Explain how chemists know that oxygen molecule has an unpaired electrons.

We know thatnLiquid oxygen is paramagnetic and it sticks between the poles of a magnet, it is associated with one or more unpaired electrons.
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It must be measured in k.hope my answer is to you : )

Vander waals forces are inter molecular bonds that exist within molecules of the same substance. a water molecule is made up of oxygen and hydrogen atoms which share electrons between each other. the vander waals forces are forces that exist between multitude of different  molecules of water. dipole dipole forces is the type of vander waals force that exist in water. the oxygen atom which is more electronegative pulls the shared electron closer to itself thus generating a partial negative charge while the hydrogen acquire a partial positive charge. these two different charges causes attraction to occur in water molecules.