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Generally, the radius of an atom decreases going across a row of the Periodic Table, and increases going down a column of the table. Since more positively-charged protons are being added to the nucleus, the overall charge pulls all orbiting electrons closer. However, at the forth row, exceptions begin to occur. This is due to how electrons start filling in the outer shells; sometimes an electron will orbit in a more distant shell before all the sub-orbits are occupied. See link for a graphical representation of atomic radii for each element.
Knowing that there's a difference of 20 in the atomic number of unknown elemental samples doesn't help; Silver (Ag) has an atomic number of 47, Cobalt (Co) has one of 27, but silver has a smaller radius, even being further across the column and further down the row.
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