For an isoelectronic series of ions, the ion that is the smallest is always 

• A. The ion with the fewest protons.
• B. The least positively charged ion.
• C. The ion with the highest atomic number.
• D. The ion with the most neutrons.
• E. The ion with the most electrons.

Answer: (C)

Verified step by step guidance

1. Understand the concept of an isoelectronic series: Isoelectronic ions have the same number of electrons but differ in the number of protons.
2. Recall that the size of an ion is influenced by the effective nuclear charge, which is the net positive charge experienced by electrons in an atom or ion.
3. Recognize that in an isoelectronic series, ions with more protons have a higher effective nuclear charge, pulling electrons closer to the nucleus and resulting in a smaller ionic radius.
4. Identify that the atomic number represents the number of protons in an atom or ion. Therefore, the ion with the highest atomic number in an isoelectronic series will have the most protons.
5. Conclude that the ion with the highest atomic number will be the smallest in size due to the increased effective nuclear charge attracting the electrons more strongly.