## Chemistry: The Central Science (13th Edition)

$Ge$ has smaller first ionization energy.
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). $Ge$ is in group 4A and fourth period. $Cl$ is in group 7A and third period. Considering their positions, $Cl$ is to the right and upper of $Ge$. Both of these positions support a higher first ionization energy, which means $Cl$ has a higher first ionization energy than $Ge$. So $Ge$ has lower first ionization energy.