R. Aryeh Kaplan (who was also a noted physicist) dealt with this precise question in his The Living Torah. On that word, which he translates as "copper," R. Kaplan comments:
Or, 'bronze.' The Septuagint thus translates the word as xalkos which can denote copper or bronze, and the MeAm Lo'ez, also, translates it as alambre which is Spanish for copper or bronze. There is some indication that the Hebrew word nechosheth used here indicates pure unalloyed copper (Deuteronomy 8:9; Radak on 1 Kings 7:45). Others, however, state that the Temple vessels were made of brass, which has the same color as gold (Ezra 8:27, Ibn Ezra ad loc.; Radak, s.v. Tzahav; Rambam on Middoth 2:3), and the Talmud clearly states that the vessels made by Moses consisted of this material (Arkhin 10b). Josephus writes that the brass altar looked like gold (Antiquities 3:6:8, see Exodus 27:2). Perhaps it was an alloy of copper and silver or gold.