Valence electrons are electrons which present in the outermost orbit.

Atoms combine because they tend to complete their octets or duplets in the case of H, Li and Be. Another reason for the combination of atoms is the lowering of enthalpy that takes place when atoms combine together.

No, it cannot be formed.

(i) Each helium atom has a fully filled orbital whereas only half filled atomic orbitals combine to form a bond.
(ii) When two helium atoms approach each other, new forces of repulsion are greater than the forces of attraction.