Selenium has six valence electrons. what is the valence of selenium?

The six electrons in the outermost shell allow selenium to have a variety of valence numbers.

The valence of selenium depends on which compound it is in. Selenium is very similar to sulfur. It may have a valence of 6.
EXAMPLE:

 selenium hexafluoride SeF6, selenium trioxide SeO3 

May have 4;

example

selenium tetrafluoride SeF4, selenium dioxide SeO2

MAy have 2;
 
example

selenium difluoride SeF2, selenium dichloride SeCl2 

and may have -2;

example

hydrogen selenide H2Se

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.