VSEPR stands for Valence Shell Electron Pair Repulsion and is grounded in the idea that electron pairs surrounding a central atom repel each other, leading to a three-dimensional arrangement. Central to VSEPR theory is the distinction between bonding and non-bonding electron pairs. Bonding pairs arise from shared electrons between atoms which form chemical bonds, while non-bonding pairs, also known as lone pairs, are localized on a specific atom without participating in bonding. To begin, identify the central atom in a molecule and then determine the number of electron domains around it. This is achieved by considering both the bonding pairs and the lone pairs associated with the central atom. The total number of domains influences the 3-D molecular form, and categorizes geometries into specific shapes, including linear, trigonal planar, tetrahedral, trigonal bi-pyramidal, and octahedral, among others. The electron domain geometry and the hybridization around a central atom will always stay the same for a given number of domains. However, molecular geometries of a particular hybridization will change based on whether the domains are bonded or non-bonded.
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Chemical hybridization is a concept in chemistry that involves the combination of atomic orbitals to form new, hybrid orbitals with different shapes and properties. This phenomenon occurs when an atom undergoes hybridization to achieve a set of hybrid orbitals that better align with the geometry of the surrounding atoms in a molecule. The most common types of hybridization involve the combination of s and p orbitals, resulting in sp, sp², or sp³ hybrid orbitals. The sp hybrid orbitals have a linear shape, sp² hybrid orbitals form a trigonal planar arrangement, and sp³ hybrid orbitals create a tetrahedral configuration. Hybridization is often invoked to explain the geometry and bonding in molecules, especially those with multiple bonds or lone pairs around a central atom.
Trigonal planar is a geometric configuration that describes the spatial arrangement of three points or vertices in a plane, forming an equilateral triangle. In Valence Shell Electron Pair Repulsion (VSEPR) theory, trigonal planar refers to the molecular geometry assumed by a molecule when a central atom is surrounded by three bonded electron domains. All atoms and electron pairs lie in the same plane. The ideal bond angles in a trigonal planar arrangement are approximately 120 degrees. An example of a molecule exhibiting trigonal planar geometry is boron trifluoride (BF₃), where a boron atom at the center is bonded to three fluorine atoms. If a bonded pair of electrons is replaced with a lone pair, the domain geometry and hybridization on the central atom remain the same, but the molecular geometry becomes bent.
A tetrahedral is a geometric shape characterized by a symmetrical arrangement of four points or vertices in three-dimensional space, forming the corners of a tetrahedron. In VSEPR theory, a tetrahedral refers to the molecular geometry adopted when a central atom is surrounded by four bonded electron domains. The tetrahedral molecular geometry is idealized with bond angles of approximately 109.5 degrees between adjacent pairs. A classic example of a molecule exhibiting a tetrahedral geometry is methane (CH₄), where a carbon atom at the center is bonded to four hydrogen atoms. If a bonded pair of electrons is replaced with a lone pair, the domain geometry and hybridization on the central atom remain the same, but the molecular geometry becomes trigonal pyramidal. If two bonding pairs of electrons on a tetrahedral are replaced with lone pairs, the molecular geometry becomes bent.
Trigonal bi-pyramidal is a molecular geometry that describes the arrangement of five points or vertices in three-dimensional space, forming the shape of two triangular pyramids joined at their bases. VSEPR theory states that a trigonal bipyramidal form has 5 bonded electron domains distributed in axial and equatorial positions around a central atom. This arrangement consists of three electron pairs in a trigonal plane and two additional electron pairs along the axis perpendicular to this plane. The bond angles in a trigonal bipyramidal structure are typically 120 degrees between equatorial positions and 90 degrees between axial and equatorial positions. Molecules exhibiting trigonal bipyramidal geometry include phosphorus pentachloride (PCl₅) and sulfur hexafluoride (SF₆). Hybridization and domain geometry always remain constant for a given set of domains. However if one bonded pair of electrons is replaced with a lone pair , the replacement happens at an equatorial position where the lone pair will have more space available and the molecular geometry becomes a see-saw. When two bonded equatorial positions are replaced with lone pairs, the molecular geometry become t-shaped. Last, when three bonded equatorial positions are replaced with lone pairs leaving only the bonded axial electrons, the molecular geometry becomes linear.
Octahedral is a geometric configuration that describes the arrangement of six points or vertices in three-dimensional space, forming the shape of two square pyramids joined at their bases. VSEPR theory states that an octahedral refers to the molecular geometry assumed by a molecule when a central atom is surrounded by six bonded electron pairs. The idealized bond angles in an octahedral arrangement are approximately 90 degrees between adjacent pairs regardless of whether they’re axial or equatorial. A classic example of a molecule with octahedral geometry is sulfur hexafluoride (SF₆), where a sulfur atom at the center is bonded to six fluorine atoms. Replacing one bonded pair of electrons with a lone pair results in the molecular geometry changing to a square pyramidal shape, and replacing two bonded pairs with lone pair results in a square planar shape with the two lone pairs on opposite faces of the molecule.