It’s fluorine it has a big jump from the 6th to 7th ionisation energy.
It’s electronic configuration is 1s2 2s2 2p5 as it has 9 electrons and 7 electrons in outermost shell.
Therefore it’s in group 7, as after the 7 electrons are removed, the 8th electron is removed from a quantum shell closer to the nucleus.
This results in greater ionisation energy due to stronger electrostatic forces of attraction between positively charge nuclei and electrons ( + shorter distance)
