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Nitrogen (N)

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Chemistry Nitrogen Periodic Table p-block elements JEE NEET CBSE ICSE Chemical Reactions

Introduction: The Ubiquitous Element

Nitrogen (N) is a fundamental chemical element, essential for life and numerous industrial processes. It comprises approximately 78% of Earth’s atmosphere as diatomic nitrogen gas (N₂). Its real-life significance spans from being a critical component of biological molecules like DNA and proteins to its use in fertilizers, explosives, and as an inert atmosphere.

CBSE/JEE Quick Revision Notes

  • Atomic Number: 7
  • Atomic Mass: 14.01 g/mol
  • Electronic Configuration: [He] 2s² 2p³
  • Valency: Typically 3 (forms 3 covalent bonds). Exhibits oxidation states from -3 to +5.
  • Group: 15 (Pnictogens)
  • Period: 2
  • Nature: Non-metallic, diatomic gas (N₂).
  • Electronegativity: 3.04 (Pauling scale).
  • Ionization Enthalpy: High.
  • Atomic Radius: Small.
  • Bonding in N₂: Forms a very strong triple covalent bond (N≡N), contributing to its high stability and inertness.
  • Allotropes: Does not exhibit common allotropy under normal conditions, unlike phosphorus.

Electron Configuration & Bonding Behavior

Nitrogen’s electronic configuration, 1s² 2s² 2p³, indicates the presence of three unpaired electrons in its 2p orbitals. This allows it to form three covalent bonds.

  • Stability: The half-filled p-orbitals in the valence shell contribute to its relative stability.
  • Diatomic Molecule (N₂): Nitrogen forms a diatomic molecule where two nitrogen atoms are linked by a triple bond (one sigma and two pi bonds). The high bond dissociation enthalpy (946 kJ/mol) of the N≡N bond makes nitrogen gas highly unreactive at room temperature.
  • Oxidation States: Nitrogen exhibits a wide range of oxidation states from -3 to +5 due to its ability to share or gain electrons.
    • -3: In ammonia (NH₃), nitrides (e.g., Mg₃N₂).
    • 0: In diatomic nitrogen (N₂).
    • +1: In nitrous oxide (N₂O).
    • +2: In nitric oxide (NO).
    • +3: In dinitrogen trioxide (N₂O₃), nitrous acid (HNO₂).
    • +4: In nitrogen dioxide (NO₂), dinitrogen tetroxide (N₂O₄).
    • +5: In dinitrogen pentoxide (N₂O₅), nitric acid (HNO₃), nitrates (e.g., KNO₃).
  • Pπ-Pπ Bonding: Nitrogen can readily form multiple bonds (double or triple) with itself and with other small, electronegative elements like carbon and oxygen due to its small size and high electronegativity.

Crucial Chemical Reactions

1. Preparation of Ammonia (Haber Process)

This is the industrial synthesis of ammonia, crucial for fertilizers. $\text{N}_2 \text{(g)} + 3\text{H}_2 \text{(g)} \rightleftharpoons 2\text{NH}_3 \text{(g)}$ ($\Delta H = -92.4 \text{ kJ/mol}$) Conditions: High pressure (200 atm), optimal temperature (400-450 °C), catalyst (finely divided iron with molybdenum as a promoter).

2. Reaction with Oxygen

Nitrogen reacts with oxygen only at very high temperatures, typically encountered during lightning or in internal combustion engines. $\text{N}_2 \text{(g)} + \text{O}_2 \text{(g)} \rightleftharpoons 2\text{NO (g)}$ (at ~2000 K)

3. Formation of Nitrides

Nitrogen reacts with highly electropositive metals to form ionic nitrides. $3\text{Mg (s)} + \text{N}_2 \text{(g)} \rightarrow \text{Mg}_3\text{N}_2 \text{(s)}$ (Magnesium nitride)

4. Ostwald Process (Key Nitrogen Reactions for Nitric Acid)

This process involves the catalytic oxidation of ammonia to produce nitric acid.

  • Step 1: Catalytic Oxidation of Ammonia $4\text{NH}_3 \text{(g)} + 5\text{O}_2 \text{(g)} \xrightarrow{\text{Pt/Rh catalyst, 800 °C}} 4\text{NO (g)} + 6\text{H}_2\text{O (g)}$
  • Step 2: Oxidation of Nitric Oxide $2\text{NO (g)} + \text{O}_2 \text{(g)} \rightarrow 2\text{NO}_2 \text{(g)}$
  • Step 3: Absorption of Nitrogen Dioxide in Water $3\text{NO}_2 \text{(g)} + \text{H}_2\text{O (l)} \rightarrow 2\text{HNO}_3 \text{(aq)} + \text{NO (g)}$

5. Laboratory Preparation of Nitrogen Gas

Nitrogen can be prepared in the laboratory by heating an aqueous solution of ammonium chloride and sodium nitrite. $\text{NH}_4\text{Cl (aq)} + \text{NaNO}_2 \text{(aq)} \rightarrow \text{N}_2 \text{(g)} + 2\text{H}_2\text{O (l)} + \text{NaCl (aq)}$

Another method involves the decomposition of ammonium dichromate. $(\text{NH}_4)_2\text{Cr}_2\text{O}_7 \text{(s)} \xrightarrow{\Delta} \text{N}_2 \text{(g)} + \text{Cr}_2\text{O}_3 \text{(s)} + 4\text{H}_2\text{O (g)}$

Industrial and Biological Importance

Industrial Importance

  • Ammonia Production: The most significant industrial use is in the Haber process for synthesizing ammonia (NH₃), which is then used to produce fertilizers (e.g., urea, ammonium nitrate) and nitric acid.
  • Inert Atmosphere: Gaseous nitrogen is used to provide an inert atmosphere for chemical reactions, packaging sensitive food items, and filling incandescent light bulbs to prevent filament oxidation.
  • Cryogenic Agent: Liquid nitrogen, with a boiling point of -196 °C (77 K), is used as a cryogenic agent for rapid freezing of biological samples (e.g., semen, eggs, tissues), in medical procedures (cryosurgery), and for cooling electronic components.
  • Explosives: Nitrogen compounds like trinitrotoluene (TNT) and nitroglycerine are key components in explosives due to the high energy released upon the formation of stable N₂ gas.
  • Metal Treatment: Used in nitriding steel to improve surface hardness and wear resistance.

Biological Importance

  • Essential Biomolecule Component: Nitrogen is a fundamental component of:
    • Proteins: Forms the amino group (-NH₂) in amino acids, the building blocks of proteins.
    • Nucleic Acids: A key part of the nitrogenous bases (adenine, guanine, cytosine, thymine, uracil) in DNA and RNA.
    • ATP: A component of adenosine triphosphate (ATP), the primary energy currency of cells.
    • Chlorophyll: Essential for photosynthesis in plants.
    • Vitamins: Found in many vitamins (e.g., B group vitamins).
  • Nitrogen Cycle: Nitrogen is continuously cycled through the atmosphere, soil, and living organisms.
    • Nitrogen Fixation: Atmospheric nitrogen (N₂) is converted into ammonia or ammonium ions by nitrogen-fixing bacteria (e.g., Rhizobium in legume root nodules, Azotobacter in soil) or by lightning.
    • Nitrification: Ammonia is oxidized to nitrites and then nitrates by nitrifying bacteria. Plants absorb nitrates for growth.
    • Denitrification: Some bacteria convert nitrates back to gaseous nitrogen, returning it to the atmosphere.
N

Nitrogen (N)

Atomic Number 7

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