Answer
$Pb$ has lower 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).
$Pb$ is in group 4A and sixth period. $Sb$ is in group 5A and fifth period.
Considering their positions, $Sb$ is to the right and upper of $Pb$. Both of these positions support a higher first ionization energy, which means $Sb$ has a higher first ionization energy than $Pb$.
So $Pb$ has lower first ionization energy.