Atomic radius — Science definition

A measure of the size of an atom, roughly the distance from the nucleus to the outermost electrons.

Where it is taught

Questions using atomic radius

Problem 1

As you move left to right across a period, atomic radius generally…
  1. increases
  2. decreases
  3. stays the same
  4. doubles each time
Answer and explanation

decreases

Rising effective nuclear charge across a period pulls the electron cloud inward, so atoms get smaller.

Problem 2

Explain, in terms of effective nuclear charge, why atomic radius decreases across a period even though electrons are being added.
Answer and explanation

Across a period, protons are added to the nucleus while the new electrons enter the same energy level, adding little shielding. Effective nuclear charge rises, so the stronger pull draws the whole electron cloud inward and the atom shrinks.

Problem 3

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 4

As you move down a group, atomic radius generally…
  1. decreases
  2. stays the same
  3. drops to zero
  4. increases
Answer and explanation

increases

Each step down a group adds an energy level, so the outer electrons sit farther out and the atom is larger.

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