How to Use This Electron Configuration Calculator
- Neutral atoms: type a symbol, name, or atomic number from 1 to 118.
- Ions: add the charge after the symbol, such as Fe3+, Cu+, or N3-.
- Results: the full configuration, the noble-gas shorthand, an orbital diagram for the electrons beyond the noble-gas core, the number of unpaired electrons, and the period and block.
The Filling Order the Calculator Follows
Electrons occupy the lowest-energy subshells first, in this order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. Each s subshell holds 2 electrons, p holds 6, d holds 10, and f holds 14. Chlorine’s 17 electrons give 1s2 2s2 2p6 3s2 3p5, or [Ne] 3s2 3p5. The order also follows the periodic table: the s block, then the d block one shell lower, then the p block. The method is worked by hand in how to write electron configurations.
How to Write Electron Configurations for Ions
- Anions gain electrons in the next available subshell: O is [He] 2s2 2p4, so O2− is [He] 2s2 2p6, the same as neon.
- Cations lose electrons from the shell with the highest n first. For main-group metals that is simply the last subshell filled: Na+ is 1s2 2s2 2p6.
- Transition metals lose 4s before 3d, even though 4s filled first: Fe is [Ar] 4s2 3d6, Fe2+ is [Ar] 3d6, and Fe3+ is [Ar] 3d5.
Ions with the same configuration, such as O2−, F−, Na+, and Mg2+, are isoelectronic; the calculator names the noble gas they match. Why ions form the charges they do is covered in AP Chemistry ions and valence electrons.
How to Read the Orbital Diagram and Count Unpaired Electrons
Each box is one orbital: one box for s, three for p, five for d, and seven for f. Electrons spread out one per box before any pair up (Hund’s rule), and paired electrons point in opposite directions. Nitrogen’s 2p3 has three single electrons, so 3 unpaired; oxygen’s 2p4 has one pair and two singles, so 2 unpaired. Atoms and ions with unpaired electrons are paramagnetic. The same subshells appear as peaks in photoelectron spectroscopy, where peak height counts the electrons in each.
What the AP Exam Does Not Assess
The AP Chemistry course description lists two exclusions here: assigning quantum numbers to electrons, and writing configurations for elements that are exceptions to the filling order. When you enter an exception such as chromium or copper, the calculator gives the predicted configuration first and shows the measured one separately, so you can see the difference without studying it for the exam. What the exam does test is using configurations to explain periodic trends and ionization energy. To see how your Unit 1 practice scores feed an overall 1–5 estimate, use the AP Chem Score Calculator.
Related Resources
- AP Chem Score Calculator
- All AP Chemistry Calculators
- How to Write Electron Configurations
- AP Chemistry Photoelectron Spectroscopy
- AP Chemistry Valence Electrons
- AP Chemistry Periodic Trends
- Unit 1 Review: Atomic Structure and Properties
Frequently Asked Questions
How do you write an electron configuration?
Find the number of electrons (the atomic number for a neutral atom), then fill subshells in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p. An s subshell holds 2 electrons, p holds 6, d holds 10, and f holds 14. Iron, with 26 electrons, is 1s2 2s2 2p6 3s2 3p6 4s2 3d6.
How do you write the electron configuration of an ion?
For a negative ion, add electrons to the neutral configuration. For a positive ion, remove electrons from the shell with the highest n first, so a transition metal loses its 4s electrons before its 3d electrons: Fe is [Ar] 4s2 3d6, Fe2+ is [Ar] 3d6, and Fe3+ is [Ar] 3d5.
What is noble-gas notation?
A shorthand that replaces the inner electrons with the symbol of the previous noble gas in brackets. Sulfur, 1s2 2s2 2p6 3s2 3p4, becomes [Ne] 3s2 3p4.
Why are chromium and copper exceptions?
Their measured ground states, [Ar] 4s1 3d5 and [Ar] 4s1 3d10, differ from the filling-order prediction because the 4s and 3d energies are very close. The AP Chemistry course description states that configurations of these exceptions are not assessed, so the predicted configuration is the one to know.
How many unpaired electrons does an atom have?
Fill each orbital of a subshell with one electron before pairing any (Hund's rule), then count the single electrons. Oxygen, 2p4, has 2 unpaired electrons; Fe3+, 3d5, has 5. An atom or ion with unpaired electrons is paramagnetic.
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