Question

How many atoms are directly bonded to the central atom in a tetrahedral molecule, a trigonal bipyramidal molecule, and an octahedral molecule?

Short answer

4 in tetrahedral, 5 in trigonal bipyramidal, 6 in octahedral.

Step-by-step solution

Understanding Tetrahedral Geometry

A tetrahedral molecule has a central atom bonded to four other atoms. The geometry is characterized by four bonds equally spaced around the central atom, resulting in bond angles of about 109.5 degrees. Examples include molecules like methane (CH₄).

Understanding Trigonal Bipyramidal Geometry

In a trigonal bipyramidal geometry, the central atom is bonded to five other atoms. Two of these atoms are bonded in a linear fashion, while the remaining three form a triangle in the perpendicular plane. This results in bond angles of 90 degrees between axial and equatorial atoms and 120 degrees between the three equatorial atoms. An example is phosphorus pentachloride (PCl₅).

Understanding Octahedral Geometry

An octahedral molecule features a central atom bonded to six other atoms, forming an octahedral shape. The bond angles between adjacent atoms are all 90 degrees, providing a symmetrical shape. A common example is sulfur hexafluoride (SF₆).

Key concepts

Tetrahedral Geometry

In tetrahedral geometry, a central atom is bonded to four atoms at the corners of a tetrahedron. This configuration results in symmetrical distribution of the bonds around the central atom. As a consequence, all bond angles are approximately 109.5 degrees.

• The reason for this specific geometry is that it allows the molecule to minimize electron pair repulsion according to VSEPR theory (Valence Shell Electron Pair Repulsion).
• In the case of methane (CH₄), the carbon atom is at the center with hydrogen atoms occupying the vertices of the tetrahedron. This results in uniform spatial distribution.

This arrangement balances the repulsive forces between the electrons in the outer shell, ensuring that each bond has maximized distance from others.

Trigonal Bipyramidal Geometry

Trigonal bipyramidal geometry involves a central atom surrounded by five atoms, forming two different types of bonds: axial and equatorial. In this structure:

• The two axial atoms are 180 degrees apart and perpendicular to the plane formed by the three equatorial atoms.
• The equatorial atoms form bond angles of 120 degrees between each other, creating a triangular planar shape.
• Bond angles between the axial and equatorial atoms are 90 degrees.

An example of this geometry is phosphorus pentachloride (PCl₅), where phosphorus is at the center. This arrangement provides both symmetry and stability, optimizing the positioning of electron pairs with minimal repulsion.

Octahedral Geometry

Octahedral geometry is a molecular shape where a central atom is bonded to six atoms, creating an octahedral figure. This configuration is characterized by:

• All bonds being equivalent, featuring bond angles of 90 degrees between adjacent atoms.
• Its highly symmetrical nature, which allows for six equivalent positions around the central atom.

A common example of this geometry is sulfur hexafluoride (SF₆), where the sulfur atom is at the center surrounded by six fluorine atoms. This symmetry ensures that the molecular shape is balanced and the spatial arrangement of bonds minimizes electron pair repulsions effectively.