Learning Objectives Predict the general shape of a simple covalent molecule. Predict the polarity of the molecule. Compare the properties of ionic and molecular compounds. Unlike the ions in ionic compounds, which are arranged in lattices called crystals, molecules of covalent compounds exist as discrete units with a characteristic mass and a certain three-dimensional shape. Molecular Shape: VSEPR Theory Unlike ionic compounds, with their extended crystal lattices, covalent molecules are discrete units with specific three-dimensional shapes. The shape of a molecule is determined by the fact that covalent bonds, which are composed of negatively charged electrons, tend to repel one another. This concept is called the valence shell electron pair repulsion (VSEPR) theory . For example, the two covalent bonds in \(\ce{BeCl2}\) stay as far from each other as possible, ending up 180° apart from each other. The result is a linear molecule: The three covalent bonds in BF 3 repel each other to form 120° angles in a plane, in a shape called trigonal planar : The molecules \(\ce{BeCl2}\) and \(\ce{BF3}\) actually violate the octet rule ; however, such exceptions are rare and will not be discussed in this text. Try sticking three toothpicks into a marshmallow or a gumdrop and see if you can find different positions where your “bonds” are farther apart than the planar 120° orientation. The four covalent bonds in CCl 4 arrange themselves three dimensionally, pointing toward the corner of a tetrahedron and making bond angles of 109.5°. CCl 4 is said to have a tetrahedral shape: Empty DataFrame Columns: [(Atoms Around Central Atom, 2 \(\ce{AB_2}\), 3 \(\ce{AB_3}\), 4 \(\ce{AB_4}\)), (Geometry, Linear, Trigonal Planar, Tetrahedral), (Example, \(\ce{BeCl_2}\), \(\ce{BF_3}\), \(\ce{CCl_4}\))] Index: [] In determining the shapes of molecules, it is useful to first determine the Lewis diagram for a molecule. The shapes of molecules with multiple bonds are determined by treating the multiple bonds as one bond. Thus, formaldehyde (CH 2 O) has a shape similar to that of BF 3 . It is trigonal planar . Molecules With Lone Pairs Around Central Atom Molecules with lone electron pairs around the central atom have a shape based on the position of the atoms, not the electron pairs. For example, NH 3 has one lone electron pair and three bonded electron pairs . These four electron pairs repel each other and adopt a tetrahedral arrangement. However, the shape of the molecule is described in terms of the positions of the atoms, not the lone electron pairs. Thus, NH 3 is said to have a trigonal pyramidal shape, not a tetrahedral one. Similarly, H 2 O has two lone pairs of electrons around the central oxygen atom and two bonded electron pairs . Although the four electron pairs adopt a tetrahedral arrangement, the shape of the molecule is described by the positions of the atoms only. The shape of H 2 O is bent with an approximate 109.5° angle. In summary, to determine the molecular geometry: Step 1: Draw the Lewis structure. Step 2: Count the number of bonds (a double/triple bond counts as one) and lone pairs around the central atom . Step 3: Use Table 4.5.1 to determine the molecular geometry. Table 4.5.1: The molecular geometry depends on the number of bonds and lone pairs around the central atom, A. Example \(\PageIndex{1}\) What is the geometry of the ammonium ion, NH 4 + ? Its Lewis structure is shown below. How is this different from ammonia, NH 3 ? Solution In ammonium ion, the central atom N has 4 bonds and no lone pair. It is equivalent to the below in Table 4.5.1. Hence, this is tetrahedral . In ammonia (NH 3 ), shown below, N has 3 bonds and one lone pair. It is equivalent to the below in Table 4.5.1. Hence, the shape of this molecule is trigonal pyramid . Exercise \(\PageIndex{1}\) What is the molecular shape of nitrosyl chloride, a highly corrosive, reddish-orange gas? Its Lewis structure is shown below. Answer Focus on the central atom, N. It has a double bond to O, count this as one bond. It also has a single bond to Cl. Thus, N has 2 bonds and one lone pair. These 3 electron pairs will spread out 120 degrees from each other. But, since the shape is defined by the arrangement of the atoms only, the shape is bent or angular. If you consult Table 4.5.1, this molecule is equivalent to the below. Hence, two bonds and one lone pair has a bent or angular shape. Molecular Polarity In general, a molecule is nonpolar if all its bonds are nonpolar. Examples are I 2 , O 2 , H 2 , CH 4 , C 2 H 6 and C 3 H 8 . In general, a molecule is polar if it contains polar bonds EXCEPT when the bond polarities cancel each other. As mentioned in Section 4.4, the shape of the CO 2 molecule (linear) orients the two C=O polar bonds directly opposite each other, thus cancelling each other's effect. Carbon dioxide (CO 2 ) is a nonpolar molecule. On the other hand, water (as discussed above) is a bent molecule because of the two lone pairs on the central oxygen atom. Because of the bent shape, the dipoles do not cancel each other out and the water molecule is polar . In the figure below, the individual H-O polar bonds represented by the two red arrows are not directly opposite each other. These two dipoles don't cancel each other out. In fact, the net dipole (blue arrow) points upward. There is a resultant partial positive charge at one end (between the two H atoms) and a partial negative charge on the other end (where O is located). The uneven distribution of charge or the overall dipole is shown by the blue arrow below (Figure 4.5.1). Hence, water is polar (has + and - poles) while carbon dioxide is nonpolar . Similarly, in BF 3 (trigonal planar), the effect of a B-F bond is cancelled by the sum of the other two B-F bonds (see video). Hence, a trigonal planar molecule (BF 3 ) is nonpolar because the bond polarities cancel each other, but a trigonal pyramidal molecule (NH 3 ) is polar. Some other molecules are shown in the figure below. Notice that a tetrahedral molecule such as CCl 4 is nonpolar . However, if the peripheral atoms are not of the same electronegativity, the bond polarities don't cancel and the molecule becomes polar , as in CH 3 Cl. Physical Properties of Molecular Compounds The physical state and properties of a particular compound depend in large part on the type of chemical bonding it displays. Molecular compounds , sometimes called covalent compounds, display a wide range of physical properties due to the different types of intermolecular attractions such as different kinds of polar interactions. The melting and boiling points of molecular compounds are generally quite low compared to those of ionic compounds . This is because the energy required to disrupt the intermolecular forces (discussed in Section 8.1) between molecules is far less than the energy required to break the ionic bonds in a crystalline ionic compound. Since molecular compounds are composed of neutral molecules, their electrical conductivity is generally quite poor, whether in the solid or liquid state. Ionic compounds do not conduct electricity in the solid state because of their rigid structure, but conduct well when either molten or dissolved into a solution. The water solubility of molecular compounds is variable and depends primarily on the type of intermolecular forces involved. Substances that exhibit hydrogen bonding or dipole-dipole forces are generally water soluble, whereas those that exhibit only London dispersion forces are generally insoluble. Most, but not all, ionic compounds are quite soluble in water. The table below summarizes some of the differences between ionic and molecular compounds. 0 1 2 Property Ionic Compounds Molecular Compounds Type of elements Metal and nonmetal Nonmetals only Bonding Ionic - transfer of electron(s) between atoms Covalent - sharing of pair(s) of electrons between atoms Representative unit Formula unit Molecule Physical state at room temperature Solid Gas, liquid, or solid Water solubility Usually high Variable Melting and boiling temperatures Generally high Generally low Electrical conductivity Good when molten or in solution Poor Table 4.5.2: Comparison of Ionic and Molecular Compounds In summary, covalent compounds are softer, have lower boiling and melting points, are more flammable, are less soluble in water and do not conduct electricity compared to ionic compounds. The individual melting and boiling points, solubility and other physical properties of molecular compounds depend on molecular polarity. Example \(\PageIndex{2}\) Describe the shape of each molecule. Is it polar or nonpolar? PCl 3 CO 2 Solution The Lewis diagram for PCl 3 is as follows: Focus on the central atom, P that has 3 bonds and one lone pair. The four electron pairs arrange themselves tetrahedrally, but the lone electron pair is not considered in describing the molecular shape. Like NH 3 , this molecule is pyramidal . The 3 P-Cl bonds don't cancel each other. This is polar. The Lewis diagram for CO 2 is as follows: Focus on the central atom, C. The multiple bonds are treated as one group, hence C has 2 bonds and zero lone pair. CO 2 has only two groups of electrons that repel each other. They will direct themselves 180° apart from each other, so CO 2 molecules are linear . This is highly symmetrical, with the two opposite dipoles cancelling each other. The CO 2 molecule is nonpolar . Exercise \(\PageIndex{2}\) Describe the shape of each molecule. Is it polar or nonpolar? CBr 4 BCl 3 Answer a: or The Lewis structure shows 4 groups attached to the central atom, hence tetrahedral . All the 4 groups are identical and the shape is symmetrical. Hence, it is nonpolar. Answer b: The Lewis diagram shows 3 groups attached to the central atom, hence trigonal planar . All the 3 groups are identical and shape is symmetrical, hence, it is nonpolar. Concept Review Exercises How do you determine the shape of a molecule? How do you determine whether a molecule is polar or nonpolar? Answers The shape of a molecule is determined by the position of the atoms, which in turn is determined by the repulsion of the bonded and lone electron pairs around the central atom. If all the bonds in a molecule are nonpolar, the molecule is nonpolar. If it contains identical polar bonds that are oriented symmetrically opposite each other (linear, trigonal planar or tetrahedral) then the molecule is nonpolar. If it contains polar bonds that don't cancel each other's effects, the molecule is polar. Key Takeaways Simple molecules have geometries that can be determined from VSEPR theory. Polar molecules result from differences in electronegativity of the atoms in the molecule. Dipoles that are directly opposite one another cancel each other out.