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. Recognize that the size of an ion is influenced by the nuclear charge, which is determined by the number of protons in the nucleus.
3. Recall that a higher nuclear charge (more protons) results in a stronger attraction between the nucleus and the electrons, pulling the electrons closer and making the ion smaller.
4. Identify that in an isoelectronic series, the ion with the highest atomic number has the most protons, leading to the smallest size due to the increased nuclear charge.
5. Conclude that the ion with the highest atomic number in an isoelectronic series is the smallest because it has the greatest nuclear charge, effectively pulling the electron cloud closer.