For electron configuration practice, start by counting the electrons the species actually has. Then check where those electrons belong. A configuration can add up correctly and still describe the wrong arrangement, so you need both checks.
The problems below move from neutral atoms to ions, orbital diagrams, and answers that look convincing but are wrong. They’re designed for introductory chemistry review. Use a periodic table, write your attempt before reading the explanation, and keep a short note about the step you missed. Unless a question says otherwise, find the ground-state configuration of an isolated atom or ion.
A short reference before you start
The number in front of a subshell letter identifies its shell. The superscript gives its electron count: 3p4 means four electrons in the 3p subshell. An s subshell holds at most two electrons, p holds six, d holds ten, and f holds fourteen. A single orbital holds at most two, with opposite spins.
A common introductory filling sequence is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Within equal-energy orbitals, place electrons singly with parallel spins before pairing them. The sequence has exceptions, including neutral chromium and copper. For first-row transition-metal cations, remove 4s electrons before 3d electrons. See OpenStax’s explanation of atomic and ionic configurations for the underlying rules and diagrams.
Put three headings above your working: target count, arrangement, final count. For a neutral atom, the target is its atomic number. For an ion, subtract a positive charge or add the magnitude of a negative charge. Don’t change the number of protons when making an ion.
1. Sulfur: separate a shell number from an electron count
Problem: Sulfur has atomic number 16. Write its full configuration and noble-gas shorthand. How many electrons occupy its highest occupied principal shell?
Answer: The full configuration is 1s2 2s2 2p6 3s2 3p4. The shorthand is [Ne] 3s2 3p4. Count 2 + 2 + 6 + 2 + 4 = 16. The highest occupied shell is n = 3, containing two 3s electrons and four 3p electrons, for six total.
If you answered four, you counted only the final subshell. If you answered three, you read the shell label as a quantity of electrons. Those are different mistakes and need different reminders. Write your correction beside the part of the notation you misread rather than copying the entire answer again.
2. Calcium: check what the bracket replaces
Problem: Calcium has atomic number 20. A student writes [Ne] 4s2. Is this a complete shorthand configuration? Repair it if needed.
Answer: No. [Ne] stands for ten electrons, so the proposed expression accounts for only twelve. Calcium’s ground-state shorthand is [Ar] 4s2, totaling 18 + 2 = 20. Written fully, it is 1s2 2s2 2p6 3s2 3p6 4s2.
The bracket is an abbreviation for occupied subshells, not a decoration marking the previous row. You could write the longer equivalent [Ne] 3s2 3p6 4s2, but [Ar] is more compact. On a quiz, follow the requested format. A correct electron arrangement may still need expanding if the prompt asks for the full configuration.
3. Fluoride and magnesium: different elements, matching counts
Problem: Find the configurations of F– and Mg2+. Their atomic numbers are 9 and 12. Explain why the answers match without claiming the species are identical.
Answer: Fluoride has 9 + 1 = 10 electrons. Magnesium’s 2+ ion has 12 – 2 = 10. Both have 1s2 2s2 2p6, equivalent to [Ne]. These species are isoelectronic, meaning that they have the same electron count and, here, the same configuration.
They still have different nuclei and charges: fluoride has nine protons; magnesium has twelve. A matching configuration doesn’t turn either ion into a neon atom. If you get eight electrons for fluoride, pause at the sign. A negative ion has gained electrons. If you get fourteen for magnesium, you have reversed what a positive charge means.
4. Iron ions: count correctly, then remove from the right subshell
Problem: Neutral iron has atomic number 26 and configuration [Ar] 3d6 4s2. Write Fe2+ and Fe3+. A classmate proposes [Ar] 3d4 4s2 for Fe2+. Diagnose the error.
Answer: Fe2+ is [Ar] 3d6, with 24 electrons. Fe3+ is [Ar] 3d5, with 23. Remove the two 4s electrons first; the third removal comes from 3d.
Your classmate’s answer also totals 24, so a count-only check would miss the error. The problem is the occupied subshells. This is why reversing the written order of the neutral configuration isn’t a reliable ion-making method. Some notes list 4s before 3d, while others list 3d before 4s. Look at the subshell labels and occupations, not simply the last term on the page.
5. Chromium: an exception is not permission to move electrons anywhere
Problem: Chromium has atomic number 24. Compare [Ar] 3d4 4s2 with [Ar] 3d5 4s1. Which is its ground-state configuration? Does the electron total decide?
Answer: Chromium’s ground state is [Ar] 3d5 4s1. Both candidates total 24 electrons, so arithmetic alone cannot choose between them. This is a known exception to the simplest filling pattern.
Keep exceptions on a separate reference card with the actual species named. Don’t replace the filling rules with a slogan that every d subshell must become half full or full. That would lead you to alter correct configurations of other elements. Check which exceptions your course expects, and distinguish a remembered experimental configuration from a pattern you have guessed.
6. Oxygen: a correct configuration can hide a wrong orbital diagram
Problem: Oxygen has configuration 1s2 2s2 2p4. A student draws its three 2p boxes as [up/down] [up/down] [empty]. Is the electron count correct? Is the ground-state diagram correct?
Answer: The boxes contain four electrons, but they pair too early. A ground-state arrangement is [up/down] [up] [up]. The two singly occupied boxes have parallel spins. Oxygen therefore has two unpaired electrons in this orbital picture.
It doesn’t matter which equivalent p box contains the pair. A drawing with the pair in the middle is the same occupancy pattern. However, [up/up] in one box is a different error: two electrons sharing an orbital cannot have the same spin. Label an error as counting, early pairing, or same-orbital spin so you can fix the specific rule.
7. An eighteen-electron mystery: know when the information is insufficient
Problem: An unidentified species has configuration 1s2 2s2 2p6 3s2 3p6. Can you name its element from that information alone? What if the prompt adds that it is a 2+ ion?
Answer: The configuration contains eighteen electrons. Without a charge or nuclear identity, the element isn’t uniquely determined. Argon, chloride, and potassium’s 1+ ion are examples with that electron count. If the species is explicitly a 2+ ion, it must have twenty protons, so it is Ca2+.
Write the relationship as protons minus electrons equals signed charge. Here, protons minus 18 equals +2, giving 20 protons. The University of Calgary’s configuration practice also includes questions about what information is needed to identify a species. Treat missing information as something to explain, not an invitation to guess.
8. Aluminum: distinguish an allowed arrangement from the ground state
Problem: Aluminum has atomic number 13. Compare 1s2 2s2 2p6 3s2 3p1 with 1s2 2s2 2p6 3s1 3p2. Both have thirteen electrons. Must both be correct answers to a ground-state question?
Answer: No. The first is aluminum’s ground-state configuration. The second moves a 3s electron into 3p and describes an excited configuration rather than the requested ground state. Neither expression exceeds a subshell capacity, but that doesn’t make them interchangeable.
Use two questions when marking your work: is the notation physically allowable, and does it satisfy the state requested? A different proposal such as 2p7 would fail a capacity check immediately. Sorting those cases prevents you from calling every unfamiliar expression impossible or accepting every correct total as a ground state.
Turn your corrections into useful chemistry notes
Choose three missed problems and cover their solutions. For each, write one sentence beginning with the actual decision: count the charge, expand the bracket, remove 4s first, or check the orbital boxes. Then work the question again on a clean page. A copied answer doesn’t show whether you can make that decision yourself.
Keep your course’s notation alongside chemistry study documents. If you need to fit this exercise into a broader review session, use the chemistry exam preparation guide. When browsing shared notes, compare an ion example and an orbital diagram before relying on a file. One accurate neutral-atom example doesn’t verify every answer.
Students should upload their own notes to help other students. A short page containing your original examples, corrected counts, and explanations can be more useful than a long list of unexplained configurations. Share work you have permission to distribute and identify any sources you used.
Frequently asked questions
Do I add shell numbers when counting electrons?
No. Add the superscripts, plus the electrons represented by any noble-gas bracket. In 3p4, four is the electron count. Three labels the shell.
Is writing 4s before 3d automatically wrong?
No. Different listing conventions can show the same occupied subshells. Check the superscripts and your instructor’s format. For the iron ions above, 4s electrons are removed first regardless of listing order.
Should I memorize every exception?
Use your course requirements. Learn the assigned exceptions accurately, but don’t assume a short mnemonic predicts every element. When a permitted reference gives an observed configuration, use that evidence.
What should I do if my total is right but the answer is marked wrong?
Check the requested charge and state, subshell capacities, ion-removal step, and orbital pairing. Problem four is the clearest reminder that the correct number of electrons is only one part of the answer.