Electronic structure and valence states
The outer energy level of group VA elements contains five electrons. Two paired electrons are located in the $s$-sublevel, and three unpaired electrons are in the $p$-sublevel. The outer levels are far from being completely filled.
The typical valence state for all elements is trivalent (III). Examples include ammonia ($NH_3$) and aminoethane ($C_2H_5NH_2$).
The pentavalent state (V) is characteristic of all elements except nitrogen. This is due to the presence of an unfilled $d$-sublevel in phosphorus, arsenic, antimony, and bismuth. Upon atom excitation, electrons from the $s$-sublevel unpair and transition to the $d$-sublevel.
- Ground state: $ns^2 np^3$ (e.g., $PCl_3$)
- Excited state: $ns^1 np^3 nd^1$ (e.g., $PCl_5$)
It is important to remember that nitrogen forms a maximum of four covalent bonds even in the +5 oxidation state.
Electronegativity
In general, group VA elements have a lower electronegativity than halogens (Group VII) and chalcogens (Group VI).
- Nitrogen (N): Has a high electronegativity (3.0), ranking third after fluorine and oxygen.
- Phosphorus (P): Its electronegativity (2.3) is significantly lower than that of nitrogen and only slightly exceeds that of hydrogen.
Interesting fact: the tabulated electronegativity values of nitrogen and chlorine are close. However, chemical behavior proves that nitrogen is more electronegative. For example, during the hydrolysis of $NCl_3$, the nitrogen atom binds to the positive hydrogen ion, while chlorine binds to the negative hydroxide group, forming ammonia ($NH_3$) and hypochlorous acid ($HClO$).
Variation of metallic properties
Moving down the group, there is a regular increase in metallic properties:
- Nitrogen (N) and Phosphorus (P): Typical nonmetals.
- Arsenic (As): Forms allotropic modifications. Yellow arsenic is an unstable nonmetal, while gray arsenic exhibits metallic properties but is brittle.
- Antimony (Sb) and Bismuth (Bi): Predominantly metals.
The acid-base properties of compounds also change: $Sb(III)$ compounds are amphoteric, whereas $Bi(III)$ compounds exhibit exclusively basic properties.
Biological and practical significance
The elements of the group can be divided into two categories based on their significance:
- Biogenic elements (N, P): Nitrogen and phosphorus are essential for life, widely distributed in nature, and of paramount importance.
- Arsenic subgroup (As, Sb, Bi): Significantly less abundant and non-essential. Arsenic and antimony are used as additives in alloys and semiconductors. Bismuth is used in ultra-low-melting alloys alongside lead, tin, and cadmium.
Class and exam questions
Why cannot nitrogen be pentavalent?
The nitrogen atom lacks a vacant d-sublevel in its outer energy level, making the unpairing of s-electrons and the formation of five covalent bonds impossible. The maximum covalency of nitrogen is four.
How to prove that nitrogen is more electronegative than chlorine?
This is proven by the hydrolysis reaction of nitrogen(III) chloride ($NCl_3$). The products are ammonia ($NH_3$) and hypochlorous acid ($HClO$), which indicates a negative charge on the nitrogen atom in the starting compound.
What is the "inert pair effect" using bismuth as an example?
For bismuth, the highest oxidation state (V) is extremely unstable, unlike the lighter elements of the group. $Bi(V)$ compounds act as very strong oxidizing agents (e.g., $Bi_2O_5$).
What else to study on this topic
- Structure and properties of allotropic modifications of phosphorus
- Properties of ammonia and ammonium salts
- Characteristics of nitric acid and its salts
- Applications of semiconductor materials based on arsenic and antimony
- Hydrolysis reactions of group V element halides
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