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Difficulty: MediumMolecular Shapes, VSEPR Theory, and Hybridization

When ammonia (NH3NH_3) reacts with an acid to form the ammonium ion (NH4+NH_4^+), the central nitrogen atom shares its lone pair of electrons to form a dative covalent bond with a proton (H+H^+). According to Valence Shell Electron Pair Repulsion (VSEPR) theory, what is the molecular geometry and hybridization state of the nitrogen atom in the NH4+NH_4^+ ion?

  1. Tetrahedral geometry with sp3sp^3 hybridizationAnswer
  2. B
    Trigonal pyramidal geometry with sp3sp^3 hybridization
  3. C
    Trigonal planar geometry with sp2sp^2 hybridization
  4. D
    Square planar geometry with sp3d2sp^3d^2 hybridization

Answer

Tetrahedral geometry with sp3sp^3 hybridization
In the ammonium ion (NH4+NH_4^+), the central nitrogen atom is surrounded by four single covalent bonds (including one dative covalent bond formed with H+H^+) and zero lone pairs. Four electron domains arrange themselves to minimize repulsion into a symmetric tetrahedral arrangement with bond angles of 109.5∘109.5^\circ. Mixing one s orbital and three p orbitals gives sp3sp^3 hybridization.

Step-by-Step Solution

1
Determine the valence electron count and electron domain total around the central nitrogen atom in NH4+NH_4^+.
Nitrogen brings 5 valence electrons, four hydrogen atoms contribute 1 electron each, and the +1 charge indicates the loss of 1 electron. Total valence electrons = 5+4−1=85 + 4 - 1 = 8 electrons (4 pairs).
VSEPR theory uses the total number of electron pairs around the central atom to predict spatial geometry.
2
Identify the number of bonding pairs and lone pairs on the nitrogen atom.
All 4 electron pairs are involved in single covalent bonds (3 covalent, 1 dative covalent) with hydrogen atoms, leaving 0 lone pairs.
Molecular geometry depends specifically on the arrangement of bonding pairs when 4 electron domains are present.
3
Determine the molecular geometry and hybridization from the electron pair arrangement.
Four bonding domains with zero lone pairs yield a symmetrical regular tetrahedral shape with a bond angle of 109.5∘109.5^\circ and sp3sp^3 hybridization.
Four equivalent steric domains require one s orbital and three p orbitals to hybridize into four sp3sp^3 hybrid orbitals.

Key Concept

Molecular geometry and hybridization of polyatomic ions using VSEPR theory
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Topics

Molecular Shapes, VSEPR Theory, and Hybridization

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