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Molecular geometry is a term describing 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 when determining the electron-pair geometry and must be considered in the shape of the molecule due to their repulsive action on the bonded pairs of electrons. This repulsion between electrons is what affects the angles between the bonded atoms and any 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. Charts comparing the electron-pair geometry and molecular geometry are commonly used to show the effects of lone pairs on the shape of the molecule, as molecules with no lone pairs have the same molecular and electron-pair geometry.
A simple theory of how electrons behave 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 position themselves as far apart from each other as they possibly can. Utilizing this theory, the geometric shape of simple molecular compounds can be accurately determined. Other methods, such as x-ray crystallography, are needed when describing the shape of complex organic molecules including genetic material and proteins.
The simplest molecule has one 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 pairs have a trigonal planar shape. This molecule has angles of 120 degrees between the three attached atoms and lies flat in a single plane.
In order to position each bonded atom as far apart as possible, a molecule with four atoms surrounding a central atom and no lone pairs has a tetrahedral shape. Each bond angle is 109.5 degrees, forming a tetrahedron with the central atom on the inside. In this same manner, with each additional atom bonded to the central atom, the shape changes as the bonded atoms push 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 one lone pair surrounding a central atom will have a trigonal pyramidal shape, with the central atom at the top of the pyramid and the three attached atoms pushed by the lone pair into a position below the central atom.
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