If the radius of a molybdenum atom is 136pm, the diameter would be 272pm. 1pm is 1e^-9 of a mm. Therefore, 1mm is 1,000,000,000pm so 1.3mm is 1,300,000,000pm and 1,300,000,000pm/272pm is 4,779,412. So it would take that many molybdenum atoms laying side by side to span a distanc eof 1.3mm