Group Fifteen Overview

Introduction

Group 15 contains nitrogen, phosphorus, arsenic, antimony and bismuth. Each has an ns²np³ outer configuration: five valence electrons. The shared pattern supports three bonds and a lone pair in many simple compounds, yet the group moves from nonmetallic nitrogen through metalloids toward metallic bismuth. Oxidation states and structures diversify down the column.

Core explanation

Nitrogen is a small nonmetal. It forms strong N≡N bonds in dinitrogen and many N–O, N–H and N–C bonds. Phosphorus is a larger nonmetal with important allotropes and phosphate chemistry. Arsenic and antimony are often classed as metalloids, while bismuth is a metal. Labels such as “metalloid” summarize properties, but actual oxides, halides and hydrides still require compound-specific descriptions.

The simple neutral hydrides NH₃ and PH₃ both have three bonds and a lone pair around the central atom in a basic Lewis model. They are not equally basic in water. Ammonia's nitrogen lone pair is readily available for proton acceptance; phosphine is a much weaker aqueous base. Geometry and electron density matter beyond a matching formula EH₃.

Common negative oxidation state −3 occurs when nitrogen is bonded to less electronegative hydrogen or metals, as in NH₃ or many nitrides. Positive states range through +3 and +5 in oxides and oxyanions. For instance, phosphorus is +3 in PCl₃ and +5 in PCl₅ under conventional chloride −1 assignment. Nitrogen reaches +5 in nitrate, NO₃⁻. The actual stability and bonding are not determined solely by the group number.

The +5 state becomes less favored relative to +3 among heavier group-15 members in many contexts. This parallels the inert-pair trend: retaining the outer ns² pair can favor a lower positive state. Bismuth(III) chemistry is common, though higher-state bismuth compounds exist. As with group 14, this is a qualified tendency, not a prohibition.

Atomic size increases down the group, and first ionization energy generally falls. Electronegativity and nonmetallic character generally decline. However, simple smooth trends can be disturbed by inner-shell shielding and specific structures. A periodic prediction should be anchored to at least one observed compound rather than treated as a universal law.

Oxoacids are especially important for nitrogen and phosphorus. Nitric acid contains nitrate and is a strong acid in water. Phosphoric acid has three P–OH groups and can lose protons stepwise. The number of hydrogens in an acid formula, the formal oxidation state of the central atom and the acid strength are three separate questions. Later pages treat these distinctions explicitly.

Step-by-step reasoning

1. Write ns²np³ and count five outer electrons. 2. Draw three bonds and one lone pair for a typical EH₃ molecule. 3. Assign states from partner electronegativities and total charge. 4. Compare nonmetallic and metallic tendencies down the group. 5. Qualify +5 versus +3 stability for heavier members.

Visual explanation

Draw the N–P–As–Sb–Bi column with a color shift from nonmetal to metal. Beside each place an ns² pair and three p electrons. Show example labels NH₃, PCl₅ and Bi₂O₃ to display −3, +5 and +3 possibilities without suggesting every member has identical chemistry.

Real-world analogy

Five identical-looking tool slots may be used differently in different workshops. Three can connect to partners while two remain together as a pair, but the material and surroundings decide which connections are stable. Electron configuration gives a starting inventory, not a finished compound list.

Real-world example

Nitrogen gas makes up most of ordinary air, while phosphate minerals are important sources of phosphorus for fertilizers. Their different forms arise from N≡N molecular bonding versus phosphorus-oxygen structures, despite both elements occupying group 15.

Why?

Why can many pnictogen molecules have a lone pair? Five outer electrons allow three to participate in three simple bonds, leaving two as a nonbonding pair in a Lewis picture, as in NH₃.

Common misconception

“Group 15 means oxidation state −3 in every compound.” Nitrogen in nitrate is +5, phosphorus in PCl₃ is +3 and bismuth commonly has +3. Partners and formula determine the formal state.

Worked example

Assign phosphorus in PCl₃ and PCl₅. Each chlorine is −1 in these compounds. For PCl₃, x+3(−1)=0, so phosphorus is +3. For PCl₅, x+5(−1)=0, so phosphorus is +5. Both share the same central element; the surrounding ligands and bonding determine the observed compound.

Quick check

1. What is the group-15 outer-electron pattern? Answer: ns²np³, or five valence electrons.

Exam focus

Name the group members, describe the nonmetal-to-metal change, give -3, +3 and +5 examples with calculations, and use the inert-pair effect only as a relative trend.

Advanced insight

For heavier pnictogens, compounds may show different coordination numbers and stereochemical activity of a lone pair. Formal oxidation state alone cannot tell whether a lone pair strongly shapes molecular geometry.

Summary

Pnictogens share ns²np³ but display varied bonding and states. Nitrogen and phosphorus are nonmetals; metallic character grows downward. Three bonds plus a lone pair are common in simple molecules, while +3 and +5 compete in heavier-element chemistry.

Practice questions

1. What is nitrogen's oxidation state in NH₃ if H is +1? Answer: −3, since x+3=0. 2. Give one group-15 +5 example. Answer: Nitrate has nitrogen +5, or PCl₅ has phosphorus +5. 3. Why does the EH₃ formula not guarantee equal basicity for NH₃ and PH₃? Answer: Lone-pair availability and bonding differ; ammonia is much more basic in water.