ArsenicElectron Configuration, Bohr Model, Valence Electrons & Orbital Diagram

The electron configuration of Arsenic is written as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³. Applying the Aufbau principle — filling orbitals from lowest to highest energy — plus the Pauli Exclusion Principle and Hund's Rule, we systematically place all 33 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³. The p-subshell adds three dumbbell-shaped orbitals (p_x, p_y, p_z) that collectively hold up to 6 electrons. In Arsenic, these outermost p-orbitals are the seat of its chemical personality — more than half-filled, driving electron acceptance.

Arsenic follows the standard Aufbau filling order without exception. The noble gas shorthand [Ar] 3d¹⁰ 4s² 4p³ replaces the inner-shell electrons with the symbol of the preceding noble gas, highlighting that only the outer electrons — 3d¹⁰ 4s² 4p³ — 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, Arsenic's 33 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): 5 electrons. The N-shell (n=4) is the valence shell, containing 5 electrons.

Chemically, this configuration places Arsenic in Group 15 with oxidation states of 5, 3, -3. This configuration directly predicts Arsenic's bonding mode, reactivity toward oxidizing and reducing agents, and the stoichiometry of its most common compounds.

The SPDF orbital model describes Arsenic'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. Arsenic's 33 electrons occupy 8 distinct subshells: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³, governed by three quantum mechanical rules.

The Pauli Exclusion Principle ensures no two electrons in Arsenic 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 33 electrons would collapse into the 1s orbital. Hund's Rule of Maximum Multiplicity is critical in Arsenic's p-subshell: the three p-orbitals (p_x, p_y, p_z) must each receive one electron before any pairing occurs. This minimizes electron-electron repulsion and explains Arsenic's 2 paired and 1 empty p-orbital.

Following standard orbital filling, Arsenic 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 4p³ subshell, making Arsenic a p-block element with 5 valence electrons in Group 15.

The outermost electrons — 4p³ — are Arsenic's chemical agents. Understanding the 4p³ occupancy — how many electrons, whether paired or unpaired, the orbital shape involved — is the foundation for predicting Arsenic's bonding geometry, oxidation behavior, and compound formation.

Arsenic's chemical reactivity is shaped by three interlocking properties: electronegativity (2.18 Pauling), first ionization energy (9.815 eV), and electron affinity (0.814 eV). Its electronegativity is moderate (2.18) — capable of both polar covalent and some ionic bonding. This mid-scale electronegativity enables Arsenic to participate in both polar covalent and ionic bonding depending on its partner.

The first ionization energy of 9.815 eV sits in the moderate range, allowing some ionic character in the right partner combinations. The electron affinity of 0.814 eV represents the energy released when Arsenic gains one electron, indicating a meaningful but moderate acceptance of electrons.

In standard chemical conditions, Arsenic forms diverse compounds across multiple oxidation states, consistent with its 5 valence electrons and p-block character.

Placing Arsenic between Germanium (Z=32) and Selenium (Z=34) reveals the incremental property changes that make the periodic table a predictive tool.

Germanium → Arsenic: adding one proton and one electron increases nuclear charge by 1. Valence electrons shift from 4 to 5 (Group 14 → Group 15). Electronegativity: 2.01 → 2.18 | Ionization energy: 7.9 → 9.815 eV. Atomic radius decreases from 125 pm to 114 pm, consistent with increasing nuclear pull across a period.

Arsenic → Selenium: the additional proton and electron in Selenium changes the valence electron count from 5 to 6, crossing from Group 15 to Group 16. This boundary also marks a categorical transition from Metalloid to Nonmetal. These comparisons confirm that Arsenic sits at a well-defined chemical inflection point in the periodic table.