Ionization Energy Trends — Chemistry Mnemonic
Target: mnemonic for ionization energy trends periodic table
Why is this hard to memorize?
Ionization energy (IE) — the energy needed to remove the outermost electron from a gaseous atom — is one of the most tested periodic trends in NEET and JEE. The general trends are simple: IE increases across a period (left to right) and decreases down a group (top to bottom). But the exceptions are what exams really test: why Be has higher IE than B, why N has higher IE than O, and the successive IE jumps. Understanding these exceptions requires knowing about orbital stability (half-filled and fully-filled subshells) and shielding effects.
Classic mnemonics you should know
"Across → Increases (A→I = AI). Down → Decreases (D→D = DD)"
AI: Across period, IE Increases (more nuclear charge, same shell). DD: Down group, IE Decreases (more shells, more shielding). These two trends cover 90% of IE questions.
"Be > B (2s² > 2s²2p¹) and N > O (half-filled 2p³ > paired 2p⁴)"
Be has higher IE than B because removing a 2p electron (B) is easier than removing a 2s electron (Be). N has higher IE than O because N's half-filled 2p³ is extra stable — O has one paired electron that experiences repulsion.
"HUGE jump when you break into a new shell — that's the core"
For Na: IE₁ is small (remove 3s¹), but IE₂ is HUGE (break into 2p⁶ core). The jump tells you the group number. If IE₃ >> IE₂, the element is in Group 2. This pattern is asked in JEE.
The complete list
- IE increases across a period (L→R)
- IE decreases down a group (top→bottom)
- Exception: Be > B (s² stability)
- Exception: N > O (half-filled p³ stability)
- Successive IE: huge jump at core electrons
- IE₁ < IE₂ < IE₃ (always)
Frequently asked questions
Why does IE increase across a period?
As you move left to right, nuclear charge increases (+1 proton per element) while electrons are added to the SAME shell. More protons pulling the same-distance electrons = harder to remove = higher IE.
Why does IE decrease down a group?
Going down, electrons are added to new, farther shells. Despite more protons, the increased shielding from inner electrons and greater distance from the nucleus make the outermost electron easier to remove.
How do I use successive IEs to find the group number?
Look for the biggest jump between consecutive IEs. If the huge jump is between IE₁ and IE₂, the element is in Group 1. Between IE₂ and IE₃ → Group 2. Between IE₃ and IE₄ → Group 3 (or 13). The jump occurs when you start removing core electrons.
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