Electron configurations: count electrons and inspect pairing

MCAT content library / Chemistry & physics / Electron configurations: count electrons and inspect pairing

An arrangement can be allowed without being the ground state. Count electrons, check orbital occupancy, and then use pairing to make a magnetic prediction.

About 21 minutes, including the worked examples and checks.

Useful foundations

• Atomic identity and nuclear change: count what is conserved

On this page

• Count electrons before arranging them
• Separate a forbidden occupancy from a higher-energy one
• Worked application: ionization and excitation of the same element
• Connect pairing to qualitative magnetic response
• Write ions from their electron count and removal order
• Use orbitals without turning them into orbits
• Check your reasoning
• Continue learning
• Sources

Count electrons before arranging them

An electron configuration answers where electrons are assigned among atomic orbitals. It does not identify an isotope or describe a tiny planetary trajectory. Start with the electron count: a neutral atom has Z electrons; a cation has fewer and an anion more. Then ask which arrangement is allowed and whether it is a ground or excited state. AAMC category 4E links these ideas to magnetic behavior and the limits of atomic models.¹

The superscript in 2p⁴ means four electrons in the 2p subshell, not four p orbitals. The leading 2 is the principal quantum number n, identifying the shell. An s subshell has one orbital, p has three, d has five, and f has seven. Because each orbital holds at most two electrons, their respective capacities are 2, 6, 10, and 14.²

A shell can include several subshells: n = 2 contains 2s and 2p, whereas n = 1 contains only 1s. The maximum capacity of shell n is 2n², but filling successive atoms does not mean completing every possible orbital of one shell before starting the next. Orbital energies in many-electron atoms depend on electron interactions as well as n.

Separate a forbidden occupancy from a higher-energy one

Pauli exclusion means that two electrons in the same atom cannot share all four quantum numbers. In a box diagram, two arrows may occupy one orbital only if they represent opposite spins. Three arrows in one box or two arrows with the same spin label in one box are forbidden. Electrons in different orbitals may have the same spin label.²

The ground state is the lowest-energy allowed arrangement for that species. In an introductory filling model, lower-energy available orbitals fill first. Within equal-energy orbitals of one subshell, Hund’s rule gives separate occupancy with parallel spins before pairing. For example, three p electrons occupy three separate p orbitals in the ground-state pattern. A fourth must pair in one of them.³

Pairing two p electrons in one orbital while other equivalent p orbitals are empty can obey Pauli yet fail to represent the lowest-energy arrangement. Similarly, promoting an electron to a higher available orbital can create an excited configuration without changing the number of electrons. “Not the ground state” and “forbidden by Pauli” therefore require different explanations.

Worked application: ionization and excitation of the same element

The following cards and comparisons are original instructional constructions using beryllium, Z = 4. For the isolated neutral ground-state atom, 1s²2s² places four electrons in two paired orbitals. The diagram omits the filled 1s core so that changes in the outer electrons remain visible.

Card	Full configuration	Electron count	Unpaired electrons
A: neutral ground state	1s22s2	4	0
B: ground-state Be+	1s22s1	3	1
C: specified neutral excited arrangement	1s22s12p1	4	2

Going from A to B removes an electron. The nucleus remains Z = 4, so the three-electron species has charge +1. Going from A to C moves an electron into a higher subshell without removing it; C remains neutral. Both changes can require energy, so an energy input alone does not distinguish excitation from ionization. Detecting a released electron or determining the resulting charge would address that difference.

Card C specifies parallel spins in the two singly occupied outer orbitals. Other excited arrangements are possible, and not every excitation must change the number of unpaired electrons. The configuration and spin pattern actually supplied are the basis of the prediction.

Connect pairing to qualitative magnetic response

In the simple isolated-species treatment used here, unpaired electrons produce a paramagnetic contribution: attraction toward the stronger part of an applied magnetic field. When all electrons are paired, the species is diamagnetic and weakly repelled from the stronger-field region. These responses do not mean that the sample remains a permanent magnet when the external field is removed.⁴

The cards predict a closed-shell diamagnetic pattern for A and unpaired-electron paramagnetic patterns for B and the specified C. This is a prediction from electron occupancy, not a reported measurement. A response magnitude would also require information such as the amount of material, temperature, and the state populations. The number of arrows alone does not supply that quantitative force.

Magnetism also does not uniquely identify a configuration. A neutral nitrogen atom has three singly occupied 2p orbitals, while a neutral oxygen atom has four 2p electrons arranged as one pair and two singles. Their p-electron counts and their unpaired-electron counts are different quantities. A molecular or coordination environment can alter the relevant orbital pattern, so do not automatically transfer an isolated-atom prediction to every compound containing that element.

Write ions from their electron count and removal order

A noble-gas abbreviation replaces an already filled inner configuration: [Ne] represents ten electrons, 1s²2s²2p⁶. Thus [Ne]3s¹ has eleven electrons in total. If the nucleus has Z = 13, that configuration belongs to a 2+ ion rather than a neutral eleven-proton atom.³

For common transition-metal cations, outer ns electrons are removed before the (n − 1)d electrons. For example, starting from cobalt’s [Ar]3d⁷4s², removal of two electrons gives Co²⁺ as [Ar]3d⁷. A memorized neutral-atom filling sequence should not be mechanically reversed to remove d electrons first. Real ground configurations also contain exceptions to the simplest filling mnemonic; use a supplied configuration when it is given.

Use orbitals without turning them into orbits

The Bohr model captures discrete energies in hydrogen and hydrogen-like one-electron ions, but its fixed circular paths are not a general description of many-electron atoms. In quantum mechanics, an orbital is a wavefunction associated with a probability distribution for position. The uncertainty relation limits simultaneous sharp position and momentum; it is not merely a statement that current microscopes are insufficiently powerful.²

A box diagram deliberately compresses this model into occupancy and spin labels. It can reveal a wrong electron count, a Pauli violation, or an unpaired electron without claiming to draw the atom’s shape. Keep those representational limits attached to the conclusions the diagram can support.

Check your reasoning

Choose an answer before opening its feedback. Each option explains the reasoning behind that choice.

Continue learning

Related lessons

• Atomic identity and nuclear change: count what is conserved
• Electromagnetic light: wavelength, photons, and polarization

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Sources

1. AAMC: C/P content category 4E

2. OpenStax Chemistry 2e: Development of Quantum Theory

3. OpenStax Chemistry 2e: Electronic Structure of Atoms

4. OpenStax Chemistry 2e: Molecular Orbital Theory