Molecules naming molecular compounds

Now you are ready to practice naming molecular compounds. Several different molecules can be formed when different numbers of nitrogen and oxygen atoms combine. Look at their formulas in the first colrnnn of Table 5.6, and try to name them without looking at the names listed in the second column. The brown gas pictured is NO2. 

Unlike ionic compounds, molecular compounds are composed of individual covalently bonded units, or molecules. Chemists use two nomenclature systems to name binary molecules. The newer system is the Stock system for naming molecular compounds, which requires an understanding of oxidation numbers. This system will be discussed in Section 2. 

The old system of naming molecular compounds is based on the use of prefixes. For example, the molecular compound CCl is named carbon tetrachloride. The prefix tetra- indicates that foiu chloride atoms are present in a single molecule of the compound. The two oxides of carbon, CO and CO2, are named carbon monoxide and carbon rftoxide, respectively. These prefix-based names are often the most widely-recognized names for some molecular compounds. However, either naming system is acceptable, unless specified otherwise. 

Some covalent compounds do not consist of individual molecules. Instead, each atom is joined to aU its neighbors in a covalently bonded, three-dimensional network. There are no distinct units in these compounds, just as there are no such units in ionic compounds. The subscripts in a formula for a covalent-network compound indicate the smallest whole-number ratio of the atoms in the compound. Naming such compounds is similar to naming molecular compounds. Some common examples are given below. 

Just as was the case for the reactant filtering, many methodologies exist for selecting library subsets for synthesis (18,20-29). For this example library we used a simple Rule of 5 type filter to select a subset of compounds (6). After filtering, the final step in the process is to extract the reactant lists from the selected library subset. Because in each step of the procedure outlined above we have maintained the MFCD numbers for all the reactants (tagged by reactant number), this is simply a matter of extracting these numbers from the final library. For each of the compounds we provide the molecule name, list of vendors, MFCD and CAS (if available) numbers, molecular weight, and information about whether the reactant is available in-house (Fig. 13). 

Predicting how the atoms within molecules will bond with one another is a tricky endeavor because two nonmetals often can combine in multiple ratios. Carbon and oxygen, for example, can combine in a one-to-two ratio to form CO2 (carbon dioxide), a harmless gas you emit every time you exhale. Alternatively, the same two elements can combine in a one-to-one ratio to form CO (carbon monoxide), a poisonous gas. Clearly, having names that distinguish between these (and other) molecular compounds is useful. The punishment for sloppy naming can be death. Or at least embarrassment. 

Naming binary molecular compounds requires using Greek prefixes to indicate the number of atoms of each element in the compound or molecule. Prefixes are given in Table 1.4. Prefixes precede each element to indicate the number of atoms in the molecular compound. The stem of the second element takes the ide suffix. The prefix mon is dropped for the initial element that is, if no prefix is given, it is assumed that the prefix is 1. Examples of molecular compounds are carbon dioxide (C02), carbon monoxide (CO), and dinitrogen tetroxide (N204). 

Binary molecular compounds are named by assuming that one of the two elements in the molecule is more cationlike and the other element is more anionlike. As with ionic compounds, the cationlike element takes the name of the element itself, and the anionlike element takes an -ide ending. The compound HF, for example, is called hydrogen fluoride. 

Molecular compounds are easier to name than ionic compounds, so let s begin there. The molecular formula of a substance gives the number of each kind of atom in the molecule. To name a molecular substance ... 

Ionic Compounds To name an ionic compound, you just name the cation and then the anion. There is a crucial difference between naming ionic compounds and molecular compounds. In molecular compounds you must include prefix multipliers (di, tri, etc.) to indicate the number of each kind of atom in the molecule. In ionic compounds you must not include prefix multipliers, because the number of each ion in the formula unit is controlled by the charges on the ions. If the cation is a representative element, it is not necessary to indicate the charge, because (with few exceptions) these metals form cations with an ionic charge equal to the group number. 

Water molecules often occupy positions within the lattice of an ionic crystal. These compounds are called hydrates, and the water molecules are known as water of hydration. The water of hydration is added after a centered dot in a formula. In a name, a number-prefix (listed below for molecular compounds) indicating the number of water molecules is followed by the root -hydrate. 

Molecular compounds (compounds making up molecules with a neutral charge) are usually composed entirely of nonmetals and are named by placing the less electronegative atom first. See Skill 3.3 for the relationship between electronegativity and the periodic table. The suffix -ide is added to the second, more electronegative atom, and prefixes indicating numbers are added to one or both names if needed. 

Another source of difficulty is that the atoms in molecules do not form ions. Therefore, the use of Roman numerals is not possible. And because nonmetals have multiple oxidation states possible (nitrogen has 8), it is too difficult to determine the possible combinations of atoms in a molecule. All of this is meant to illustrate why a different naming system is required for molecular compounds. 

Compounds which have interior cavities which are large enough to incarcerate organic guest molecules have been named molecular container compounds. 

Many binary molecular compounds were discovered and given common names long before the modem naming system was developed. Table 9-2 lists some of these molecules, their common names, and the binary molecular compound names. 

Using the system of rules for naming binary molecular compounds, describe how you would name the molecule N2O4. 

You can now identify atoms that bond covalently and name the molecular compounds formed through covalent bonding. In order to predict the arrangement of atoms in each molecule, a model, or representation is used. Several different models can be used, as shown in Figure 9-10. Note that in the ball-and-stick and space-filling molecular models, atoms of each specific element are represented by spheres of a representative color, as shown in Table C-1 in Appendix C. These colors are used for identification of the atoms if the chemical symbol of the element is not present. 

Use Greek prefixes (Table 3.10) to specify the number of atoms of each element in the molecular formula of the compound (di- for two, tri- for three, and so forth). If the compound is a solid without well-defined molecules, name the empirical formula in this way. The prefix for one (mono-) is omitted, except in the case of carbon monoxide. 

Of the various types of higher order compounds which should be named as coordination compounds, only molecular addition compounds have not been considered. The rules for naming inorganic compounds indicate that (CHaJsN-BF would be called compound of boron trifluoride with trimethylamine., However, Davidson and Brown (4) suggest that this substance be called trimethylamine-boron trifluoride. In general, that molecule which donates a pair of electrons (base) is given first followed by the acceptor molecule (acid). Similarly, (CH3)2S-A1(CH3)3 would be known as dimethyl sulfide-trimethylaluminum. If these were named as coordination compounds, the names would be (trimethylamine)trifluoroboron and (dimethylsulfide)trimethylalu-minum. 

N. (1) Symbol for nitrogen. The names of certain compounds (such as V,lV-dibutyl urea) contain this symbol as an indication that the group or groups appearing next in the name (i.e., the butyl groups in the example cited) are joined to the nitrogen atoms in the molecule. The molecular formula is N2. (2) Mathematical symbol for Avogadro s number. (3) Abbreviation for normal solution. 

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