Question

Designate the electron-region geometry for each case from two to six electron pairs around a central atom.

Short answer

From two to six pairs: linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral geometries.

Step-by-step solution

Understanding Electron Pair Geometry

The geometry of electron regions around a central atom is determined by the number of electron pairs (bonded and lone pairs). The VSEPR (Valence Shell Electron Pair Repulsion) model predicts the shapes based on the repulsion between these pairs.

Two Electron Pairs

When there are two electron pairs, they are as far apart as possible, resulting in a linear arrangement. The angle between electron pairs is 180°. This geometry is often seen with molecules like BeCl₂.

Three Electron Pairs

With three electron pairs, the arrangement is trigonal planar, where the pairs are oriented 120° apart in a plane. An example of this geometry is BF₃.

Four Electron Pairs

For four electron pairs, the geometry is tetrahedral. The electron pairs are positioned 109.5° apart, forming a three-dimensional shape. Methane (CH₄) exhibits this geometry.

Five Electron Pairs

When there are five electron pairs, the geometry is trigonal bipyramidal. This includes three pairs in a plane 120° apart, and two pairs above and below the plane at 90° angles. An example compound is PCl₅.

Six Electron Pairs

With six electron pairs, the geometry becomes octahedral. The pairs are arranged at 90° angles, forming an eight-faced shape. An example of this geometry is SF₆.

Key concepts

Electron Pair Geometry

Electron pair geometry refers to the arrangement of electron pairs—both bonding pairs and lone pairs—around a central atom in a molecule. This concept is a cornerstone of the VSEPR (Valence Shell Electron Pair Repulsion) model. VSEPR explains that electron pairs repel each other and, as a result, arrange themselves as far apart as possible to minimize this repulsion.

• Two electron pairs around a central atom result in a linear geometry. The pairs are 180° apart to reduce repulsion.
• Three electron pairs form a trigonal planar geometry, keeping 120° apart in a symmetrical arrangement.
• Four electron pairs adopt a tetrahedral geometry with each pair positioned 109.5° apart.
• With five electron pairs, the structure becomes trigonal bipyramidal; three pairs lie in a plane (120° apart) and two are positioned above and below the plane at 90° angles.
• Six pairs of electrons create an octahedral shape, maintaining equal 90° angles.

Molecular Geometry

Molecular geometry, while closely related to electron pair geometry, refers specifically to the arrangement of atoms within a molecule. It is determined based on the positions of the atoms and omits lone electron pairs when describing the shape of a molecule.

• For instance, in a molecule like water (H₂O), the electron pair geometry is tetrahedral due to two lone pairs and two bonded pairs around oxygen, but the molecular geometry is bent or V-shaped.
• The consideration of only bonding pairs allows a clearer picture of how the molecule interacts and reacts with other substances.
• This is why analogous electron-pair and molecular geometries can differ when lone pairs are present; these pairs impact the shape by pushing bonding pairs closer together.

Bond Angles

Bond angles are the angles formed between two adjacent bonds in a molecule. They are crucial for determining the molecular shape and are directly influenced by electron pair repulsion. By understanding bond angles, we gain insight into the spatial arrangement of molecules.

• For a linear geometry, the bond angle is 180°.
• In trigonal planar structures, the bond angles are typically 120°.
• Tetrahedral shapes have bond angles of about 109.5°.
• Trigonal bipyramidal arrangements show two distinct bond angles: 120° in the plane and 90° between the plane and axial positions.
• In octahedral geometries, all bond angles are 90°.

These angles are crucial because they influence how a molecule fits into biological receptor sites, reacts with other molecules, and its physical properties like polarity and reactivity.

Central Atom

The central atom in a molecule is the atom to which all other atoms are bonded. It is pivotal in determining the electron pair geometry and, consequently, the molecular shape. Understanding the role of the central atom helps predict molecular behavior and reaction pathways.

• The central atom is usually the one with the highest capacity to form bonds, often found in the middle of the periodic table.
• In small molecules like water, the central atom is oxygen. In methane, it is carbon.
• Determining the central atom allows chemists to draw the molecule's basic skeleton and, using VSEPR theory, predict its geometry.
• The central atom's electron configuration will determine the number of electron domains, critical for applying VSEPR theory effectively.

Exploring the central atom's characteristics and bonds can reveal a lot about the physical and chemical properties of the compound.