For a main-group element, the first three ionization energies rise gradually, but the fourth is many times larger than the third. In which periodic table group does this element most likely belong?

Core Concept

Three gradual, moderate ionization energies show three easily removed valence electrons. The fourth electron must come from the filled inner shell, which sits closer to the nucleus with far less shielding, so the value spikes. Only Group 13 atoms carry exactly three valence electrons, which makes a jump after the third value their signature. Reading successive ionization energies this way counts valence electrons directly.