Full Electron Configuration: How to Find It

A full electron configuration shows how an atom’s electrons are distributed among its energy levels and subshells. Use the element’s atomic number, then fill subshells in the standard order; for ions, adjust the electron count before filling. You will get a notation such as 1s² 2s² 2p⁶.

What You’ll Need

A periodic table that gives each element’s atomic number.
A way to keep track of subshell capacities and the filling order. This guide includes both, so no calculator is required.
The element symbol and, if relevant, the charge of the ion.

Step-by-Step

Find the atomic number. The atomic number, Z, is the number of protons. A neutral atom has the same number of electrons as protons, so its electron count equals Z. For example, neutral chlorine has Z = 17 and therefore 17 electrons.
Adjust for the charge if you have an ion. A positive charge means electrons have been removed; a negative charge means electrons have been added. Subtract the positive charge from Z or add the magnitude of a negative charge. For example, Cl⁻ has 17 + 1 = 18 electrons. Fe²⁺ has 26 − 2 = 24 electrons. Use the resulting electron count for the remaining steps.
Use the subshell filling order. Add electrons to subshells in this order, stopping when you have placed the required number: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. This is the Aufbau, or building-up, order used for ground-state configurations. It does not simply go through every subshell of one principal energy level before starting the next.
Fill each subshell to its capacity. An s subshell holds up to 2 electrons, p holds 6, d holds 10, and f holds 14. Write the subshell label followed by a superscript showing how many electrons it contains. Fill in order; the final subshell may be only partly full. For 17 electrons, the count proceeds as 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), then 3p⁵ (17).
Check that the electron total is correct. Add the superscripts. For neutral atoms the sum must equal the atomic number; for ions it must equal the adjusted count. The chlorine example gives 2 + 2 + 6 + 2 + 5 = 17, so its full configuration is 1s² 2s² 2p⁶ 3s² 3p⁵.
Check whether the element is an exception. The simple filling order predicts many ground-state configurations correctly, but a few neutral atoms have slightly different arrangements because nearby subshell energies are close. Common examples include chromium, whose configuration is [Ar] 3d⁵ 4s¹, and copper, [Ar] 3d¹⁰ 4s¹. If your result is for one of these elements or another known exception, check a reliable periodic-table reference for its observed ground state.
Write the full form, expanding any shorthand. If you used a noble-gas core to work quickly, replace it with the configuration of that noble gas and retain the remaining subshells. For iron, [Ar] 3d⁶ 4s² expands to 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶. The order in which 4s and 3d are written can vary by convention; the electron count and subshell occupancies are what matter.
For transition-metal ions, remove electrons from the right place. First form the neutral atom’s configuration, then remove electrons from the highest principal energy level, n. For transition metals this usually means removing 4s electrons before 3d electrons, even though 4s fills first in the basic Aufbau sequence. Thus neutral iron is [Ar] 3d⁶ 4s², while Fe²⁺ is [Ar] 3d⁶. Expand [Ar] if the question asks for the full configuration.

Common Mistakes

Using atomic number as the electron count for every species. That works only for neutral atoms. Add electrons for anions and remove them for cations.
Filling subshells in the wrong sequence. In particular, 4s fills before 3d in the standard introductory filling order. Follow the sequence rather than assuming the principal number alone decides what comes next.
Exceeding a subshell’s capacity. A p subshell cannot have more than 6 electrons, a d more than 10, or an f more than 14. If it does, recalculate the count and filling sequence.
Leaving noble-gas shorthand in a full configuration. Brackets such as [Ne] or [Ar] are convenient abbreviations, not the expanded form. Replace the bracketed core when the question specifically says “full.”
Applying the neutral-atom order unchanged to a transition-metal ion. For common transition-metal cations, remove the outer s electrons before d electrons. Otherwise the electron count or resulting ion configuration will be wrong.
Treating the basic filling rule as exception-free. The Aufbau order is a useful guide, but it does not capture every observed ground-state arrangement. Verify known exceptions when precision matters.

Tips

Keep a running total beside each subshell. This makes it easier to spot where you reached the target electron count.
Memorize the capacities as 2, 6, 10, and 14 for s, p, d, and f. The general rule is 2(2l + 1), where l identifies the subshell, but the capacities are usually faster to use.
Use a diagonal-rule chart as a memory aid for the filling order, but check that the chart includes all subshells through 7p if you are working with heavy elements.
Write superscripts clearly and distinguish them from the leading principal number: in 3p⁵, the 3 identifies the energy level and the 5 is the electron count.
When comparing answers, do not assume that different ordering of occupied subshells always means a different configuration. Focus on the number of electrons in each subshell and the species being described.

Troubleshooting

If your superscripts add to more or fewer electrons than expected, start again from the atomic number and charge. Write the target count at the top, then update a running total after filling each subshell. If a subshell is over capacity, you may have skipped an earlier subshell or used an incorrect sequence.

If your answer disagrees with a reference for a neutral transition element, check whether it is a known ground-state exception rather than forcing the simple rule to match. If you are working with an ion, verify that the charge sign was interpreted correctly and that transition-metal electrons were removed from the outer s subshell first. Also confirm whether the problem asks for a ground-state configuration: excited states can have different arrangements.

Frequently Asked Questions

What does “full electron configuration” mean?
It is the expanded subshell notation listing occupied subshells and their electron counts, without replacing inner electrons with noble-gas shorthand.

How do I find the number of electrons in an ion?
Start with the atomic number. Subtract the charge for a positive ion, or add the magnitude of the charge for a negative ion.

Why does 4s fill before 3d?
In the usual Aufbau sequence for filling neutral atoms, 4s is occupied before 3d. Subshell energies are close and can shift with electron occupancy, which is why ion removal and some ground-state exceptions need extra care.

Should I write 4s before 3d?
Either ordering may be seen in references. Use the convention requested by your class or source, and make sure each subshell has the correct occupancy.

Do electrons in a subshell all have the same spin?
No. When describing orbitals in more detail, the Pauli exclusion principle and Hund’s rule determine how electrons occupy individual orbitals. A basic full configuration gives subshell totals, not individual orbital diagrams.

Can this method give an excited-state configuration?
The standard filling method gives the usual ground-state configuration, with exceptions checked separately. An excited-state configuration requires information about the particular excitation.

General steps for current mainstream versions; menus can differ by device and version. See the disclaimer or report an error.