Electron Dot Structures for Molecules

We saw in Chapter 19 how to draw Lewis electron dot structures for atoms and how to combine them to make diatomic molecules. In this chapter we will do the same with molecules that contain three or more atoms. To do so, we will follow two fundamental rules  

Although there are molecules that do contain an odd number of electrons, in this discussion we will limit our examples only to molecules that contain an even number of electrons. Therefore, in the Lewis structure of a molecule, all of the electrons will be shown as being paired. 

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Write electron-dot structures for molecules with the following connections ... 

Draw an electron-dot structure for each of the molecules in Problem 2.38, indicating any unshared electron pairs. 

Draw electron dot structures for each of the following molecules (a) CO, (b) C02, (c) HCN, (d) N20 (an unsymmetrical molecule, with the two nitrogen atoms adjacent to each other). 

With the information given in Table 21-1, it is possible to write an electron dot structure for an organic molecule. 

When writing electron-dot structures for covalent compounds, chemists often use a straight line to represent the two electrons involved in a covalent bond. In some representations, the nonbonding electron pairs are left out. This is done in instances where these electrons play no significant role in the process being illustrated. Here are two frequently used ways of showing the electron-dot structure for a fluorine molecule without using spheres to represent the atoms ... 

Atoms other than hydrogen also form covalent bonds by sharing electron pairs, and the electron-dot structures of the resultant molecules are drawn by assigning the correct number of valence electrons to each atom. Group 3A atoms (such as boron) have three valence electrons, group 4A atoms (such as carbon) have four valence electrons, and so on across the periodic table. The group 7 A element fluorine has seven valence electrons, and an electron-dot structure for the F2 molecule shows how a covalent bond can form ... 

PROBLEM 7.4 Draw electron-dot structures for the following molecules ... 

PROBLEM 7.7 There are two molecules with the formula C2H60. Draw electron-dot structures for both. 

Which of the two structures for 03 is correct In fact, neither is correct by itself. Whenever it s possible to write more than one valid electron-dot structure for a molecule, the actual electronic structure is an average of the different possibilities, called a resonance hybrid. Note that the different resonance forms differ only in the placement of the valence-shell electrons. The total number of valence electrons remains the same in both structures, the connections between atoms remain the same, and the relative positions of the atoms remain the same. 

Aspirin has the following connections among atoms. Complete the electron-dot structure for aspirin, tell how many cr bonds and how many tt bonds the molecule contains, and tell the hybridization of each carbon atom. 

Try writing electron-dot structures for these two species, and you will see the problem Paramagnetic molecules... 

Lewis structures are electron dot representations for molecules. There are three general rules for drawing Lewis structures. 

Some examples of electron dot structures for a few commonly encountered molecules from inorganic chemistry. 

Now imagine a 300-pound smno wrestler on one end of the rope, and another 300-pound sumo wrestler on the other end. This situation is the same as when two of the same atoms form a bond, for example, when two fluorine atoms form the fluorine molecule, F2. Here is the electron dot structure for F2. 

The geometry of a methane molecule, CH4, is shown in Fig e 9.17. Methane is the simplest hydrocarbon compormd. Hydrocarbons are organic compounds composed of only hydrogen and carbon. The electron dot structure for methane consists of a central carbon atom with four C—H single bonds, as shown in Figure 9.17. 

MiniLab 2 Below is the electron dot structure for AlBr3. What geometric shape do you predict for this molecule ... 

Organizing Information Complete the table below. Draw the best electron dot structure for each compound listed. In all cases, except H2O, the first atom in the formula is the central atom. Predict the geometric arrangement of electron clouds aroimd the central atom and use your prediction to determine the geometry of the molecule. From your predicted geometry, decide whether the molecule is polar or nonpolar. Water is given as an example. 

In order to write Lewis dot structures for molecules containing only hydrogen and second row atoms (most organic molecules), we need to know the number of electrons in the atoms. For atoms in the second row of the periodic table, we can tell immediately how many electrons are available for bonding by knowing the atomic number of the atom or by knowing the column the atom is in. The number of available electrons is equal to the total number of electrons in the atom, which is the same as the atomic number, less the two Ir electrons. One can also use the column number, which corresponds to the number of electrons in the outer shell (see Table 1.8). The electrons in the outermost shell, which are the least tightly held electrons, are called the valence electrons. 

In Section 1.3, we constructed Lewis dot structures for molecules. Our next task is to elaborate on this theme to produce better pictures of the bonds that hold atoms together in molecules. We ll need to consider how electrons act to bind nuclei together, and we ll take as our initial example hydrogen (H2), the second simplest molecule. 

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