Molecular geometry is determined by the number of bonded atoms and lone pairs surrounding a central atom. The Valence Shell Electron Pair Repulsion theory predicts that electrons will position themselves as far apart as possible. The shape of simple molecules can be precisely determined, while complex organic molecules require other methods such as X-ray crystallography. The presence of lone pairs affects the shape of the molecule, resulting in different angles between the bonded atoms and the lone pairs.
Molecular geometry is a term that describes the three-dimensional shape of a molecule, given the number of lone pairs and bonded atoms surrounding a central atom. Any lone pairs – unbonded pairs of electrons – are used to determine the geometry of the electron pair and must be considered in the shape of the molecule due to their repulsive action on the bonded electron pairs. This repulsion between electrons is what affects the angles between the bonded atoms and the lone pairs surrounding the central atom. These angles, rather than the number of atoms attached to the central atom, define the molecular geometry of covalently bonded molecules. Graphs comparing electron pair geometry and molecular geometry are commonly used to show the effects of lone pairs on the shape of the molecule, since molecules lacking lone pairs have the same molecular and electron pair geometry.
A simple theory of the behavior of electrons is used when predicting the shape of a molecule. The theory of Valence Shell Electron Pair Repulsion (VSEPR) states that bonded and lone pairs of valence electrons will locate as far apart from each other as possible. Using this theory, the geometric shape of simple molecular compounds can be precisely determined. Other methods, such as X-ray crystallography, are needed to describe the shape of complex organic molecules, including genetic material and proteins.
The simplest molecule has a central atom with two additional atoms bonded to it. According to the VSEPR theory, the two bonded atoms will position themselves as far apart from each other as possible, resulting in a linear molecular shape. The angles between the bonds are 180 degrees. Covalently bonded molecules with three atoms surrounding a central atom and no lone pair have a trigonal planar shape. This molecule has 120 degree angles between the three attached atoms and lies flat in a single plane.
To place each bonded atom as far apart as possible, a molecule with four atoms surrounding a central atom and no lone pair has a tetrahedral shape. Each bond angle is 109.5 degrees, forming a tetrahedron with the central atom inside. Similarly, with each additional atom bonded to the central atom, the shape changes as the bonded atoms move away from each other. With the presence of lone pairs, the molecular geometry of the atom changes, as the lone pair also exerts repulsion. A molecule with three atoms and a lone pair surrounding a central atom will have a trigonal pyramid shape, with the central atom at the top of the pyramid and the three attached atoms pushed by the lone pair to a position below the central atom.
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