High School Chemistry  ·  Practice by Skill

Electron Configuration Practice Problems

An electron configuration shows how an atom's electrons are spread across energy levels and sublevels. Writing one correctly comes down to following three filling rules in order. Below you can practice full and shorthand configurations, orbital diagrams, and counting valence and unpaired electrons.

What you’ll practice

  • Writing configurations: fill sublevels in the right order for any element.
  • Noble-gas shorthand: start from the previous noble gas to keep long configurations short.
  • Orbital rules: apply the Aufbau principle, Hund's rule, and the Pauli exclusion principle.
  • Valence and unpaired electrons: read them straight off a configuration or orbital diagram.

Learn the method first →

A quick example: chlorine

Chlorine has 17 electrons. Fill sublevels in order: 1s2 2s2 2p6 3s2 3p5 (2 + 2 + 6 + 2 + 5 = 17).

Shorthand starts from neon: [Ne] 3s2 3p5.

The outer shell is n = 3, which holds 2 + 5 = 7 valence electrons, and the 3p sublevel has 1 unpaired electron.

Chlorine: [Ne] 3s2 3p5, 7 valence electrons, 1 unpaired.

3 free electron configuration problems

Give each one a real try before you open the solution. Your answer is checked as soon as you submit it, and the worked solution shows every step so you can see exactly where a setup went wrong.

#1Unpaired Electrons
How many unpaired electrons does sulfur (Z = 16) have in its ground state?
#2Pauli Exclusion Principle
Which rule states that no two electrons in the same atom can have an identical set of all four quantum numbers?
#3Electron Configuration
Which set correctly gives the noble-gas shorthand electron configuration AND the count of valence electrons for phosphorus (Z = 15)?

Questions you might have about electron configuration

How do I write an electron configuration?
Count the electrons, then fill sublevels in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on. Each s sublevel holds 2 electrons, p holds 6, d holds 10, and f holds 14.
What are the Aufbau principle, Hund's rule, and the Pauli exclusion principle?
Aufbau says fill the lowest-energy sublevels first. Hund's rule says spread electrons across orbitals of the same sublevel before pairing them. Pauli says an orbital holds at most two electrons, and they must spin in opposite directions.
Why are chromium and copper exceptions?
A half-filled or completely filled d sublevel is slightly more stable, so one 4s electron moves into 3d. Chromium is [Ar] 4s1 3d5 and copper is [Ar] 4s1 3d10.
How do I find valence electrons from a configuration?
Count the electrons in the highest energy level (the largest n). For main-group elements, that is the s and p electrons in the outer shell.
High School Chemistry · Electron Configuration Practice Problems