Oxidation States and Trends in Physical and Chemical Properties — Revision Notes

⚡ 30-Second Revision

Electronic Configuration — $ns^2 np^3$
Oxidation States — 3, +3, +5 (N: wide range, no +5 ionic)
Inert Pair Effect — Stability of +3 increases, +5 decreases down the group (esp. for Sb, Bi)
Atomic/Ionic Radii — Increases down group
Ionization Enthalpy — Decreases down group
Electronegativity — Decreases down group
Metallic Character — Increases down group (N, P non-metals; As, Sb metalloids; Bi metal)
Hydrides ($EH_3$)

- Thermal Stability: $NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$ - Reducing Character: $NH_3 < PH_3 < AsH_3 < SbH_3 < BiH_3$ - Basicity: $NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$

Oxides ($E_2O_3$)

- Acidic Character: $N_2O_3 > P_2O_3 > As_2O_3 > Sb_2O_3 > Bi_2O_3$ (Acidic $ightarrow$ Amphoteric $ightarrow$ Basic)

Halides

- $EX_3$ formed by all. - $EX_5$ formed by P, As, Sb. N does not form $EX_5$. $BiF_5$ exists but unstable.

2-Minute Revision

Group 15 elements, or pnictogens, share the $ns^2 np^3$ valence configuration. Their primary oxidation states are -3, +3, and +5. Nitrogen is unique due to its small size and lack of d-orbitals, preventing it from forming pentavalent compounds like $NCl_5$ and allowing strong pπ-pπ bonding.

The inert pair effect becomes significant for heavier elements (Sb, Bi), making the +3 oxidation state more stable than +5. Physically, atomic size, metallic character, and density increase down the group, while ionization enthalpy and electronegativity decrease.

Melting and boiling points show a more complex trend. Chemically, the thermal stability of hydrides ($EH_3$) decreases down the group, leading to an increase in their reducing character. Conversely, the basicity of hydrides decreases.

For oxides ($E_2O_3$), the acidic character diminishes down the group, transitioning from acidic (N, P) to amphoteric (As, Sb) and finally basic (Bi). All elements form trihalides, but only P, As, and Sb form stable pentahalides, with the stability of pentahalides decreasing down the group.

5-Minute Revision

The Group 15 elements (N, P, As, Sb, Bi) are characterized by their $ns^2 np^3$ valence electron configuration, leading to common oxidation states of -3, +3, and +5. Nitrogen, the first member, exhibits anomalous behavior due to its small size, high electronegativity, and absence of d-orbitals.

It can form a wide range of positive oxidation states (from +1 to +4) but cannot expand its octet to form pentavalent compounds like $NCl_5$, unlike phosphorus which readily forms $PCl_5$ by utilizing its vacant d-orbitals.

Nitrogen also forms stable pπ-pπ multiple bonds (e.g., $N equiv N$).

Trends in Oxidation States: The stability of the +5 oxidation state decreases down the group, while the stability of the +3 oxidation state increases. This is primarily due to the inert pair effect, where the $ns^2$ electrons become increasingly reluctant to participate in bonding for heavier elements like Antimony and Bismuth. For example, $BiCl_3$ is much more stable than $BiCl_5$.

Physical Properties Trends:

Atomic and Ionic Radii — Increase down the group due to added electron shells.
Ionization Enthalpy — Decreases down the group as valence electrons are further from the nucleus and more shielded.
Electronegativity — Decreases down the group, making elements less able to attract electrons.
Metallic Character — Increases down the group (N, P non-metals; As, Sb metalloids; Bi metal).
Melting and Boiling Points — Generally increase from N to As, then decrease for Sb and Bi, influenced by changes in bonding and crystal structure.

Chemical Properties Trends:

Hydrides ($EH_3$) — All form hydrides. Their thermal stability decreases ($NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$) due to weakening E-H bonds. Consequently, their reducing character increases ($NH_3 < PH_3 < AsH_3 < SbH_3 < BiH_3$). Their basicity decreases ($NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$) as the lone pair becomes more diffuse.
Oxides ($E_2O_3$ and $E_2O_5$) — All form oxides. The acidic character of oxides decreases down the group. For +3 oxides: $N_2O_3$ and $P_2O_3$ are acidic, $As_2O_3$ and $Sb_2O_3$ are amphoteric, and $Bi_2O_3$ is basic. Higher oxidation state oxides are generally more acidic (e.g., $N_2O_5$ is more acidic than $N_2O_3$).
Halides ($EX_3$ and $EX_5$) — All form trihalides ($EX_3$). Only P, As, and Sb form stable pentahalides ($EX_5$). Nitrogen does not form pentahalides, and Bismuth's pentahalides (like $BiF_5$) are less stable due to the inert pair effect. The stability of pentahalides decreases down the group.

Example: Compare the basicity of $NH_3$ and $PH_3$. $NH_3$ is more basic than $PH_3$. Nitrogen is smaller and more electronegative than Phosphorus, holding its lone pair of electrons in a more concentrated region, making it more available for donation to a proton (Lewis base). As the central atom size increases, the lone pair becomes more diffuse and less available, decreasing basicity.

Prelims Revision Notes

1
Electronic Configuration — All Group 15 elements have $ns^2 np^3$ valence shell configuration.
2
Oxidation States — Common states are -3, +3, +5. Nitrogen shows a wide range (+1 to +4) but cannot form $N^{5+}$ ions or pentahalides due to absence of d-orbitals.
3
Inert Pair Effect — Becomes significant for heavier elements (Sb, Bi). It causes the +3 oxidation state to be more stable than +5. Example: $BiCl_3$ is stable, $BiCl_5$ is not.
4
Physical Properties Trends (Down the Group)

* Atomic/Ionic Radii: Increase. * Ionization Enthalpy: Decrease. * Electronegativity: Decrease. * Metallic Character: Increase (N, P non-metals; As, Sb metalloids; Bi metal). * Melting/Boiling Points: Increase from N to As, then decrease for Sb and Bi. * Density: Increase.

1
Chemical Properties Trends (Down the Group)

* **Hydrides ($EH_3$)**: * Thermal Stability: Decreases ($NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$). * Reducing Character: Increases ($NH_3 < PH_3 < AsH_3 < SbH_3 < BiH_3$). * Basicity: Decreases ($NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$).

* Bond Angle: Decreases ($NH_3 (107.8^circ) > PH_3 (93.6^circ) > AsH_3 (91.8^circ) > SbH_3 (91.3^circ)$). * **Oxides ($E_2O_3$ and $E_2O_5$)**: * Acidic Character: Decreases down the group.

$N_2O_3, P_2O_3$ (acidic); $As_2O_3, Sb_2O_3$ (amphoteric); $Bi_2O_3$ (basic). * Higher oxidation state oxides are more acidic (e.g., $N_2O_5$ is more acidic than $N_2O_3$). * **Halides ($EX_3$ and $EX_5$)**: * All form $EX_3$.

$BiF_3$ is ionic, others covalent. * $EX_5$ formed by P, As, Sb. Nitrogen does not form $EX_5$. Stability of $EX_5$ decreases down the group (e.g., $BiF_5$ is less stable).

Vyyuha Quick Recall

For Group 15 Hydrides ($EH_3$) trends: Through Reality, Bonds Decrease. (Thermal stability Decreases, Reducing character Increases, Basicity Decreases, Bond angle Decreases).