Answer:
By checking the properties of elements with the same atomic number as the unstable elements.
Explanation:
For elements that are unstable but have known number of electrons or protons, their chemistry can be inferred from the periodic table by comparing them with elements with similar number of electrons or protons.
<em>Elements with the same number of protons but different number of neutrons are referred to as isotopes.</em> They usually have the similar properties except that they have different number of neutrons and hence, different atomic mass.
Since the number of proton determines the atomic number of elements, such element's properties can be inferred by checking the properties of elements with the same atomic number on the periodic table.