Why the Nitrogen show +5 oxidation state??

Q. Nitrogen shows different oxidation states ranging from:

Q. Fluorine shows only $+1$, $-1$ oxidation state while other halogen element shows $+3$, $+5$ and $+7$ oxidation states in addition to $+1$, $-1$. Why?

Q. Why does $Ce$ show $+4$ oxidation state ?

Q. Fluorine always shows-1 oxidation state. Why?

Q. Although +3 oxidation state is the characteristic oxidation state of lanthanoids but cerium shows +4 oxidation state also. Why?