molecules are compounds formed from atoms that are attached by covalent bonds. A covalent bond is a chemical bond in which atoms share their valence electrons. The molecuar shape of a molecule is its containership in space as defined by its bond lengths (distances between nuclei of bonding atoms) bond angles (angles between b. The two types of molecular shapes of interest are: ABn Shapes and nonABn Shapes.
Molecules with ABn Shapes are called ABn molecules. ABn molecules have a single central atom bonded to two or more atoms of the same type. For example, CH4 (methane) has carbon as the central atom bonded to four hydrogen atoms. Determination of the molecular shapes of ABn molecules is best explained by examples. Consider the following problems:
(1) How meaningful is the molecular shape of the molecule with n = 1?
(2) What is the value of n for the following molecules?:
(a) CO2 (b) SO3 (c) CCl4
(3) Determine the Lewis structure for the following molecules:
(a) CO2 (b) H2O (c) BF3 (d) XeF4
(e) CCl4 (f) NH3 (g) PCl5 (h) SF6
(4) Use the Valence-Shell Electron-Pair Reduction (VSEPR) model to predict the molecular shapes and bond angles of the molecules indicated in problems 3a, 3c, 3e, 3g and 3h.
(5) Predict the molecular geometries of the molecules of 3b, 3d and 3f, from the electron-domain geometries of problem (4). Will the bond angles of these molecules be greater or smaller than the bond angles indicated in the electron-domain geometries from which their molecular geometries were derived?
(1) When n = 1, there are only two atoms in the molecule. Molecular shape is meaningful only when there are at least three atoms. Two atoms can be arranged next to each other without any special name being given to the arrangement.
(2) (a) n = 2 (b) n = 3 (c) n = 4.
(3) The Lewis structures of the molecules are illustrated in the diagram. The Lewis structure of an atom, ion or molecule is an electron dot diagram that represents the valence electrons of the atom with dots and its nucleus and all its inner shell electrons with the symbol of the atom.
(4) The VSEPR model is used to classify each nonbonding pair, single bond, or multiple bond around the central atom of an ABn molecule as an electron-domain. The electron-domains are then used to derive an electron-domain geometry from which a molecular geometry is derived. The general steps for using the VSEPR model is as follows:
(i) Determine the Lewis structure of the molecule or ion, then count the total electron-domains around the central atom. Each nonbonding pair, single bond, or multiple bond around the central atom counts as an electron-domain.
(ii) Determine the electron-domain geometry by arranging the electron-domains about the central atom in a manner that minimizes the electron repulsion among them.
(iii) Use the arrangement of the bonded atoms to determine the molecular geometry.
(3a) CO2: electron domains = 2; bonding domains = 2; nonbonding domains = 0; electron geometry = linear; molecular geometry = linear; bonding angle = 180o
(3c) BF3: electron domains = 3; bonding domains = 3; nonbonding domains = 0; electron geometry = trigonal planar; molecular geometry = trigonal planar; bonding angle =120o. In the trigonal planar, all the atoms are on the same triangular plane of an equilateral triangle.
(3e) CCl4: electron domains = 4; bonding domains = 4; nonbonding domains = 0; electron geometry = tetrahedral; molecular geometry = tetrahedral; bonding angle = 109.5o. The tetrahedron has 4 vertices and four faces all of which are equilateral triangles. The vertices of the equilateral base holds three atoms. The central atom is the center of the tetrahedron which is above the plane of the base. The fourth atom is at the fourth vertex which is above both the base of the tetrahedron and the central plane.
(3g) PCl5: electron domains = 5; bonding domains = 5; nonbonding domains = 0; electron geometry = trigonal bipyramidal; molecular geometry = trigonal bipyramidal; bonding angle between axial and equatorial bond = 90o; bond angle between equatorial bonds = 120o. The trigonal bipyramid has 5 vertices. Three of which are the vertices of an equilateral triangle. The three atoms at the vertices of the equilateral triangle are equatorial atoms. The central atom A, is at the center of the equilateral triangle. The two atoms at the vertices above and below the plane of the equilateral triangle are axial atoms.
(3h) SF6: electron domains = 6; bonding domains = 6; nonbonding domains = 0; electron geometry = octahedral; molecular geometry = octahedral; bonding angle between axial and equatorial bond = 90o; bond angle between equatorial bonds = 90o. The octahedral has six vertices and 8 surfaces which are all equilateral triangles. Four equatorial atoms are at the vertices of a square. The central atom A, is at the center of the square. The axial atoms are at the vertices above and below the plane of the square.
(5) H20: electron domains = 4; bonding domains = 2; nonbonding domains = 2; electron geometry = tetrahedral; molecular geometry = bent (tetrahedral minus 2 electron domains); bond angle will be less than 120o because of the effect of the nonbonding domains.
XeF4: electron domains = 6; bonding domains = 4; nonbonding domains = 2; electron geometry = octahedral; molecular geometry = square planar (ocahedral minus 2 axial electron domains); bond angle 90o. No axial bonds.
NH3: electron domains = 4; bonding domains = 3; nonbonding domains = 1; electron geometry = tetrahedral; molecular geometry = trigonal pyramidal (tetraahedral minus 1 electron domain); bond angle will be less than 120o because of the effect of the nonbonding domain.
Non-ABn molecules are usually molecules larger than ABn molecules. Unlike ABn molecules, they do not have a single central atom. Instead they have two or more central atoms that are also called interior atoms. An electron-domain is determined using each of the interior atoms as a central atom. These individual electron-domain geometries are then integrated to form the electron-domain geometry of the non-ABn molecule.
The solution to the following problem explains how the moleculay shape of a nonABn molecule is determned:
The above diagram is the Lewis structure of acetic acid. Predict the molecular geometry of acetic acid from its Lewis structure and the bond angles associated with its geometry.
The Lewis structure of acetic acid as shown, has three interior atoms: the leftmost C atom, the central C atom and the rightmost O atom. Each of these atoms is considered as a central atom and the electron-domain geometry is determined.
Leftmost C atom as a central atom: there are 4 electron-domains and all are bonding domains. Therefore the predicted electron-domain geometry and molecular geometry are both tetrahedral. Consequently the predicted bond angle is 109.5o.
Central C atom as a central atom: there are 3 electron-domains and all are bonding domains. Therefore the predicted electron-domain geometry and molecular geometry are both trigonal planar. The predicted bond angle will deviate slightly from the usual 120o because of the space demand of the double bond.
Rightmost O atom as a central atom: there are 4 electron-domains. 2 bonding domains and 2 nonbonding domains. Therefore the predicted electron-domain geometry is tetrahedral and its molecular geometry is bent because of the 2 nonbonding domains which also cause a slight deviation of the bond angle from 120o.
The diagram is the integrated molecular geometry.