Steric number | AP Chemistry | Khan Academy

Steric Number and Molecular Geometry

In this section, the concept of steric number is introduced, which helps determine the number of hybridized orbitals in a molecule. The steric number is calculated by adding the number of sigma bonds and lone pairs of electrons. The example of methane is used to illustrate these concepts.

Determining Steric Number for Methane

• To find the steric number for methane, we count the number of sigma bonds around carbon (four single bonds) and the number of lone pairs (zero). The steric number is four.

• A steric number of four indicates SP3 hybridization, where one S orbital and three P orbitals combine to form four SP3 hybrid orbitals.

Molecular Geometry of Methane

• The carbon atom in methane is SP3 hybridized.

• Four SP3 hybrid orbitals are drawn for carbon, each containing one valence electron. Hydrogen atoms are also drawn with one valence electron each.

• Electron pairs in methane repel each other, resulting in a tetrahedral arrangement around carbon.

• The molecular geometry of methane is also tetrahedral, with bond angles approximately 109.5 degrees.

Steric Number and Molecular Geometry: Ammonia

This section explores the steric number and molecular geometry of ammonia (NH3). By determining the steric number, we can understand the hybridization and arrangement of electron pairs in ammonia.

Determining Steric Number for Ammonia

• For ammonia, we count three sigma bonds around nitrogen and one lone pair. The steric number is four.

Hybridization and Molecular Geometry of Ammonia

• A steric number of four indicates SP3 hybridization for nitrogen.

• Four SP3 hybrid orbitals are drawn for nitrogen, with appropriate valence electrons. Hydrogen atoms are also drawn with one valence electron each.

• Electron pairs in ammonia repel each other, resulting in a tetrahedral arrangement around nitrogen.

• The molecular geometry of ammonia is not tetrahedral due to the presence of a lone pair on nitrogen.

Conclusion

The concept of steric number helps determine the hybridization and molecular geometry of molecules. By counting sigma bonds and lone pairs, we can understand the arrangement of electron pairs and predict the shape of molecules. In methane, the steric number is four, indicating SP3 hybridization and a tetrahedral molecular geometry. In ammonia, the steric number is also four, but the presence of a lone pair results in a distorted molecular geometry compared to its electron group arrangement.

Molecular Geometry of Ammonia

In this section, the molecular geometry of ammonia is discussed, including the bond angle and shape of the molecule.

Bond Angle and Shape

• The bond angle in ammonia is approximately 107 degrees, which is slightly less than the ideal tetrahedral angle of 109.5 degrees due to extra repulsion between electron pairs.

• The shape of the ammonia molecule is described as trigonal-pyramidal. Nitrogen is bonded to three hydrogen atoms, forming a pyramid-like structure.

• The term "trigonal" refers to nitrogen being bonded to three atoms, while "pyramidal" describes the arrangement of atoms around nitrogen when lone pairs are ignored in molecular geometry analysis.

• Nitrogen in ammonia is SP3 hybridized.

Molecular Geometry of Water

This section focuses on the molecular geometry of water and how it differs from ammonia.

Bond Angle and Shape

• The steric number for water is four, indicating that it requires four hybridized orbitals. This corresponds to SP3 hybridization for oxygen in water.

• Oxygen in water has four SP3 hybrid orbitals, each accommodating one electron except for two electrons in two orbitals. These orbitals form sigma bonds with hydrogen atoms.

• The electron pair arrangement in water is tetrahedral due to VSEPR theory, but the overall molecular geometry is different because lone pairs affect bond angles.

• Due to two lone pairs repelling bonding electrons, the bond angle in water decreases further compared to ammonia and becomes approximately 105 degrees. The molecular geometry of water is described as "bent" or "angular."

Overall Molecular Geometry

This section summarizes the key points discussed about molecular geometry in ammonia and water.

• The central atom in both ammonia and water (nitrogen and oxygen, respectively) is SP3 hybridized.

• Understanding the steric number and hybridization allows us to determine the overall molecular geometry of a molecule.