Arsenic (As)

The ground-state configuration of arsenic is \([Ar]3d^{10}\,4s^2\,4p^3\). With five valence electrons, As forms trivalent (As³⁺) and pentavalent (As⁵⁺) compounds. This reflects its position in Group 15, similar to phosphorus and antimony, showing both covalent and amphoteric behavior.

Arsenic exists mainly as:

• Gray (metallic) arsenic: stable, brittle, semimetallic, and most common form.
• Yellow arsenic (As₄): molecular solid, soft and waxy, formed by rapid cooling of vapor.
• Black arsenic: amorphous, glassy, and less common.

The gray allotrope conducts electricity and sublimes directly to vapor instead of melting.

Arsenic shows both metallic and nonmetallic traits—it forms metallic-looking crystals yet forms acidic oxides and covalent compounds (e.g., AsCl₃, As₂O₃). Its intermediate electrical conductivity and amphoteric oxide behavior also confirm its metalloid character.

Arsenic commonly exhibits oxidation states of −3, +3, and +5.

• −3: Arsine, \(\mathrm{AsH_3}\)
• +3: Arsenic trioxide, \(\mathrm{As_2O_3}\)
• +5: Arsenic pentoxide, \(\mathrm{As_2O_5}\)

Arsenic trioxide is widely used (and notorious) as a poison but also in small, controlled doses for medical treatment of certain leukemias.

As₂O₃ reacts with both acids and bases:

• Acidic: \(\mathrm{As_2O_3 + 6\,HCl \rightarrow 2\,AsCl_3 + 3\,H_2O}\)
• Basic: \(\mathrm{As_2O_3 + 2\,NaOH + 3\,H_2O \rightarrow 2\,NaH_2AsO_3}\)

This dual behavior is typical of metalloids that lie between metals and nonmetals in reactivity.

Arsine is a highly toxic, colorless gas formed by reaction of As compounds with nascent hydrogen:

\(\mathrm{As^{3+} + 3\,H_2 \rightarrow AsH_3}\)

It decomposes on heating to elemental arsenic and hydrogen. Even trace exposure causes hemolysis and kidney failure, making it one of the most toxic inorganic gases known.

At 1 atm, the vapor pressure of arsenic becomes high before the solid can melt, so it sublimes directly around 887 K (≈614 °C). At higher pressures, it can form a liquid phase, but under normal conditions the solid → gas transition dominates.

Controlled forms of arsenic are used in:

• Semiconductors: GaAs (gallium arsenide) for LEDs, lasers, and solar cells.
• Alloys: small additions to lead for battery plates and ammunition.
• Glass and pigments: to remove bubbles and add opacity (historical).

Usage is tightly regulated to prevent environmental contamination.

Exposure may come from contaminated groundwater (AsO₄³⁻), occupational hazards, or old pesticides. Symptoms include skin lesions, abdominal pain, vomiting, and in chronic cases, cancers of the skin, lung, and bladder. Arsenic trioxide and arsenates interfere with ATP production by replacing phosphate groups in metabolism.

Arsenic pollution in groundwater (notably in Bangladesh and India) affects millions. Long-term ingestion causes arsenicosis—a chronic poisoning. Mitigation involves filtration (iron-based adsorbents), switching wells, and monitoring As levels below WHO limits (10 µg/L).