Arsenic, Antimony and Bismuth

Introduction

Moving from phosphorus to arsenic, antimony and bismuth brings a change from nonmetal toward metal. Arsenic and antimony are commonly classified as metalloids; bismuth is a metal. The +3 and +5 oxidation states remain important, but the heavy end of the group increasingly favours +3. The trend links visible physical character with electron-configuration and redox ideas.

Core explanation

Group 15 atoms have outer ns²np³ configurations. Three np electrons can participate in bonding while the ns² pair remains comparatively less involved, giving a formal +3 state with electronegative partners. In a +5 state, all five valence electrons enter the formal oxidation-state account. Nitrogen and phosphorus readily show +5 compounds such as nitrate and phosphate. Arsenic and antimony form both +3 and +5 compounds. For bismuth, +3 is especially stable in many ordinary compounds, and +5 compounds generally require strong oxidising conditions or particularly stabilising partners.

Metallic character increases down the group as size increases and valence electrons are less tightly held overall. Arsenic and antimony have semimetallic or metalloid properties in common elemental forms; bismuth is clearly metallic, though its detailed electronic behaviour is distinctive. The family is not a smooth series of identical structures with one number changing. Allotropy, crystal structures and bonding vary. The useful broad statement is that oxide and halide chemistry becomes more metallic and ionic in tendency toward Bi, while the lower oxidation state becomes more important.

The inert-pair effect provides an electronic explanation for the +3 trend. Inner d and f electrons shield poorly, and heavy s orbitals are comparatively stabilised, especially when relativistic effects become appreciable. Involving the ns² pair in a +5 product may cost more than the resulting bonds repay. Bismuth(III) therefore predominates in many salts. This does not imply Bi(V) is impossible: high-oxidation-state bismuth compounds exist and can be strong oxidants. Likewise, As(V) and Sb(V) chemistry is not merely hypothetical; arsenate and antimonate species are familiar examples.

Oxides show changing acid–base behaviour. Arsenic(III) oxide has substantial acidic/amphoteric character depending on conditions, antimony oxides can be amphoteric, and bismuth(III) oxide is more basic. The exact aqueous reactions depend on oxide structure and pH; a one-word label cannot predict a dissolution rate. A helpful qualitative pattern is that nonmetal oxides tend to be acidic and metal oxides basic, with metalloids near the boundary often amphoteric. Higher oxidation-state oxides tend to be more acidic than lower-state oxides of the same element.

Hydride behaviour also changes. NH₃ is a relatively stable basic molecule; PH₃, AsH₃ and SbH₃ are less basic in water and generally less thermally stable down the group. Their toxicity and handling are significant in real chemistry, but classification questions should focus on bond strength, lone-pair availability and thermodynamic conditions. A hydride's formula EH₃ alone does not imply NH₃-like aqueous basicity.

The Group 15 trend is a guide to likely states and reactivity, not a deterministic rule. Ligand electronegativity, oxygen chemical potential, solvation and lattice energy can stabilise an oxidation state. Bismuth(V) may be present in suitable oxides or complex compounds, and As(III) versus As(V) forms depend strongly on redox environment.

Step-by-step reasoning

1. Place As, Sb and Bi in order down Group 15 and note increasing metallic character. 2. Use ns²np³ to identify +3 and +5 as common positive oxidation states. 3. Explain why +3 gains relative stability down the group through the inert-pair effect. 4. Classify an oxide only after checking element and oxidation state; identify whether it reacts with acid, base or both. 5. Treat stability as conditional on ligand and medium, especially for high oxidation states.

Visual explanation

Draw a vertical N–P–As–Sb–Bi column. Shade As and Sb as metalloids and Bi as metallic. Beside it place two arrows: metallic character increasing downward, and relative +3 stability increasing downward. Write “+5 still possible” beside As, Sb and Bi to avoid turning a tendency into a ban.

Real-world analogy

A family may share a common set of options, but one option becomes increasingly attractive for later members. Group 15 elements all have valence electrons that can support several oxidation states, while heavy members more often favour the +3 choice. The actual choice still depends on the environment, just as a preference is not an absolute rule.

Real-world example

Bismuth(III) compounds are used in several chemical and materials contexts because Bi³⁺ is a readily accessible state. Arsenic can occur in both As(III) and As(V) forms in environmental waters, with different mobility and chemical behaviour. A treatment or analysis method must distinguish the oxidation state rather than referring to “arsenic” as one solution species.

Why?

Why is Bi(III) more characteristic than Bi(V) in many compounds? The heavy 6s² electron pair is relatively stabilised and less readily involved in bonding. A +3 state can use the outer p electrons while retaining that pair, whereas +5 requires extra energetic investment that not every bonding environment compensates.

Common misconception

“Arsenic and antimony are ordinary metals because they lie below phosphorus” ignores their metalloid character. Another error is to say Bi(V) cannot exist; it is less commonly stable than Bi(III), but suitable oxidising environments and ligands can support it.

Worked example

Compare oxidation states in As₂O₃, As₂O₅ and Bi₂O₃. With O at −2, As₂O₃ has two As totaling +6, hence As(III). As₂O₅ gives As(V), showing both states exist. Bi₂O₃ gives Bi(III), consistent with the heavier-element tendency toward the lower state. The formulas alone do not say which oxide dissolves fastest; that requires reaction conditions.

Quick check

1. Which is most metallic among As, Sb and Bi, and which positive state is relatively favoured there? Answer: Bismuth is the most metallic. Its +3 oxidation state is relatively favoured over +5 in many ordinary compounds, consistent with the heavy Group 15 inert-pair effect.

Exam focus

Use “relative stability” rather than “only oxidation state.” Calculate formal states before assigning redox roles. State arsenic and antimony as metalloids and bismuth as a metal, then connect the +3 trend to lower participation of the ns² pair. For oxide classifications, specify acid or base reactions and conditions instead of relying only on periodic position.

Advanced insight

The +3/+5 preference reflects both atomic orbital energetics and stabilisation from bonds, solvation and crystal packing. Relativistic stabilisation is strongest for the heavy end, but a strong oxidant or strongly electronegative ligand can still support higher states. Environmental arsenic speciation also involves protonation of oxoanions, so oxidation state and acid–base state are separate dimensions.

Summary

Arsenic and antimony are metalloids, while bismuth is metallic. All can show +3, and +5 remains possible, but +3 becomes increasingly important down Group 15 because the heavy ns² pair participates less readily. Oxide acid–base behaviour also shifts toward basicity with increasing metallic character, subject to oxidation state and conditions.

Practice questions

1. Determine arsenic's oxidation state in As₂O₅. Answer: Five oxide oxygens total −10, so two As atoms total +10 and each As is +5. This confirms that As(V) compounds exist despite a trend toward +3 lower in the group.

2. Why is a simple statement that “all Group 15 hydrides are like ammonia” unreliable? Answer: EH₃ bond strengths, lone-pair availability, basicity and thermal stability change down the group. PH₃ and heavier hydrides do not show NH₃'s characteristic aqueous basicity to the same extent.

3. State one way bismuth differs from arsenic in broad periodic behaviour. Answer: Bismuth is a metal and commonly favours Bi(III), while arsenic is a metalloid with substantial As(III) and As(V) chemistry. The difference follows growing metallic character and relative +3 stability down the group.