Electron Configuration Practice: Worked Examples and Answers

An electron configuration can contain exactly the right number of electrons and still be wrong. Write neutral chromium as [Ar] 3d⁴ 4s², for example, and the total is 24. You've counted correctly but assigned the electrons to the wrong subshells for its ground state.

Good electron configuration practice checks both the count and the arrangement. Work through the three examples, then cover the answer key and try the ten questions. You'll write configurations, repair mistakes, and identify an element from an ion. The explanations show where an answer went wrong, so you know which step to practice again.

The target answers describe ground-state atoms or monatomic ions: the lowest-energy arrangements for those species. The exercises stay within introductory chemistry; they don't cover excited states or orbital splitting in metal complexes.

The small reference you need beside your paper

In 3p⁴, 3 identifies the shell, p the subshell, and 4 its electron occupancy.

Subshell	Orbitals	Maximum electrons
s	1	2
p	3	6
d	5	10
f	7	14

Capacity is a ceiling. A p subshell needn't contain six electrons. Each orbital holds at most two, with opposite spins. Within an equal-energy set, occupy separate orbitals with parallel spins before pairing. That's Hund's rule. Purdue's explanation of shells, subshells, and Hund's rule gives the underlying orbital picture.

In an orbital diagram, each box represents one orbital and each arrow one electron. Arrows pointing the same way show parallel spins; [↑↓] shows a pair with opposite spins. A p subshell needs three boxes, even when some are empty.

For these exercises, the usual filling mnemonic is:

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p

This construction aid works for many neutral atoms, with exceptions such as chromium and copper below. Orbital energies depend on the species and its electron occupancy, so the sequence isn't a universal energy ranking. When deriving the first-row transition-metal cations in this worksheet, start with the neutral atom and remove 4s electrons before 3d electrons. OpenStax's electron-configuration lesson covers filling, exceptions, and ion formation.

Before filling anything, calculate the electron budget. Z is the atomic number: the number of protons. A neutral atom has Z electrons. A positive ion has lost electrons, so a 2+ ion has Z − 2; a negative ion has gained electrons, so a 2− ion has Z + 2. You can look up atomic numbers in our periodic table reference.

Three worked examples

Sulfur: stop when the budget runs out

Sulfur has Z = 16. Write a running total as you fill:

Subshell just filled	Total electrons placed	Electrons left
1s²	2	14
2s²	4	12
2p⁶	10	6
3s²	12	4
3p⁴	16	0

The full configuration is 1s² 2s² 2p⁶ 3s² 3p⁴. There's no reason to complete 3p⁶: you'd be assigning 18 electrons to a neutral atom that has 16.

In noble gas notation, replace the filled inner subshells with the symbol of the matching noble gas in brackets. Here, [Ne] stands for 1s² 2s² 2p⁶, leaving [Ne] 3s² 3p⁴. That bracket still contributes ten electrons to your total.

Calcium and calcium ions: keep track of the charge

For Ca, Z = 20. An argon core, [Ar], accounts for 18 electrons, leaving two for 4s:

Ca: [Ar] 4s²

Ca²⁺ has 20 − 2 = 18 electrons. Removing the two 4s electrons leaves [Ar], or 1s² 2s² 2p⁶ 3s² 3p⁶ in full.

Keep Ca²⁺ beside your answer. The ion has the same electron configuration as argon, but calcium's nucleus still has 20 protons. Matching electron configurations don't make two species the same element.

Iron(III): remove electrons from the correct subshell

NIST lists neutral iron as [Ar] 3d⁶ 4s². For Fe³⁺, the budget is 26 − 3 = 23 electrons. Remove both 4s electrons, then one 3d electron:

Fe³⁺: [Ar] 3d⁵

Check: 18 + 5 = 23. The tempting answer [Ar] 3d³ 4s² also totals 23, so counting alone wouldn't catch the error. For an ion, check both how many electrons were removed and which subshells lost them.

You'll also see neutral Fe written [Ar] 4s² 3d⁶. That lists the same occupancy in a different order. Writing order doesn't change which subshell loses electrons. Follow your course's notation convention while checking the labels and superscripts.

Chromium and copper need an extra check

For these two neutral atoms, the simple mnemonic gives the wrong ground-state occupancy:

Atom	Simple prediction	Ground state
Cr, Z = 24	[Ar] 3d⁴ 4s²	[Ar] 3d⁵ 4s¹
Cu, Z = 29	[Ar] 3d⁹ 4s²	[Ar] 3d¹⁰ 4s¹

The ground states are recorded in NIST's data for chromium and copper. Remember this pair explicitly. A half-filled or filled d subshell is a useful cue here, but it isn't a rule that predicts every exception across the periodic table.

Electron configuration worksheet

Use a blank sheet and keep the reference and worked examples available on your first attempt. For questions 1–9, include the total electron count with your answer. For question 10, count only the electrons in the 3p subshell. In repair questions, name the error before correcting it. Everything you need is above the worksheet.

#	Problem
1	Write full and noble-gas configurations for Si, Z = 14.
2	Write the configuration of S²⁻, Z = 16. Explain how it differs from the worked sulfur example.
3	A learner writes Ca²⁺ as [Ar] 4s². Diagnose and repair it.
4	Neutral Ti has Z = 22. Write its noble-gas configuration, then derive Ti²⁺.
5	An answer for Fe³⁺ reads [Ar] 3d³ 4s². Does it fail the electron-count check, the removal check, or both?
6	Repair neutral Cr written as [Ar] 3d⁴ 4s². Does your correction change the total?
7	Derive Cu⁺ from neutral Cu. State which subshell loses the electron.
8	A 2+ ion has configuration [Ar] 3d². Identify its element and atomic number.
9	Are [Ar] 4s² 3d² and [Ar] 3d² 4s² different occupancies? Count the electrons in each.
10	For sulfur's 3p⁴ subshell, a diagram shows [↑↓] [↑↓] [ ]. Repair the diagram and count unpaired electrons.

Answers, including the mistake to catch

1. Si: 14 electrons; 1s² 2s² 2p⁶ 3s² 3p², shortened to [Ne] 3s² 3p². After the ten-electron core and 3s², two remain. Don't fill 3p to capacity.
2. S²⁻: 18 electrons; [Ne] 3s² 3p⁶, equivalent to [Ar]. The negative charge adds two electrons to sulfur's 3p⁴. Subtracting two would reverse the meaning of the charge.
3. Ca²⁺: 18 electrons; [Ar]. The proposed answer contains 20 electrons and describes neutral calcium. Its unchanged 4s² exposes the missed ionization step.
4. Ti: [Ar] 3d² 4s²; Ti²⁺: [Ar] 3d². The neutral count is 18 + 2 + 2 = 22; the ion count is 20. Remove the two 4s electrons.
5. Only the removal check fails. The proposed count is 18 + 3 + 2 = 23, as required, but the electrons were removed from 3d while 4s stayed occupied. The answer is [Ar] 3d⁵.
6. Cr: [Ar] 3d⁵ 4s¹. Both versions total 24. Move one electron in your written assignment from 4s to 3d; don't add a twenty-fifth electron.
7. Cu⁺: [Ar] 3d¹⁰, with 29 − 1 = 28 electrons. Neutral Cu has one 4s electron; removing it leaves the filled 3d subshell. The NIST copper table confirms this under Cu II, its label for singly ionized copper.
8. Titanium, Z = 22. The ion contains 18 + 2 = 20 electrons. A 2+ charge means two fewer electrons than protons, so Z = 20 + 2. The electron total alone is insufficient to identify an element unless you know its charge.
9. Same occupancy; 22 electrons each. Both assign two electrons to 3d and two to 4s outside the argon core. Moving a whole term on the page doesn't move an electron between subshells.
10. [↑↓] [↑] [↑]: four electrons, two unpaired. Any of the three orbitals may hold the pair. The two single arrows must point the same way; drawing both downward is also valid. The proposed diagram has the right total and opposite spins within each pair, but leaves one 3p orbital empty while two are paired, contrary to Hund's rule.

Turn a missed answer into one useful review card

Before making a card, classify the mistake. Otherwise, you may memorize the corrected configuration without fixing the step that produced it.

What went wrong	Focused card prompt	Answer to recall
Charge changed the count the wrong way	Sulfur has Z = 16. How many electrons does S²⁻ have?	18; the 2− charge means two added electrons.
A subshell was filled past the budget	After [Ne] 3s², how many electrons remain for neutral Si?	Two, giving 3p².
The wrong subshell lost electrons	In these first-row transition-metal cations, which goes first: 4s or 3d?	4s.
An exception was missed	Neutral Cr: ground-state occupancy outside [Ar]?	3d⁵ 4s¹.
Electrons were paired too soon	Four electrons in three equal-energy p orbitals: how many remain unpaired?	Two; one pair and two parallel-spin single electrons.

For neutral-atom recall, Nibomo's electron configuration flashcards provide a separate practice set. Keep written ion problems alongside it: recalling an atom's configuration and deriving an ion are different tasks.

On your next attempt, redo the questions you missed without looking at the reference, worked examples, or cards. Write one sentence explaining each correction: “The count was right, but I removed 3d electrons before 4s” tells you much more than “I got iron wrong.”