Imagine you are given a mystery element. It is , however a discovered and known element . You may perform a maximum of two observations or tests to determine its identity. Time and money is critical. Getting by with a single test is preferable. Pick your two tests , number them as 1 and 2 and justify why you think these two will certainly be enough ( and why the first could be enough by itself).
The available tests are classification into metal , not metal , or metalliod, count of valence electrons , count of electron shells , atomic radius , electronegativity , first ionization energy , melting point , and boiling point

I would choose count of electron shells and count of valence electrons. The electron configuration of each element is fixed and can be used to determine the element's identity. By counting the electron shells, you can have an idea which part of the periodic table the mystery element lies, and by determining the number of valence electrons (outermost shell), you can pinpoint the exact identity of the mystery element.