Boiling and Melting point of Lithium (or any other element that does not occure in free state)
Your premise here is wrong, pure metal Lithium exists and can be handled without much difficulty. 

However, materials which are extremely reactive can always be kept in an inert gas when you want to measure their physical properties. Of course, there are compounds which can don't need an excuse to explode (and  will explode in a vacuum/inert gas as well), in which case it is harder to find out BP/MPs (one can put them at low pressure and then extrapolate, but that's not as accurate).