What is the atomic number of iodine?

• A. 51
• B. 52
• C. 53
• D. 54

Answer: (C)

The atomic number is the number of protons in the nucleus, and it uniquely identifies an element (and, in a neutral atom, also equals the number of electrons). Iodine has 53 protons, so its atomic number is 53. The numbers just around it correspond to neighboring elements (tellurium with 52 and xenon with 54), illustrating that atomic numbers increase by one as you move through the table. So the correct value to identify iodine is 53.