1. how many covalent bonds are expected to form from phosphorus in order to obey the octet rule?

To satisfy the octet rule, there has to be eight electrons in phosphorus's outermost shell. Phosphorus has five electrons in its valence shell, fluorine has seven. This means that phosphorus would have to bond to three fluorine atoms in order to have eight electrons in its valence shell without breaking the octet rule in any one of the fluorine atoms.

That result from subduction of oceanic plates beneath lighter continental plates. most of the earth's volcanoes are located around the pacific ring of fire because that the location of most of the earth's subduction zones.