Non-Octet Molecules

Introduction

In chemistry, the octet rule is a fundamental concept that states that atoms tend to gain, lose, or share electrons in order to achieve a full valence shell of eight electrons. However, not all molecules follow this rule. Some molecules have five or six electron pairs around the central atom, leading to a non-octet configuration. In this study guide, we will discuss the properties and behaviors of non-octet molecules with 5 or 6 electron pairs and formal charge.

Formal Charge

Formal charge is the charge assigned to an atom in a molecule or polyatomic ion based on the number of valence electrons it has and the number of electrons it shares in a covalent bond. It is used to determine the most stable resonance structures of a molecule or ion.

The formal charge of an atom is calculated using the following formula:

Formal Charge = Valence electrons - Non-bonding electrons - (1/2)Bonding electrons

Non-Octet Molecules with 5 Electron Pairs

Trigonal Bipyramidal

A molecule with five electron pairs around the central atom has a trigonal bipyramidal geometry. This geometry is characterized by a central atom surrounded by three atoms in a plane and two atoms on opposite ends of the molecule.

An example of a molecule with a trigonal bipyramidal geometry is phosphorus pentafluoride (PF5).

Seesaw

Another possible geometry for a molecule with five electron pairs is the seesaw shape. This shape is characterized by a central atom surrounded by two atoms in a plane and one atom on top and one on the bottom.

An example of a molecule with a seesaw shape is sulfur tetrafluoride (SF4).

T-Shaped

A molecule with five electron pairs can also have a T-shaped geometry. This shape is characterized by a central atom surrounded by three valence electron pairs in a plane and two atoms on opposite ends of the molecule, forming a T shape.

An example of a molecule with a T-shaped geometry is bromine trifluoride (BrF3).

Linear

A molecule with five electron pairs can also have a linear geometry. This shape is characterized by a central atom surrounded by three atoms in a plane and two atoms on opposite ends of the molecule, forming a T shape.

An example of a molecule with a T-shaped geometry is bromine trifluoride (BrF3).

Non-Octet Molecules with 6 Electron Pairs

Octahedral

Octahedral geometry is a shape with a central atom surrounded by six other atoms or electron pairs that are placed at the vertices of an octahedron.

An example of a molecule with octahedral geometry is sulfur hexafluoride (SF6). In this molecule, the central sulfur atom is surrounded by six fluorine atoms, which are evenly spaced around the central atom. The bond angles in an octahedral geometry are 90 degrees between adjacent atoms or electron pairs.

Square Pyramidal

Square pyramidal geometry is a shape with a central atom surrounded by five other atoms, where four of them are arranged in a square plane and one atom is located above the plane.

An example of a molecule with square pyramidal geometry is the compound Xeon pentafluoride (XeF5+). The bond angle in square pyramidal geometry is approximately 90 degrees between the atoms in the square base and the atom above the plane.

Square Planar

Square planar geometry is a shape with a central atom surrounded by four other atoms or electron pairs, which are arranged in a square planar configuration

An example of a molecule with square planar geometry is the compound Xeon tetrafluoride (XeF4). The bond angle in square planar geometry is approximately 90 degrees between adjacent atoms or electron pairs in the square base.

Table of Molecules

Electron Domains	Bonding Pairs	Lone Pairs	Basic Shape	Actual Shape	Examples
5	5	0	Trigonal Bipyramidal	Trigonal Bipyramidal	PF5
5	4	1	Trigonal Bipyramidal	See-Saw	SF4
5	3	2	Trigonal Bipyramidal	T-Shape	BrF3
5	2	3	Trigonal Bipyramidal	Linear	XeF2
6	5	1	Octalhedral	Octalhedral	SF6
6	4	2	Octalhedral	Square Pyramidal	BrF5
6	3	3	Octalhedral	Square Linear	XeF4

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