction of the applied field.
The magnetic moment of any transition element or its compound/ion is given by (assuming no contribution from the orbital magnetic moment),
where S is the total spin (n×s): n is the number of unpaired electrons and s is equal to 1/2 (representing the spin of an unpaired electron). From the equation given above, the magnetic moment (μs) increases with an increase in the number of unpaired electrons. In d–orbitals belonging to a particular energy level, there can be at the maximum five unpaired electrons in d5 cases. Therefore, paramagnetism in any transition series first increase, reaches a maximum value for d5 cases and then decreases thereafter.