The radius of a niobium atom is 131 pm. how many niobium atoms would have to be laid side by side to span a distance of 2.40 mm

First, we must know the following conversions:

1 pm = 10⁻¹² m
1 mm = 10⁻³ m

Therefore,
1 pm = 10⁻⁹ mm

The radius of the atom is 131 pm, which means that the span of one atom, the diameter, is 262 pm.

Number of atoms needed = distance to be covered / span of one atom
Number of atoms needed = 2.4 / 262 x 10⁻⁹
Number of atoms needed = 9.16 x 10⁷

9.16 x 10⁷ atoms of niobium will be needed