George Martins
Newton Junior College Newtonville, Mossochusetts
Atomic Orbital Molecular Models
Thc student of beginning chemistry is now presented with a more refined approach to the structures of molecules. He can proceed from the orbital spatial configurations of electrons in atoms to the varied molecular geometry of the chemical bond. Models provide a qualitative way of "seeing" these structures. This article considers a set of styrofoam models of covalently bonded structures developed from atomic orbitals, for lecture presentation and student use. A survey of recent literature on molecular models indicates the interest and concern in molecular structure. Ogrydo and Porter produced an extensive set of atomic orbitals and a set of molecular models from molecular orbital considerations ( 1 ) . Baker depicts an ethylene molecule showing orbital configuration and pi bonding (2). Molecular models based on electronpair repulsion were made by Bromlilc (9). Charge cloud models, theoretically developed by K i b a l l , are described by Bent (4), and used in a chemistry text (5). And in the treatment of organic compounds, ball and stick models have evolved into framework models for
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con~plexstructures (6-8). A models kit similar in approach to the set described in this article has been developed by Stone and Siegelman (9). The kit differs in the use of diierent construction materials, the procedure for assembly, and the necessity to purchase it as a kit. The production of the following models follows from an understanding of the spatial configuration of electrons in atoms and of the formation of bonds between atoms by orbital overlap. Features of signla and pi bonding from Molecular Orbital Theory are considered, but the models portray the pedagogical simplicity of orbital overlap. Important points denlonstrated in the models are the following: (1) single and multiple bonding, (2) hybridization, (3) structural geometry, (4) electron delocalization, ( 5 ) electron boolckeeping. The models depict best the outer p or hybridized orbital inclusion in molecular structure but do not included orbitals and, consequently, do not apply to transition metal bonding. Also, no scale of dimension is used and thus one model can be used to
demonstrate several structures, as in the case of the hydrogen halides. Molded white expanded polystyrene' in the f6rm of 51-mm-diameter spheres and 51 X 66-mm teardrops are the basic construction pieces. Some teardrops are color coded for the benefit of electron bookkeeping. Spheres used to represent overlapping s orbitals are cut (sawed) 13 nnu from the edge and punctured in the center of the flat faces. Teardrops used for overlapping p or hybridized orbitals are cut 13 mm from the wide end. All teardrops are punctured with holes in the thin ends, in the faces of those cut, and on the sides for orbitals involved in pi bonding. In Figure 1 the various construction pieces are described. In addition to the plast~cpieces are the frames and connectors
Figure I . Construction parts. Top, I. to r.. half of half-filled p orbital lyellowi; half of full orbit01 (white); full hybridized orbitol I b l ~ e l . Center, I. to r., trig planor plus p frame; overlopping p or hydbridired orbital Iwhite); trig plonor and tetrahedral framer; overlapping r orbital lwhite); coordinate oxer. Bottom, connectors, long for pi bonding and short for sigma bonding.
uscd in assembly. The frames are made from chenille #4 pipe cleaners, cut, twisted together, and wrapped with wire tightly around the centers, their arms extcuding 3-4 cm from center. Connectors are cut in 5 and 13 cm lengths. AIodels are easily constructed by sliding the thin ends of teardrops onto frames and using small connectors between faces of teardrops. Models
With the color coding for determining electron distribution in p orbitals, a variety of atomic structures All materials used, including paint colors, were obtained from Plasteel Cnrporetinn, 26070 Princet,on, Inkster, Michigan.
Figure 2. Structure of otomr: corbon (two pair of yellow teordraprl and oxygen (two pair yellow and one pair whitel.
Figure 3. Single bondr: Ruorine moleculer.
can be represented in their outermost orbital configurations. From these, covalent bonded molecules can be constructed from the principle of orbital overlap, end-toend for sigma bonding and side-to-side for pi bonding. Atoms. Models of the atoms of elements, atomic numbers 1 to 20, can be constructed. The hydrogen atom is represented by a single yellow sphere. (The alkali and alkaline earth metals are not of much concern in a treatment of the covalent bond, but their outermost orbital configuration could be represented by a yellow sphere in the case of the former and a white sphere, full outer s orbital, for the latter.) The carbon atom, i s 2 2s2 2pZ12pV1,consists of two pair of yellow teardrops on coordinate axes; and the oxygen atom, l s 2 2sZ2p4,consists of two pair of yellow teardrops and one of white to indicate two half-filled p orbitals and one full p orbital (Fig. 2 ) . These two structures can be used also to represent the silicon and sulfur atoms. A group of these models representing most of the elements in a period can be used in the discussion of periodic properties. Simple single bonding. The hydrogen molecule and its sigma bond of two overlapping orbitals can be shown by two cut spheres joined by a small connector (Fig. 3). In the same figure is depicted the fluorine molecule. In the construction of the fluorine atom a bonding p orbital is available. The two yellow teardrops are replaced by white cut teardrops and two similarly modified F atoms can be connected to form the molecule. With a cut sphere and the halogen atom the hydrogen halides can be represented as in Figure 4. The water molecule has two covalently bonded hydrogens; but since its bond angle is better described by a hybridized structure, it is described later. However, hydrogen sulfide, H2S, can be demonstrated with p orbitals and the coordinate axes as shown in Figure 4. Multiple bonding. The oxygen molecule, because of its paramagnetic nature, does not have a single simple structure with a double bond. The nitrogen molecule. Nz, with its triple bond can be used to show multiple bonding (Fig. 6). From the electron configuration of the nitrogen atom it can be seen that a sigma bond and two pi bonds account for the structure of the molecule. For side-to-side overlap a long connector is pushed into the sides of yellow teardrops. Hybridizatirm. Methane, CH4, is the most common example of sp3 hybridization. Four cut teardrops on a tetrahedral frame are connected to cut spheres, representing hydrogen atoms (Fig. 6). In the discussion of this model from the structure of the carbon atom.
the hydrogen and
Figure 4. Single bondr: hydrogen holider, HX, and hydmgen sulfide, H&.
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8.
Figure 9.
Figure Hybridirotion and multiple bonds; ethylene, C&, upper left; and acetylene, CIH1. lower right.
electron promotion is considered leading to the hybridized low energy state in the molecule. This model can he modified to show the ammonia, NHI, and water, H20, molecules by replacing a sp3-s combination with blue teardrops indicating electron pairs (Fig. 6). I n this way, models for the latter two molecules show bond angles closer to experimental values than the 90' angle expected from nilhybridized stmctures. Boron trifluoride, BF8, with its sp2 hybridization and trigonal planar shape, is depicted in Figure 7. The other hybridized states of carbon can be seen in ethylene, CzH,, and acetylene, C2H2,which also show multiple bonding (Fig. 8). The former requires two sp2 plus p axes for the carbon centers, whiie the latter has two coordinate axes for each of the pairs of sp hybrid orbitals and the two pairs of p orbitals. As in N2, pi bonding is shown by long connectors between yellow teardrops. De2oealizatia (Resaanee). There are structures that do not give a complete bookkeeping of electrons by the above methods. NO3- and COi- are isoelectronic and can be represented by the same structure (Fig. 9). For NO3-, the N atom is considered as sp2 hybridized in a trigonal plane bonded with three oxygens. By counting electrons the four vertical p orbitals can be thought of as containing six electrons, all entering into a single pi hond in the ion, each N-0 bond contributing pi hond. For complete bookkeeping a model can he made with a pi bond between the N atom and one of the 0 atoms, but the other contributing structures should be discussed. Other molecules showing delocalization of electrons, such as ozone and benzene, can be constructed; the benzene, however, is unwieldy in size. Student Exercise
Because of the significant time spent in lecture on 660
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Delocalization:
nitrote ion, NO;
structure and bonding and because of the lack of enough suitable student experiments on these topics, a student exercise was developed in which students actually constructed their own models from given formulas. Using paper drawings for their electron bookkeeping, they constrncted some of the above-described niodels and others, e.g., BeH2, SiH,, OFa, C12, OKC1. CO?, HaO+, OH-, H202,O-, SO
