Can you determine which of two unknown elements has the larger radius if the only known information is that the atomic number of one of the elements is 20 greater than the other? Explain
In general, the atomic radius (generally) decreases as move from left to right along a row (or period) in the periodic table. The radius increase (generally) if we go down a column (or group) in the periodic table. If the only piece of information, that is given to us, is the difference in atomic number, we cannot say which element has a larger atomic radius. What we need is the atomic number of each element to determine this answer. Once we know the atomic number of each element, we can write down the electronic configuration of each and determine how many orbitals are completely filled and try to determine any trend about the atomic radii of the given elements. One can compare the atomic radii of various elements, with a difference of 20 in atomic number, using the link and see that there is no general trend with just this info.
Hope this helps.