How Electron Configuration Works

Quick Answer

An electron configuration says which orbitals an atom's electrons occupy. Electrons fill in Madelung order, lowest n plus l first, which gives 1s, 2s, 2p, 3s, 3p, 4s, 3d and onward. Iron is therefore 1s2 2s2 2p6 3s2 3p6 4s2 3d6, or [Ar] 4s2 3d6 in noble gas shorthand. Two things trip people up. Ions lose electrons from the outermost shell first, so Fe2+ is [Ar] 3d6 rather than [Ar] 4s2 3d4. And twenty ground states, chromium and copper among them, break the aufbau order to buy a half-filled or filled subshell. Build any element here and watch the electrons land in order.

How Do You Read An Electron Configuration?

Each term carries three facts. The number is the principal shell, the letter is the subshell, and the superscript is how many electrons sit in it. So 3p6 means six electrons in the p subshell of the third shell. Read the terms left to right and you have read the order they filled, which is why the written string and the orbital diagram carry the same information.

• The s subshell holds 2 electrons, p holds 6, d holds 10 and f holds 14.
• The last terms are the ones that bond, therefore a noble gas configuration written as shorthand keeps only those.
• The shell picture and the orbital diagram show the same electron shells two ways, so watch both fill together.

Why Does 4s Fill Before 3d?

Energy depends on the shell number and the subshell shape together, not on the shell number alone. Madelung order ranks subshells by n plus l, with the lower n winning a tie. That puts 4s at 4 and 3d at 5, therefore 4s fills first. The two sit close in energy, which matters again as soon as you make an ion.

• For example, potassium is 4s1 rather than 3d1, even though the third shell has room for ten more electrons.
• The same rule puts 6s before 4f, which is where the lanthanides come from.

How Do You Find The Electron Configuration Of An Ion?

Atoms vs ions is where most marks are lost. The electron configuration of ions follows a different rule from filling: electrons are removed from the outermost shell first, not by running the filling order backwards. Iron fills 4s then 3d, however Fe2+ loses both 4s electrons and keeps all six 3d ones. For example, set the charge slider to plus two and watch which bar empties.

• Fe2+ is [Ar] 3d6. Writing [Ar] 4s2 3d4 is the classic error.
• Anions are simpler: the extra electrons just continue down the filling order.
• An ion that reaches a noble gas configuration is called isoelectronic with it, therefore chloride is written [Ar].

Which Elements Break The Rule?

Twenty ground states do not match aufbau, and every one of them buys a half-filled or completely filled subshell. Chromium moves an electron from 4s to 3d to reach 3d5. Copper does the same to reach 3d10. However, these are measured ground states rather than a pattern you can derive, so they have to be learned or looked up.

• Chromium: 4s1 3d5, not 4s2 3d4.
• Copper: 4s1 3d10, not 4s2 3d9.
• Palladium goes furthest and empties 5s completely, ending at 4d10.

Which Electrons Actually Bond?

The valence electrons, meaning the ones in the outermost occupied shell. Chlorine has seven and forms one bond to complete an octet. Calcium has two and loses both. A transition metal is the interesting case: iron has two valence electrons in 4s, but its incomplete 3d subshell joins in as well, therefore iron shows several oxidation states rather than one. Used as a valence electrons calculator, the readout above the orbital diagram gives that count directly.

• The valence shell is drawn highlighted on both the shell picture and the orbital diagram, so the electrons that bond are never buried in the middle of a string.
• However, for a transition metal read the d count as well, because those electrons bond too.

Where Does This Get Used?

Atomic structure is the foundation the rest of chemistry sits on, and predicting the ion an element forms is the everyday use, because that is what a formula needs. Sodium has one valence electron, loses it, and becomes Na+, so the formula with chloride is NaCl. Configuration also explains why transition metals have several oxidation states and why some ions are coloured. For an exam, the practical use is checking a configuration before answering anything on periodic trends.

• Main group elements gain or lose electrons to reach the nearest noble gas count.
• Unpaired electrons make a species paramagnetic, which a magnetic measurement can check.

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