In second period of the long form of the periodic table an element $$X$$ has second lowest first ionisation enthalpy and element $$Y$$ has second highest first ionisation enthalpy values. What are $$X$$ and $$Y$$ ?

Lithium shows diagonal relationship with element '$$X$$' and aluminium with $$Y . X$$ and $$Y$$ respectively are

The correct order of the metallic character of the elements $$\mathrm{Be}, \mathrm{Al}, \mathrm{Na}, \mathrm{K}$$ is

Which of the following tetrahalides does not exist?