chemistry

Electron Configuration

Electron configuration describes how an atom’s electrons are arranged in shells, subshells and orbitals. The arrangement is predicted using the aufbau principle, Hund’s rule and the Pauli exclusion principle, although a few elements have exceptions because some arrangements are especially stable.

Visual note, a 3:28 episode with transcript, a mind map, 12 flashcards and a 5 question quiz.

Electron Configuration at a glance

Visual note summarising electron configuration

Listen: Electron Configuration

Two hosts talking it through, 3:28. The full transcript is below, so you can read along or skip the audio.

Transcript

AnnaWhat does electron configuration actually describe?

MarcoIt describes how an atom’s electrons are arranged in shells, subshells, and orbitals. Shells are energy levels, numbered one, two, three, and so on. Each shell contains subshells called s, p, d, and f.

AnnaDo all shells contain all four subshells?

MarcoNo. The first shell has only s. The second has s and p. The third has s, p, and d. The fourth has s, p, d, and f. Each subshell is divided into orbitals. An s subshell has one orbital, p has three, d has five, and f has seven.

AnnaAnd each orbital can hold two electrons, right?

MarcoExactly, and the electrons must have opposite spins. So the capacities are s squared, p to the sixth power, d to the tenth power, and f to the fourteenth power. That restriction comes from the Pauli exclusion principle.

AnnaWhat are the other rules for filling orbitals?

MarcoThe aufbau principle says electrons occupy available orbitals in order of increasing energy. The commonly used order starts one s, two s, two p, three s, three p, four s, three d, and four p. It continues through the higher subshells. The order is based on orbital energies, not simply shell numbers.

AnnaWhere does Hund’s rule fit in?

MarcoOrbitals with equal energy are filled singly before any pairing happens. In a p cubed subshell, for example, three electrons occupy the three p orbitals separately, with parallel spins. They do not pair in one orbital first.

AnnaHow would we write a configuration from scratch?

MarcoFirst find the atomic number. For a neutral atom, that is also the number of electrons. Then fill the orbitals in aufbau order, follow the capacity limits, and apply Hund’s rule. The superscripts show the number of electrons in each subshell, and they must add to the total.

AnnaCan we test that with oxygen?

MarcoOxygen has atomic number eight, so it has eight electrons. Its configuration is one s squared, two s squared, two p to the fourth power. In that two p subshell, the four electrons form one pair and two single electrons across the three orbitals.

AnnaWhat about larger atoms?

MarcoA preceding noble gas can be used as shorthand. Chlorine is one s squared, two s squared, two p to the sixth, three s squared, three p to the fifth. Or it is written as neon in brackets, three s squared, three p to the fifth.

AnnaAre transition elements different?

MarcoThe four s orbital is generally filled before three d in a neutral atom. Iron can be written as argon in brackets, four s squared, three d to the sixth. But when iron forms iron two plus, electrons are removed from four s first, giving argon in brackets, three d to the sixth.

AnnaAre there exceptions to the filling order?

MarcoYes. Nearby subshell energies can be very similar. Chromium is argon in brackets, three d to the fifth, four s to the first, rather than the simple prediction. Copper is argon in brackets, three d to the tenth, four s to the first. Half-filled or completely filled d subshells can be especially stable.

AnnaAnd for ions, we should check the electron count carefully?

MarcoAlways. Add electrons for a negative ion and remove them for a positive ion. Oxygen two minus has ten electrons, so its configuration is one s squared, two s squared, two p to the sixth, the same as neon. For transition-metal positive ions, remove four s electrons before three d electrons.

The notes

Shells, subshells and orbitals

Electrons occupy energy levels called shells. Shells are numbered 1, 2, 3 and so on. Each shell contains subshells, named s, p, d and f. The first shell has only an s subshell, the second has s and p, the third has s, p and d, and the fourth has s, p, d and f.

A subshell is divided into orbitals. An s subshell contains one orbital, a p subshell contains three orbitals, a d subshell contains five orbitals and an f subshell contains seven orbitals. Each orbital can hold a maximum of two electrons, so the maximum capacities are s2, p6, d10 and f14.

The three rules for filling orbitals

The aufbau principle states that electrons occupy the available orbitals in order of increasing energy. A commonly used filling order is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d and 7p. The order is based on orbital energies in many-electron atoms, not simply on shell number.

Hund’s rule states that orbitals of equal energy are occupied singly before any pairing occurs. For example, the three electrons in a p3 subshell occupy the three p orbitals separately, with parallel spins, rather than pairing in one orbital. The Pauli exclusion principle states that an orbital can contain at most two electrons, and the two electrons must have opposite spins. No two electrons in one atom can have the same set of four quantum numbers.

How to write an electron configuration

To write a configuration for an element, first find its atomic number. This equals the number of electrons in a neutral atom. Fill orbitals in the aufbau order, obeying the maximum capacity of each subshell and applying Hund’s rule within p, d and f subshells. The superscripts show how many electrons are in each subshell, and the superscripts must add to the total number of electrons.

For example, oxygen has atomic number 8, so it has eight electrons. Its configuration is 1s2 2s2 2p4. The 2p subshell contains three orbitals. Four electrons occupy these as one pair and two single electrons, rather than pairing in two orbitals while leaving the third empty.

For larger atoms, a noble gas can be used as a shorthand. Chlorine has the full configuration 1s2 2s2 2p6 3s2 3p5, or the abbreviated configuration [Ne] 3s2 3p5. The symbol in brackets represents the electron configuration of the preceding noble gas.

Orbital diagrams and transition elements

An orbital diagram represents each orbital by a box or line and each electron by an arrow. Two arrows in the same orbital must point in opposite directions because of the Pauli exclusion principle. In a set of equal-energy orbitals, such as the three p orbitals, single arrows are placed in separate orbitals before arrows are paired.

For transition elements, the 4s orbital is generally filled before the 3d orbital when building the neutral atom. For example, iron is written as [Ar] 4s2 3d6, although [Ar] 3d6 4s2 is also commonly seen. When transition elements form positive ions, electrons are usually removed from the outer s orbital before the d orbital. Thus Fe2+ is [Ar] 3d6, not [Ar] 4s2 3d4.

Common exceptions to the filling order

Some atoms have configurations that differ from the simple aufbau prediction because the energies of nearby subshells are very similar. A half-filled d subshell and a completely filled d subshell can provide extra stability. The most commonly taught exceptions are chromium, which is [Ar] 3d5 4s1 rather than [Ar] 3d4 4s2, and copper, which is [Ar] 3d10 4s1 rather than [Ar] 3d9 4s2.

Other transition elements also have exceptions, especially among the heavier elements. Examples include molybdenum, [Kr] 4d5 5s1, silver, [Kr] 4d10 5s1, and gold, [Xe] 4f14 5d10 6s1. Lists of exceptions can vary in detail because orbital energies depend on the atom and on whether it is neutral or ionised. The safest method is to learn the common examples required by the course and check unusual cases against reliable data.

Configurations of ions

For a negative ion, add electrons to the neutral atom until the ion’s total charge is obtained. For example, oxygen has eight electrons, so O2− has ten electrons and the configuration 1s2 2s2 2p6, which is the same as [Ne].

For a positive ion, remove electrons from the neutral atom. In main group elements, this often means removing electrons from the highest occupied shell. In transition elements, remove 4s electrons before 3d electrons, even though 4s was filled first in the neutral atom. Always check that the number of electrons in the written configuration matches the atomic number adjusted for the ion’s charge.

What to remember

The aufbau principle says that electrons fill orbitals in order of increasing energy.
Hund’s rule says that equal-energy orbitals fill singly before electrons pair.
The Pauli exclusion principle limits each orbital to two electrons with opposite spins.
The subshell capacities are s2, p6, d10 and f14.
A neutral atom has the same number of electrons as its atomic number.
The common configurations of chromium and copper are [Ar] 3d5 4s1 and [Ar] 3d10 4s1.
When transition-metal cations form, 4s electrons are removed before 3d electrons.