Arsenic (As) Revision Guide

Atomic Model Details

Arsenic (As) is a p-block element belonging to Group 15 and Period 4 of the periodic table. It exhibits properties of metalloids.

Fundamental Particles

• Atomic Number (Z): 33
• Mass Number (A): Approximately 75 (for the most common isotope, Arsenic-75)
• Number of Protons: 33
• Number of Electrons: 33 (in a neutral atom)
• Number of Neutrons: 42 (for Arsenic-75, calculated as A - Z)

Atomic Size

• Covalent Radius: 119 pm
• Van der Waals Radius: 185 pm
• Metallic Radius: 124.5 pm

Electronic Configuration & Shell Diagram

Electronic Configuration

• Ground State Electronic Configuration: [Ar] 3d¹⁰ 4s² 4p³
• Orbital-wise Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³

Shell Diagram Representation

The electronic configuration describes how electrons are distributed in different energy shells (K, L, M, N, etc.):

• K-shell (n=1): 2 electrons (1s²)
• L-shell (n=2): 8 electrons (2s² 2p⁶)
• M-shell (n=3): 18 electrons (3s² 3p⁶ 3d¹⁰)
• N-shell (n=4): 5 electrons (4s² 4p³)

Valence Shell: The outermost shell (N-shell) contains 5 electrons (4s² 4p³). This configuration, particularly the half-filled p-subshell (4p³), contributes to the relative stability of Arsenic and its common oxidation states (+3, +5, -3).

Periodic Trends

Ionization Enthalpy

• First Ionization Enthalpy: 947 kJ/mol
• Trend in Group 15: Ionization enthalpy generally decreases down Group 15. Arsenic’s value is lower than Phosphorus (1012 kJ/mol) but higher than Antimony (834 kJ/mol). This decrease is due to increasing atomic size and increased shielding effect from inner electrons, making it easier to remove the outermost electron despite increasing nuclear charge.

Electronegativity (Pauling Scale)

• Electronegativity: 2.18
• Trend in Group 15: Electronegativity generally decreases down Group 15. Arsenic is less electronegative than Phosphorus (2.19) but more electronegative than Antimony (2.05). This trend is consistent with increasing atomic size and decreased nuclear attraction on valence electrons.

Electron Gain Enthalpy

• Electron Gain Enthalpy: -78 kJ/mol
• Trend in Group 15: Electron gain enthalpy becomes less negative (or more positive) down Group 15. Arsenic’s value is less negative than Phosphorus (-72 kJ/mol, although some sources report P as slightly more negative) and more negative than Antimony (-101 kJ/mol, again, some sources vary). This indicates a decreasing tendency to accept an additional electron as atomic size increases and electron-electron repulsion in the valence shell becomes more significant relative to nuclear attraction.

Atomic Radius

• Covalent Radius: 119 pm
• Trend in Group 15: Atomic radius (covalent radius) increases down Group 15. Arsenic has a larger atomic radius than Phosphorus (107 pm) and a smaller atomic radius than Antimony (139 pm). This increase is due to the addition of new electron shells with increasing atomic number.

Key Physical Properties

Density

• Grey (Metallic) Arsenic: 5.73 g/cm³ (at 20 °C)
• Yellow (Molecular) Arsenic: 1.97 g/cm³ (at 20 °C)

State at Standard Temperature and Pressure (STP)

• Solid

Color

• Grey Arsenic: Greyish-silver, metallic luster.
• Yellow Arsenic: Waxy, yellow. (This allotrope is unstable and converts to grey arsenic upon heating or exposure to light).

Melting Point

• Arsenic sublimes at atmospheric pressure.
• Sublimation Point: 615 °C
• Melting Point (under pressure): 817 °C (at 2.8 MPa, where it can exist as a liquid)

Boiling Point

• Arsenic sublimes before reaching a conventional boiling point at atmospheric pressure. Its sublimation point (615 °C) is often cited in lieu of a distinct boiling point.