MCAT topics → Atomic Structure and Nuclear Phenomena → Electron Configuration and Aufbau Principle

Electron Configuration and Aufbau Principle

MCAT trap: Applies simple n-ordering and misses that 4s fills before 3d in the Aufbau sequence. 4s fills before 3d in neutral atoms because its effective energy is lower, though 3d electrons are lost first when forming cations.

Electron configuration is tested on the MCAT at multiple levels — from straight recall of the Aufbau filling order to writing configurations for transition metal cations and predicting magnetic behavior in passage-based questions. The most common error: students carry over "4s fills before 3d" into cation formation and remove 3d electrons first during ionization. That's wrong — in cations, 4s electrons are removed first, not 3d. The three core rules — Aufbau filling order, Pauli exclusion principle, and Hund's rule — work together to determine ground-state configuration for any atom or ion.

What makes this topic tricky isn't memorizing the rules — it's knowing when they interact in non-obvious ways. The 4s/3d energy relationship flips depending on whether you're talking about a neutral atom or a cation, and the Cr and Cu exceptions also catch students off guard because they look like violations of Aufbau when they're actually a consequence of half-filled and fully-filled d-subshell stability.

On the MCAT, electron configuration questions usually aren't isolated — they show up embedded in passages about enzyme active sites, redox chemistry, or coordination compounds, where you need to quickly determine whether a metal ion is paramagnetic or diamagnetic. That means you need to be able to write transition metal cation configurations fast and accurately, without second-guessing yourself on the 4s-vs-3d removal order. Getting the rules solid now saves you a lot of time under pressure.

Common misconceptions

Common mistake
Wrong: 3d fills before 4s because n = 3 is lower than n = 4.
Right: 4s fills before 3d in neutral atoms because its effective energy is lower, though 3d electrons are lost first when forming cations.
The n = 3 shell is lower than n = 4 in principal quantum number, but orbital energy isn't determined by n alone — effective nuclear charge and electron-electron repulsion both matter. In neutral atoms, 4s sits lower in energy than 3d due to greater penetration of the nucleus, so 4s fills first in the Aufbau sequence. However, once you form a cation and remove electrons, the relative energies shift and 3d drops below 4s, which is why 4s electrons are always removed first during ionization. Keep these two scenarios — neutral atom filling vs. cation formation — mentally separate.
Common mistake
Wrong: Electrons pair up in the first available orbital before occupying empty orbitals of the same subshell.
Right: Hund's rule requires one electron in each degenerate orbital before any pairing occurs, maximizing unpaired spins.
Pairing two electrons in one orbital forces them into the same small region of space, which costs energy due to electron-electron repulsion. Hund's rule exists precisely to minimize that cost: every degenerate orbital in a subshell must receive one electron before any orbital gets a second. So when filling p or d orbitals, spread electrons out first with parallel spins, then start pairing. A student who pairs immediately is essentially ignoring that repulsion cost and will get magnetic behavior questions wrong as a result.
Common mistake
Wrong: Cr is [Ar] 3d⁴ 4s² and Cu is [Ar] 3d⁹ 4s² following normal Aufbau filling.
Right: Cr is [Ar] 3d⁵ 4s¹ and Cu is [Ar] 3d¹⁰ 4s¹ because half-filled and fully filled d subshells confer extra stability.
Strict Aufbau predicts Cr as [Ar] 3d⁴ 4s² and Cu as [Ar] 3d⁹ 4s², but the actual configurations are [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹. The reason is that a exactly half-filled d subshell (d⁵) and a completely filled d subshell (d¹⁰) have extra stability from exchange energy and symmetry — stable enough that it's energetically favorable to 'borrow' one electron from 4s to achieve those configurations. These are the two exceptions you need to know for the MCAT; don't try to generalize this rule to other elements.
Common mistake
Wrong: When a transition metal loses electrons to form a cation, 3d electrons are removed first.
Right: 4s electrons are removed before 3d electrons when transition metals are ionized, because 4s is higher in energy in the presence of the nuclear charge experienced by cations.
This is the most common error on transition metal ion questions. When writing the neutral atom, 4s fills before 3d — that part is correct. But ionization doesn't simply reverse the filling order. In cations, the higher nuclear charge experienced by remaining electrons stabilizes 3d more than 4s, so 4s becomes the highest-energy occupied subshell and is removed first. Concretely: Fe is [Ar] 3d⁶ 4s², Fe²⁺ is [Ar] 3d⁶ (both 4s electrons gone), and Fe³⁺ is [Ar] 3d⁵. If you mistakenly remove 3d electrons first, you'll get the wrong unpaired electron count and the wrong magnetic prediction every time.
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What the exam tests

  1. Know the Aufbau filling sequence, Pauli exclusion principle, and Hund's rule well enough to apply all three simultaneously when building orbital diagrams.
  2. Write correct ground-state electron configurations for both neutral atoms and ions, using either full notation or noble gas shorthand.
  3. Recognize that Cr ([Ar] 3d⁵ 4s¹) and Cu ([Ar] 3d¹⁰ 4s¹) are exceptions to standard Aufbau filling because half-filled and fully filled d subshells are especially stable.
  4. Determine whether a transition metal ion is paramagnetic or diamagnetic by correctly writing its electron configuration — remembering that 4s electrons are removed before 3d when forming cations.

Can you avoid these mistakes?

Write the full ground-state electron configuration for Fe²⁺ and determine whether it is paramagnetic or diamagnetic. How many unpaired electrons does it have?
A student writes the electron configuration of Cr as [Ar] 3d⁴ 4s². What is wrong with this, and what is the correct configuration? What principle explains the actual configuration?
You are filling the 2p subshell with 4 electrons. Draw the orbital diagram. Which orbitals are paired and which are singly occupied? Which rule governs your answer?
Manganese (Mn, Z = 25) forms a Mn²⁺ ion in solution. Write the electron configuration of Mn²⁺ using noble gas shorthand and predict whether it is paramagnetic or diamagnetic, explaining which electrons were removed first and why.

Related topics

Quantum Numbers and Atomic Orbitals Subatomic Particles and Isotopes Photoelectric Effect and Bohr Model Periodic Trends (Atomic Radius, IE, EA, Electronegativity)

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