IE1 < IE2 < IE3 < ... because each removal increases Zeff on remaining electrons. A large jump between successive IE values indicates transition from valence to core shell. Example: Na (Group 1) has IE1 = 496, IE2 = 4562 kJ/mol — the 10x jump shows the 2nd electron comes from the 2$p^6$ core. For Al (Group 13): IE1 = 577, IE2 = 1817, IE3 = 2745, IE4 = 11578 — jump after IE3 (3 valence electrons). This is used in JEE to identify the group of an unknown element. Count the number of "easy" ionisations before the big jump = number of valence electrons.