MolDraw › Chemistry tools › Electron Configuration Calculator
Electron Configuration Calculator
Pick an element and optional ion charge to get the ground-state electron configuration in full and noble-gas (shorthand) notation, an orbital box diagram, electrons per shell and the number of unpaired electrons. Known exceptions such as Cr, Cu, Pd and Au are built in.
How to use the electron configuration calculator
- Type an element symbol (
Cu), name (copper) or atomic number (29). - For an ion, enter its charge:
2for Cu2+,-1for Cl−. - Read the noble-gas shorthand and the full configuration. Orbitals are listed in shell order (3d before 4s).
- Use the orbital diagram to count unpaired electrons and predict whether the atom or ion is paramagnetic.
Worked examples
Oxygen (Z = 8)
1s2 2s2 2p4 = [He] 2s2 2p4
Two unpaired 2p electrons (Hund's rule).
Copper (Z = 29), an exception
Aufbau predicts [Ar] 3d9 4s2
Actual: [Ar] 3d10 4s1
Cu2+ = [Ar] 3d9
Iron(III) ion, Fe3+
Fe = [Ar] 3d6 4s2; remove 2 × 4s then 1 × 3d
Fe3+ = [Ar] 3d5 (5 unpaired)
How electron configurations are built
Electrons fill orbitals in order of increasing energy (the Aufbau principle), following the Madelung (n + l) rule:
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p
Each orbital holds two electrons of opposite spin (Pauli exclusion), and electrons spread out singly across orbitals of the same subshell before pairing (Hund's rule). Capacities are s = 2, p = 6, d = 10 and f = 14.
Exceptions: Cr, Cu and others
About 20 elements have measured ground states that differ from the simple order, usually because a half-filled or completely filled d or f subshell is lower in energy. The common ones are Cr ([Ar] 3d5 4s1), Cu ([Ar] 3d10 4s1), Mo, Ag, Pd ([Kr] 4d10), Pt, Au, La, Ce, Gd and several actinides. The calculator uses the experimental (NIST) ground states for these.
Ions
Anions add electrons to the next available orbital (Cl− = [Ar]). Transition-metal cations lose their outer s electrons before d electrons, so Fe2+ is [Ar] 3d6, not [Ar] 3d4 4s2.
Explore trends across the table with the interactive periodic table.
Building teaching material? Draw molecules, orbitals and apparatus for your slides with the free MolDraw editor.
Open MolDraw editorFrequently asked questions
What is an electron configuration?
It lists how an atom's electrons are distributed among orbitals, for example 1s2 2s2 2p4 for oxygen. Superscripts give the number of electrons in each subshell.
What is noble gas notation?
A shorthand that replaces the core electrons with the preceding noble gas in brackets: sodium 1s2 2s2 2p6 3s1 becomes [Ne] 3s1.
Why is copper 3d10 4s1 and not 3d9 4s2?
A completely filled 3d subshell is especially stable, so one 4s electron moves into 3d. Chromium does the same to reach a half-filled 3d5.
Which electrons are removed first when forming a cation?
Electrons in the highest principal shell are removed first. For transition metals the 4s electrons go before the 3d electrons.
How do you find the number of valence electrons?
For main-group elements, count the electrons in the outermost shell (highest n). For example, [Ne] 3s2 3p5 for chlorine gives 7 valence electrons.
What is the electron configuration of Fe3+?
[Ar] 3d5. Iron is [Ar] 3d6 4s2, and the three electrons removed are the two 4s electrons and one 3d electron.
Related chemistry tools
Need to sketch molecules for a report? Draw chemical structures online with MolDraw, a free online ChemDraw alternative (see the ChemDraw vs MolDraw comparison).