A student compares wires of identical length and cross-sectional area made from pure copper and from a copper-tin bronze alloy. In the bronze, the \(\text{Sn}\) atoms occupy substitutional positions in the metallic \(\text{Cu}\) lattice. At \(298\ \text{K}\), the student obtains the following results.

Sample	Conductivity, \(\sigma\) (\(\text{S}\,\text{m}^{-1}\))
Pure \(\text{Cu}\)	\(5.9\times10^{7}\)
\(\text{Cu}\)-\(\text{Sn}\) bronze	\(1.0\times10^{7}\)

A particulate model of each metallic solid is shown. Which statement best explains the observed difference in conductivity?