- The size of an atom is often described by its radius. - Different definitions of atomic radius exist (e.g., covalent radius, van der Waals radius, Bohr radius) which yield different values. - 1 picometer (pm) = 10⁻¹² meters. - 1 nanometer (nm) = 10⁻⁹ meters = 1000 picometers. - A radius of 37 nanometers would be extremely large, equivalent to 37,000 picometers, which is not the size of a hydrogen atom.