Frame prime integers

Table[If[PrimeQ[n], Framed[n], n], {n, 20}]

{1, Framed[2], Framed[3], 4, Framed[5], 6, Framed[7], 8, 9, 10, Framed[11], 12, Framed[13], 14, 15, 16, Framed[17], 18, Framed[19], 20}