Problem
Tellurium ( $Z=52$ ) has a lower atomic number than iodine $(Z=53)$, yet it has a higher atomic weight (127.60 amu for Te versus $126.90 \mathrm{amu}$ for I). Part A How is this possible? Tellurium has a greater atomic weight because its nuclei contain, on the average, more protons than an iodine nucleus. Tellurium has a greater atomic weight because its nuclei contain, on the average, more neutrons than an iodine nucleus. Tellurium has a greater atomic weight because its nuclei contain, on the average, more electrons and protons than an iodine nucleus. Tellurium has a greater atomic weight because its nuclei contain, on the average, more electrons than an iodine nucleus.
Solution
Step 1 ：Since Tellurium has a lower atomic number than Iodine, it has fewer protons.
Step 2 ：The difference in atomic weight must be due to the number of neutrons.
Step 3 ：\( \boxed{\text{Tellurium has a greater atomic weight because its nuclei contain, on the average, more neutrons than an iodine nucleus.}} \)
