Watch the electrons fill in the order they really go in.
Electron Configuration simulator
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Live measurements
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Formula
electrons = Z − charge
- e Electrons
- —
Observations
What's happening
Real-world examples
Iron
Iron fills 4s before 3d, ending at 4s2 3d6. For example, that is why the fourth shell is already occupied while the third is still eight electrons short of full.
The iron(II) ion
Fe2+ is [Ar] 3d6, not [Ar] 4s2 3d4. The 4s electrons leave first because they sit in the outer shell, even though they arrived before the 3d ones.
Chromium
Aufbau predicts 4s2 3d4. The real ground state is 4s1 3d5, because a half-filled d subshell costs less than the order predicts.
The chloride ion
Adding one electron to chlorine completes the third shell, giving exactly the argon configuration. That is what isoelectronic means, and it is why chloride is stable.
Uses of electron configuration
- Predicting the ion an element forms before writing a formula
- Explaining why transition metals have several oxidation states
- Checking a configuration before an exam question on periodic trends
electron configuration pitfalls
- Removing electrons from an ion in reverse aufbau order instead of from the outermost shell
- Applying the aufbau order to chromium and copper without checking the exceptions
- Confusing the filling order with the order the shells sit in
electron configuration questions (8)
What is an electron configuration?
An electron configuration lists which orbitals an atom's electrons occupy, in order, with the count in each. Iron is 1s2 2s2 2p6 3s2 3p6 4s2 3d6. The notation packs three facts into each term: the shell number, the subshell letter, and how many electrons are in it. Everything about how an element bonds follows from the last few terms.
What is the aufbau principle?
Aufbau means building up. Electrons occupy the lowest energy subshell available, and the order follows n plus l, with lower n breaking a tie. That gives 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p and onward. Notice that 4s comes before 3d, which is the step that surprises people most.
Why does 4s fill before 3d?
Because 4s has a lower n plus l value: 4 plus 0 is 4, while 3d is 3 plus 2, which is 5. Energy depends on both quantum numbers rather than on the shell number alone. The two subshells sit very close together, which is also why several elements swap an electron between them.
How do you write the electron configuration of an ion?
Start from the neutral atom, then remove electrons from the highest principal shell first. For iron that means taking the 4s electrons before the 3d ones, so Fe2+ is [Ar] 3d6. This is the step most people get wrong, because running the filling order backwards would wrongly give [Ar] 4s2 3d4. For anions, simply continue down the filling order.
Which elements break the aufbau rule?
Twenty ground states do, and chromium and copper are the two every course covers. Chromium is 4s1 3d5 rather than 4s2 3d4, and copper is 4s1 3d10 rather than 4s2 3d9. Each swap buys a half-filled or completely filled d subshell. Palladium goes further and empties its 5s entirely.
What are valence electrons?
Valence electrons are the ones in the outermost occupied shell, and they are the electrons that form bonds. Chlorine has seven, calcium has two. For a transition metal the picture is wider: iron has two in the 4s shell, but its incomplete 3d subshell takes part in bonding as well, which is why it shows several oxidation states.
What is noble gas shorthand?
Noble gas shorthand replaces the filled inner shells with the symbol of the noble gas that matches them. Iron becomes [Ar] 4s2 3d6 instead of the full string. It keeps the part that matters, because the electrons beyond the core are the ones that react. Writing a noble gas as its own core is circular, so the element itself keeps the full form.
How many electrons fit in each shell?
Electron shells hold up to 2n squared electrons: 2 in the first, 8 in the second, 18 in the third, 32 in the fourth. However, the outermost shell of a real atom never holds more than 8, because the next shell starts before the current one fills. That gap between capacity and occupancy is the whole reason the filling order looks strange.