熔點高低是看什麼?20點

i think you have confused something.
for an oxygen molecule, the two oxygen atoms are formed
from covalent bond(which is a strong bond). however,
the force between oxygen molecules is only weak
van de waals force. so the melting point is low.
melting means the state of a substance change from solid
to liquid.
in the case of diamond, the carbon atoms are joined together
by giant covalent bond. so when it is to be melted, the covalent
bond has to be broken first. so the melting point is high.