CadmiumElectron Configuration, Bohr Model, Valence Electrons & Orbital Diagram

The electron configuration of Cadmium is written as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s². Applying the Aufbau principle — filling orbitals from lowest to highest energy — plus the Pauli Exclusion Principle and Hund's Rule, we systematically place all 48 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s². Transition metals like Cadmium are defined by d-orbital filling. The five d-orbitals can hold up to 10 electrons and are responsible for Cadmium's characteristic bonding behavior, colored compounds, and catalytic activity.

Cadmium follows the standard Aufbau filling order without exception. The noble gas shorthand [Kr] 4d¹⁰ 5s² replaces the inner-shell electrons with the symbol of the preceding noble gas, highlighting that only the outer electrons — 4d¹⁰ 5s² — 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, Cadmium's 48 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): 18 electrons; O-shell (n=5): 2 electrons. The O-shell (n=5) is the valence shell, containing 12 electrons.

Chemically, this configuration places Cadmium in Group 12 with oxidation states of 2. The partially (or fully) filled d-subshell is the source of Cadmium's variable valency, colored compounds, and catalytic behavior.

The SPDF orbital model describes Cadmium'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. Cadmium's 48 electrons occupy 10 distinct subshells: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s², governed by three quantum mechanical rules.

The Pauli Exclusion Principle ensures no two electrons in Cadmium 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 48 electrons would collapse into the 1s orbital. For Cadmium's d-electrons, Hund's Rule requires filling each of the five d-orbitals singly before pairing. This maximizes electron spin, producing Cadmium's characteristic magnetic moment and explaining its tendency toward specific oxidation states.

Following standard orbital filling, Cadmium 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 5s² subshell, making Cadmium a d-block element with 12 valence electrons in Group 12.

The outermost electrons — 5s² — are Cadmium's chemical agents. Understanding the 5s² occupancy — how many electrons, whether paired or unpaired, the orbital shape involved — is the foundation for predicting Cadmium's bonding geometry, oxidation behavior, and compound formation.

Cadmium's chemical reactivity is shaped by three interlocking properties: electronegativity (1.69 Pauling), first ionization energy (8.994 eV), and electron affinity (0 eV). Its electronegativity is low-to-moderate (1.69) — predominantly metallic character, electropositive tendency. This mid-scale electronegativity enables Cadmium to participate in both polar covalent and ionic bonding depending on its partner.

The first ionization energy of 8.994 eV sits in the moderate range, allowing some ionic character in the right partner combinations.

In standard chemical conditions, Cadmium forms predominantly +2 oxidation state compounds, consistent with its 12 valence electrons and d-block character.

Placing Cadmium between Silver (Z=47) and Indium (Z=49) reveals the incremental property changes that make the periodic table a predictive tool.

Silver → Cadmium: adding one proton and one electron increases nuclear charge by 1. Valence electrons shift from 11 to 12 (Group 11 → Group 12). Electronegativity: 1.93 → 1.69 | Ionization energy: 7.576 → 8.994 eV. Atomic radius decreases from 165 pm to 161 pm, consistent with increasing nuclear pull across a period.

Cadmium → Indium: the additional proton and electron in Indium changes the valence electron count from 12 to 3, crossing from Group 12 to Group 13. Both elements share Post-Transition Metal character, with Indium exhibiting slightly higher electronegativity. These comparisons confirm that Cadmium sits at a well-defined chemical inflection point in the periodic table.