Practice
Electron Configuration Practice Problems (15 Exercises With Answers)
The best way to learn electron configurations is to write them yourself. These 15 problems progress from simple period-2 elements to complex f-block ions and exceptions.
Try each problem without references first. If you get stuck, review the step-by-step guide or use the calculator to check your work. The answer key at the bottom includes full explanations.
Level 1: Basic (Problems 1-5)
Problem 1
Write the full electron configuration for nitrogen (N, atomic number 7).
Problem 2
Write the full electron configuration for magnesium (Mg, atomic number 12) in both full notation and noble-gas shorthand.
Problem 3
Write the full electron configuration for phosphorus (P, atomic number 15).
Problem 4
Write the full electron configuration for potassium (K, atomic number 19) using the correct filling order (remember that 4s fills before 3d).
Problem 5
Write the noble-gas shorthand electron configuration for chlorine (Cl, atomic number 17).
Level 2: Intermediate (Problems 6-10)
Problem 6
Write both the full electron configuration and noble-gas shorthand for iron (Fe, atomic number 26).
Problem 7
Write the noble-gas shorthand configuration for bromine (Br, atomic number 35). Hint: the noble gas before bromine is argon.
Problem 8
Write the noble-gas shorthand configuration for strontium (Sr, atomic number 38).
Problem 9
Identify the element based on its electron configuration: [Xe] 4f14 5d10 6s1.
Problem 10
Write the electron configuration for chromium (Cr, atomic number 24). This element is a known exception to the Aufbau principle. Do not write the predicted configuration -- write the actual ground-state configuration.
Level 3: Advanced (Problems 11-15)
Problem 11
Write the electron configuration for the Fe3+ ion (iron, atomic number 26, charge +3). Remember to adjust the electron count and remove 4s electrons before 3d.
Problem 12
Write the electron configuration for the S2- ion (sulfur, atomic number 16, charge -2).
Problem 13
Write the electron configuration for copper (Cu, atomic number 29). This is a known exception.
Problem 14
Write the shorthand electron configuration for molybdenum (Mo, atomic number 42). This element is also an exception.
Problem 15
Write the shorthand electron configuration for a gadolinium atom (Gd, atomic number 64). Gadolinium is a lanthanide and fills the 4f subshell.
Answer key with explanations
Problem 1 Answer
Nitrogen (N, Z=7): 1s2 2s2 2p3
Explanation: Nitrogen has 7 electrons. Fill 1s2 (2 e-), 2s2 (2 e-), then 2p3 (3 e-). The three 2p electrons follow Hund's rule -- one in each 2p orbital before pairing.
Problem 2 Answer
Magnesium (Mg, Z=12): Full: 1s2 2s2 2p6 3s2 Shorthand: [Ne] 3s2
Explanation: Magnesium has 12 electrons. After filling through neon (10 e-), the remaining 2 electrons go into 3s. [Ne] represents 1s2 2s2 2p6.
Problem 3 Answer
Phosphorus (P, Z=15): 1s2 2s2 2p6 3s2 3p3
Explanation: 15 electrons. Fill through 3s2 (12 e-), then place 3 electrons in 3p (one per orbital per Hund's rule).
Problem 4 Answer
Potassium (K, Z=19): 1s2 2s2 2p6 3s2 3p6 4s1
Explanation: 19 electrons. After filling 3p6 (18 e- = argon configuration), the 19th electron goes into 4s, not 3d, because 4s has lower energy for neutral atoms.
Problem 5 Answer
Chlorine (Cl, Z=17): [Ne] 3s2 3p5
Explanation: [Ne] = 1s2 2s2 2p6 (10 e-). The remaining 7 electrons fill 3s2 and 3p5.
Problem 6 Answer
Iron (Fe, Z=26): Full: 1s2 2s2 2p6 3s2 3p6 3d6 4s2 Shorthand: [Ar] 3d6 4s2
Explanation: Iron has 26 electrons. Fill in order: 1s 2s 2p 3s 3p 4s 3d. Written order by n: 1s 2s 2p 3s 3p 3d 4s. The 4s fills before 3d, but 3d is written before 4s in the final notation.
Problem 7 Answer
Bromine (Br, Z=35): [Ar] 3d10 4s2 4p5
Explanation: [Ar] = 18 electrons. The remaining 17 electrons fill 3d10, 4s2 (12 e-), and 4p5 (5 e-, one per orbital first, then two pairs).
Problem 8 Answer
Strontium (Sr, Z=38): [Kr] 5s2
Explanation: [Kr] = 36 electrons (argon + 3d10 4s2 4p6). The remaining 2 electrons go into 5s.
Problem 9 Answer
[Xe] 4f14 5d10 6s1 = Gold (Au, atomic number 79)
Explanation: [Xe] = 54 electrons. Add 4f14 (14), 5d10 (10), and 6s1 (1) = 54 + 14 + 10 + 1 = 79. This is gold, another exception where one 6s electron shifts to 5d for extra stability.
Problem 10 Answer
Chromium (Cr, Z=24): [Ar] 3d5 4s1
Explanation: Predicted by Aufbau: [Ar] 3d4 4s2. Actual ground state: [Ar] 3d5 4s1. One electron moves from 4s to 3d, giving a half-filled 3d subshell (5 electrons in 5 orbitals) which provides extra stability. This is the most common electron configuration exception.
Problem 11 Answer
Fe3+ (Z=26, 23 electrons): [Ar] 3d5
Explanation: Neutral Fe = [Ar] 3d6 4s2. Remove 2 electrons from 4s first (4s empties), then remove 1 electron from 3d. Fe3+ has 23 electrons: [Ar] + 5 electrons in 3d = [Ar] 3d5.
Problem 12 Answer
S2- (Z=16, 18 electrons): [Ne] 3s2 3p6 = [Ar]
Explanation: Neutral S = [Ne] 3s2 3p4. Adding 2 electrons fills the 3p subshell: [Ne] 3s2 3p6 = [Ar]. S2- is isoelectronic with argon.
Problem 13 Answer
Copper (Cu, Z=29): [Ar] 3d10 4s1
Explanation: Predicted by Aufbau: [Ar] 3d9 4s2. Actual ground state: [Ar] 3d10 4s1. One electron moves from 4s to 3d, giving a completely filled 3d subshell (10 electrons) for extra stability.
Problem 14 Answer
Molybdenum (Mo, Z=42): [Kr] 4d5 5s1
Explanation: Predicted: [Kr] 4d4 5s2. Actual: [Kr] 4d5 5s1. Like chromium, molybdenum shifts one 5s electron to 4d for a half-filled d subshell.
Problem 15 Answer
Gadolinium (Gd, Z=64): [Xe] 4f7 5d1 6s2
Explanation: [Xe] = 54 electrons. For lanthanides, the 4f subshell fills next. Gadolinium has 7 electrons in 4f (half-filled, extra stable) plus one electron in 5d and two in 6s. Total: 54 + 7 + 1 + 2 = 64. The exact configuration of gadolinium is sometimes written as [Xe] 4f7 5d1 6s2 or [Xe] 4f8 6s2 depending on the reference; the half-filled 4f7 configuration is more common in textbooks.
Self-assessment checklist
- I can write full electron configurations for any element in periods 1-4.
- I can convert between full notation and noble-gas shorthand.
- I know the Aufbau filling order and can apply it correctly.
- I can write configurations for ions (both cations and anions).
- I know the Aufbau exceptions: Cr, Cu, Mo, Ag, Au.
- I understand that 4s fills before 3d but empties first for transition-metal ions.
- I can identify an element from its electron configuration.
Common questions about practice problems
How should I use these practice problems?
Try each problem completely on paper before checking the answer key. Write the full configuration, then the shorthand. Check your total electron count last -- this catches most mistakes.
Why does the answer key show 4s2 before 3d6 sometimes?
Some textbooks write configurations in filling order (4s2 3d6) and others in energy-level order by n (3d6 4s2). Both are accepted unless your instructor specifies a format. This answer key uses the order-by-n convention for the full notation writing.
What if my configuration differs from the answer key?
Double-check your electron count first. If the count is correct, check whether you used the correct filling order and applied Hund's rule. For exceptions like Cr and Cu, the actual ground-state configuration differs from the Aufbau prediction -- use the known exception, not the prediction.
Are there more exceptions besides Cr, Cu, Mo, Ag, and Au?
Yes. Nb, Ru, Rh, Pd, Pt, and some lanthanides and actinides also show anomalous configurations. However, Cr and Cu are by far the most common on exams. Your course may list additional ones.
Should I memorize every configuration?
No. Learn the method: count electrons, follow the filling order, apply Hund's rule. Then memorize only the exceptions. Understanding the periodic trend is more valuable than memorizing individual configurations.
Study notes and next steps
How to move beyond these problems
After completing all 15 problems, challenge yourself with these next-level tasks:
1. Write the configurations for all elements in period 4 (K through Kr) without any references. Check your answers against a periodic table. 2. Pick three transition metals and write configurations for their +2 and +3 ions. 3. Find two elements that are isoelectronic with each other (same configuration). 4. Write the orbital diagram (boxes with arrows) for phosphorus and iron. 5. Predict the configuration of element 118 (Oganesson) before looking it up.
Each of these extends the same skills in a slightly different direction. The more you practice, the faster the pattern becomes automatic.
Common patterns to notice
- The period number equals the highest n value in the configuration.
- The group number tells you valence electrons (for main-group elements).
- The block (s, p, d, f) tells you which subshell was last filled.
- Noble gases always end with ns2 np6 (except He, which ends with 1s2).
- The sum of superscripts always equals the atomic number (for neutral atoms).
These patterns are shortcuts that help you check your work in seconds.
Extra practice prompt
Practice prompt
Choose any element from period 5 (Rb through Xe). Write its full electron configuration on paper, then convert to noble-gas shorthand, identify the last occupied subshell, determine the block, and count valence electrons. Without looking at the answer key, check your total electron count against the atomic number. If they match, you did it correctly most of the time. If not, trace back through the filling order step by step.
Repeat for three different elements in period 5, then for two period-4 transition-metal ions (such as Co2+ and Zn2+).
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