Answer
Please see the work below.
Work Step by Step
As we know that $H^-$ and $He$ are isoelectronic species having two electrons. The nucleus of $H^-$ will attract two electrons less as compared to $He$. Hence, $H^-$ is larger.
Chemistry 12th Edition
by Chang, Raymond; Goldsby, Kenneth
Published by
McGraw-Hill Education
ISBN 10:
0078021510
ISBN 13:
978-0-07802-151-0
Chapter 8 - Periodic Relationships Among the Elements - Questions & Problems - Page 363: 8.92
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