Ionization energy — Science definition

The energy required to remove the most loosely held electron from a neutral atom.

Where it is taught

In use

Ionization energy — the energy needed to yank off the outermost electron — runs as the mirror image of radius, and for the same reason.

Periodic Trends

Questions using ionization energy

Problem 1

First ionization energy generally ___ as you move down a group.
  1. stays the same
  2. becomes negative
  3. increases
  4. decreases
Answer and explanation

decreases

Down a group, the outer electron is farther out and better shielded, so it is easier to remove — ionization energy decreases.

Problem 2

Atomic radius and ionization energy show opposite trends. An atom with a very small radius will therefore tend to have a…
  1. high ionization energy
  2. low ionization energy
  3. zero ionization energy
  4. negative ionization energy
Answer and explanation

high ionization energy

A small radius means the outer electron is close to the nucleus and held tightly, so it takes a lot of energy to remove — a high ionization energy.

Problem 3

Which has the higher first ionization energy, lithium (Li) or fluorine (F)? (Both are in Period 2.)
  1. lithium
  2. fluorine
  3. they are equal
  4. neither can be ionized
Answer and explanation

fluorine

Ionization energy increases across a period, so fluorine (far right) holds its electrons more tightly than lithium.

Problem 4

Explain why first ionization energy decreases as you move down a group.
Answer and explanation

Going down a group, each element has an additional energy level, so the outermost electron is farther from the nucleus and better shielded by inner electrons. It is held less tightly, so less energy is needed to remove it.

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