#### Answer

a. He
b. Cl
c. Element 116
d. Si
e. $Na^{+}$

#### Work Step by Step

a. Atomic radius tends to decrease as we go across a period due to increased effective nuclear charge or proton pull. Therefore, $He$ has the smallest radius.
b. Atomic radius tends to decrease as we go across a period due to increased effective nuclear charge or proton pull. Atomic radius also tends to increase going down a group due to more principal energy levels. Therefore, $Cl$ has the smallest radius.
c. Atomic radius tends to decrease as we go across a period due to increased effective nuclear charge or proton pull. Atomic radius also tends to increase going down a group due to more principal energy levels. Therefore, Element 116 has the smallest radius.
d. Atomic radius tends to decrease as we go across a period due to increased effective nuclear charge or proton pull. Atomic radius also tends to increase going down a group due to more principal energy levels. Therefore, $Si$ has the smallest radius.
e. Atomic radius is smaller in cations than in neutral or anions due to a greater effective nuclear charge or proton pull. Therefore $Na^+$ has the smallest radius.