ProtactiniumElectron Configuration, Bohr Model, Valence Electrons & Orbital Diagram

The electron configuration of Protactinium is written as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f¹⁴ 5d¹⁰ 6s² 6p⁶ 5f² 6d¹ 7s². Applying the Aufbau principle — filling orbitals from lowest to highest energy — plus the Pauli Exclusion Principle and Hund's Rule, we systematically place all 91 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f¹⁴ 5d¹⁰ 6s² 6p⁶ 5f² 6d¹ 7s². Protactinium fills f-orbitals — seven orbitals accommodating up to 14 electrons — that are energetically shielded by outer s and d electrons, which explains why lanthanide and actinide elements have such similar surface chemistry despite differing nuclear charges.

Protactinium follows the standard Aufbau filling order without exception. The noble gas shorthand [Rn] 5f² 6d¹ 7s² replaces the inner-shell electrons with the symbol of the preceding noble gas, highlighting that only the outer electrons — 5f² 6d¹ 7s² — are chemically active. Note: for Period 4+ elements, the 4s orbital fills before 3d per Madelung's rule, even though 3d ends at a lower energy in the final atom.

Shell-by-shell, Protactinium's 91 electrons are distributed as: K-shell (n=1): 2 electrons; L-shell (n=2): 8 electrons; M-shell (n=3): 18 electrons; N-shell (n=4): 32 electrons; O-shell (n=5): 20 electrons; P-shell (n=6): 9 electrons; Q-shell (n=7): 2 electrons. The Q-shell (n=7) is the valence shell, containing 5 electrons.

Chemically, this configuration places Protactinium in Group 3 with oxidation states of 5, 4. This configuration directly predicts Protactinium's bonding mode, reactivity toward oxidizing and reducing agents, and the stoichiometry of its most common compounds.

The SPDF orbital model describes Protactinium's electrons not as planetary orbits but as three-dimensional probability clouds — each orbital a region of space where an electron is most likely to be found. Protactinium's 91 electrons occupy 18 distinct subshells: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f¹⁴ 5d¹⁰ 6s² 6p⁶ 5f² 6d¹ 7s², governed by three quantum mechanical rules.

The Pauli Exclusion Principle ensures no two electrons in Protactinium share the same four quantum numbers (n, l, m_l, m_s). This is why the 1s orbital holds only 2 electrons, the full p-subshell holds 6, d holds 10, and f holds 14. Without this rule, all 91 electrons would collapse into the 1s orbital. In Protactinium, Hund's Rule applies to seven f-orbitals — each occupied singly before pairing. The energetic near-degeneracy of 4f/5d/6s (or 5f/6d/7s) orbitals means minor perturbations determine the exact filling order, causing the configurational complexity of f-block elements.

Following standard orbital filling, Protactinium fills orbitals in the sequence: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p. The final electron enters the 7s² subshell, making Protactinium a f-block element with 5 valence electrons in Group 3.

The outermost electrons — 7s² — are Protactinium's chemical agents. Understanding the 7s² occupancy — how many electrons, whether paired or unpaired, the orbital shape involved — is the foundation for predicting Protactinium's bonding geometry, oxidation behavior, and compound formation.

Protactinium's chemical reactivity is shaped by three interlocking properties: electronegativity (1.5 Pauling), first ionization energy (5.89 eV), and electron affinity (0.55 eV). Its electronegativity is low-to-moderate (1.5) — predominantly metallic character, electropositive tendency. This mid-scale electronegativity enables Protactinium to participate in both polar covalent and ionic bonding depending on its partner.

The first ionization energy of 5.89 eV is relatively low, confirming Protactinium's readiness to lose electrons — a quintessentially metallic trait. The electron affinity of 0.55 eV represents the energy released when Protactinium gains one electron, indicating a meaningful but moderate acceptance of electrons.

In standard chemical conditions, Protactinium forms predominantly +5 oxidation state compounds, consistent with its 5 valence electrons and f-block character.

Placing Protactinium between Thorium (Z=90) and Uranium (Z=92) reveals the incremental property changes that make the periodic table a predictive tool.

Thorium → Protactinium: adding one proton and one electron increases nuclear charge by 1. Valence electrons shift from 4 to 5 (Group 3 → Group 3). Electronegativity: 1.3 → 1.5 | Ionization energy: 6.307 → 5.89 eV. Atomic radius decreases from 206 pm to 200 pm, consistent with increasing nuclear pull across a period.

Protactinium → Uranium: the additional proton and electron in Uranium changes the valence electron count from 5 to 6, crossing from Group 3 to Group 3. Both elements share Actinide character, with Uranium exhibiting slightly different electronegativity. These comparisons confirm that Protactinium sits at a well-defined chemical inflection point in the periodic table.