FermiumElectron Configuration, Bohr Model, Valence Electrons & Orbital Diagram

The electron configuration of Fermium is written as <strong>1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f¹⁴ 5d¹⁰ 6s² 6p⁶ 5f¹² 7s²</strong>. Applying the Aufbau principle — filling orbitals from lowest to highest energy — plus the Pauli Exclusion Principle and Hund's Rule, we systematically place all 100 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f¹⁴ 5d¹⁰ 6s² 6p⁶ 5f¹² 7s². Fermium 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.

Fermium follows the standard Aufbau filling order without exception. The noble gas shorthand <strong>[Rn] 5f¹² 7s²</strong> replaces the inner-shell electrons with the symbol of the preceding noble gas, highlighting that only the outer electrons — 5f¹² 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, Fermium's 100 electrons are distributed as: K-shell (n=1): <strong>2</strong> electrons; L-shell (n=2): <strong>8</strong> electrons; M-shell (n=3): <strong>18</strong> electrons; N-shell (n=4): <strong>32</strong> electrons; O-shell (n=5): <strong>30</strong> electrons; P-shell (n=6): <strong>8</strong> electrons; Q-shell (n=7): <strong>2</strong> electrons. The Q-shell (n=7) is the valence shell, containing 3 electrons.

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

The SPDF orbital model describes Fermium'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. Fermium's 100 electrons occupy 17 distinct subshells: <strong>1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f¹⁴ 5d¹⁰ 6s² 6p⁶ 5f¹² 7s²</strong>, governed by three quantum mechanical rules.

<strong>The Pauli Exclusion Principle</strong> ensures no two electrons in Fermium 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 100 electrons would collapse into the 1s orbital. <strong>In Fermium, 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.</strong>

Following standard orbital filling, Fermium 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 <strong>7s²</strong> subshell, making Fermium a f-block element with 3 valence electrons in Group 3.

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

Fermium's chemical reactivity is shaped by three interlocking properties: electronegativity (1.3 Pauling), first ionization energy (6.5 eV), and electron affinity (0 eV). Its electronegativity is low-to-moderate (1.3) — predominantly metallic character, electropositive tendency. Fermium donates electrons to partners rather than accepting them — the hallmark of electropositive metals.

The first ionization energy of 6.5 eV is relatively low, confirming Fermium's readiness to lose electrons — a quintessentially metallic trait.

In standard chemical conditions, Fermium forms predominantly +3 oxidation state compounds, consistent with its 3 valence electrons and f-block character.

Placing Fermium between Einsteinium (Z=99) and Mendelevium (Z=101) reveals the incremental property changes that make the periodic table a predictive tool.

Einsteinium → Fermium: adding one proton and one electron increases nuclear charge by 1. Valence electrons remain at 3 — both occupy Group 3. Electronegativity: 1.3 → 1.3 | Ionization energy: 6.42 → 6.5 eV. Atomic radius increases from 186 pm to 190 pm, consistent with descending a group with additional shells.

Fermium → Mendelevium: the additional proton and electron in Mendelevium maintains 3 valence electrons but shifts subshell occupancy. Both elements share Actinide character, with Mendelevium exhibiting slightly different electronegativity. These comparisons confirm that Fermium sits at a well-defined chemical inflection point in the periodic table.