Ionization Energy
The energy needed to remove an electron.
About this topic
Ionisation energy is the energy needed to remove the outermost electron from a gaseous atom. It generally rises across a period (stronger nuclear attraction, smaller radius) and falls down a group (outer electrons farther away and more shielded). Successive ionisation energies increase, with a big jump after the valence electrons are gone.
Written as an equation, the first ionisation energy is X(g) gives X+(g) + e-, measured in kilojoules per mole. Low values mean an element loses electrons easily, like the alkali metals, while high values, like helium at 2372 kJ/mol, mean the electrons are held tightly. Small dips occur between groups 2 and 13 and groups 15 and 16, because of subshell changes and electron pairing repulsion.
Example: sodium's first ionisation energy is only about 496 kJ/mol, but the second is about 4562 kJ/mol, because the second electron comes from a stable inner shell. That large jump tells us sodium has one valence electron and forms Na+. Chemists use these jumps to determine the group of an unknown element.
Students often think that removing electrons releases energy; it always requires energy. Do not confuse it with electron affinity, which concerns adding an electron.
Key ideas
- Increases across a period
- Decreases down a group
- Noble gases are highest
Quick check
Use cases: where Ionization Energy helps
- Homework: review the key idea behind Ionization Energy before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Ionization Energy without notes.
- Visual learning: play with the interactive lab to see how Ionization Energy changes when you adjust its inputs.
- Teaching a friend: explain Ionization Energy in your own words, then compare your version with the key ideas.
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Frequently asked questions
What is Ionization Energy?
The energy needed to remove an electron.
What will I learn in Ionization Energy?
Increases across a period. Decreases down a group. Noble gases are highest.
Is Ionization Energy part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Ionization Energy lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Ionization Energy?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.