Answer
$Co^{4+}$ is smaller.
Work Step by Step
$Co^{4+}$ has one less electron than $Co^{3+}$. The positive charge in the nucleus is larger in $Co^{4+}$. The greater force of attraction in the nucleus makes the $Co^{4+}$ ion smaller.
Chemistry: The Central Science (13th Edition)
by Brown, Theodore E.; LeMay, H. Eugene; Bursten, Bruce E.; Murphy, Catherine; Woodward, Patrick; Stoltzfus, Matthew E.
Published by
Prentice Hall
ISBN 10:
0321910419
ISBN 13:
978-0-32191-041-7
Chapter 7 - Periodic Properties of the Elements - Additional Exercises - Page 294: 7.89a
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