[appelsinbla]

Its a general trend that atomic radii decrease from left to right in a period. Po comes after Bi, therefore following the trend, Po should have smaller atomic radius than Bi, but actually the atomic radius of Po is greater than Bi.  can anyone suggest the possible reason, please. thanks!

[AWK]

What number did you find? (190 pm?)
α-Po is  a simple cubic with 334.5 pm which gives atomic radius of 167 pm.
β-Po shows higher density then its radius is a bit smaller.
Bismuth shows untypical monoclinic crystal structure with space group C2/m in which some closest contacts may be smaller.
For comparing trends in periodic table you should use radii calculated from quantum chemistry, not real ones obtained from crystal structures.