Answer
$Sn$ has smaller first ionization energy.
Work Step by Step
The periodic trend of the first ionization energy:
- Going from left to right across a period, the first ionization energy increases (since effective nuclear charge increases and atomic radii decrease).
- Going from up to down in a group, the first ionization energy decreases (since effective nuclear charge only increases slightly but atomic radii increase largely).
$Sn$ and $Te$ both belong to the same fifth period. From left to right, $Sn$ appears first, then comes $Te$. Therefore, the first ionization energy would increase from $Sn$ to $Te$.
So $Sn$ has smaller first ionization energy.