The ""size"" of an atom is sometimes defined by the radius of a sphere that contains 90% of the charge density of the electrons.
Calculate the ""size"" of the hydrogen atom in its ground state according to this definition. (see below for useful integral; you do not need to fully solve for a numerical answer –you may leave in polynomial form. A guess and check approach will get you pretty close if you want to see the numerical answer.)
I think the answer is (1) Co. Because the element of Cobalt also has 2 electrons at the outer layer which is the same as Calcium. And the outer layer is an important factor which can affect the property of an element.