Elements molecule/formula unit

Atomic mass refers to the mass of one mole of an element. Molar mass refers to the mass of one mole of molecules, formula units, or ions. 

The meaning of a chemical formula was discussed in Chapter 5, and we learned how to interpret formulas in terms of the numbers of atoms of each element per formula unit. In this chapter, we will learn how to calculate the number of grams of each element in any given quantity of a compound from its formula and to do other calculations involving formulas. Formula masses are presented in Section 7.1, and percent composition is considered in Section 7.2. Section 7.3 discusses the mole—the basic chemical quantity of any substance. Moles can be used to count atoms, molecules, or ions and to calculate the mass of any known number of formula units of a substance. Section 7.4 shows how to use relative mass data to determine empirical formulas, and the method is extended to molecular formulas in Section 7.5. 

The remaining four elements form molecular solids. The atoms of white phosphorus, sulfur, and chlorine are strongly bonded into small molecules (formulas, P4, S8, and Cl2, respectively) but only weak attractions exist between the molecules. The properties are all appropriate to this description. Of course there is no simple trend in the properties since the molecular units are so different. 

Formulas for ionic compounds represent one formula unit. However, molecules too have formulas, and thus formula units. Even uncombined atoms of an element have formulas. Thus, formula units may refer to uncombined atoms, molecules, or atoms combined in an ionic compound one formula unit may be... 

The equation states that elementary sodium reacts with elementary chlorine to produce sodium chloride, table salt. (The fact that chlorine is one of the seven elements that occur in diatomic molecules when not combined with other elements is indicated.) The numbers before the Na and NaCI are coefficients, stating how many formula units of these substances are involved. If there is no coefficient in a balanced equation, a coefficient of 1 is implied, and so the absence of a coefficient before the Cl2 implies one Cl2 molecule. The equation thus states that when the two reagents react, they do so in a ratio of two atoms of sodium to one molecule of chlorine, to form two formula units of sodium chloride. In addition, it states that when the two reagents react, they do so in a ratio of 2 mol of sodium to 1 mol of chlorine molecules, to form 2 mol of sodium chloride. The ratios of moles of each reactant and product to every other reactant or product are implied ... 

By far the most insidious source of error, and also a common one, in making inferences about molecular symmetry from space group symmetry is the occurrence of disorder. This usually takes the form of what we may call a systematic disorder, as opposed to a random one. A molecule, or a component of the formula unit, will lie on a crystallographic symmetry element that would seem to require more symmetry of it than it is capable of having. It accomodates to this by systematically taking each of two (or more) orientations an equal number of times in different unit cells. 

The molar mass of an element is the mass per mole of its atoms the molar mass of a molecular compound is the mass per mole of its molecules the molar mass of an ionic compound is the mass per mole of its formula units. The units of molar mass in each case are grams per mole (g-moD1). 

The molar masses of molecular and ionic compounds are calculated from the molar masses of the elements present the molar mass of a compound is the sum of the molar masses of the elements that make up the molecule or the formula unit. We need only note how many times each atom or ion appears in the molecular formula or the formula unit of the ionic compound. For example, the molar mass of the ionic compound Na2S04 is... 

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. 

The nomenclature for molecular compounds is much less complicated than for ionic compounds. Molecular compounds are formed from covalently bonded nonmetallic elements. The formula for a molecule represents a stable unit of atoms, unlike a formula for an ionic compound, which only represents the simplest whole number ratio of ions. As a result, molecular formulas cannot be simplified like formulas for ionic compounds. An example would... 

To indicate that a binary compound is made up of two non-metals, a prefix is usually added to both non-metals in the compound. This prefix indicates the number of atoms of each element in one molecule or formula unit of the compound. For example, P205 is named diphosphorus pen-toxide. Alternatively, the Stock System may be used, and P205 can be named phosphorus (V) oxide. AsBr3 is named phosphorus tribromide. 

As part of his atomic theory, John Dalton stated that atoms combine with one another in simple whole number ratios to form compounds. For example, the molecular formula of benzene, C6H6, indicates that one molecule of benzene contains 6 carbon atoms and 6 hydrogen atoms. The empirical formula (also known as the simplest formula) of a compound shows the lowest whole number ratio of the elements in the compound. The molecular formula (also known as the actual formula) describes the number of atoms of each element that make up a molecule or formula unit. Benzene, with a molecular formula of C6H6, has an empirical formula of CH. Table 6.1 shows the molecular formulas of several compounds, along with their empirical formulas. 

I Because formulas are used to represent unbonded atoms, covalently bonded molecules (Section 5.5), and ionically bonded compounds (Section 5.2), a formula unit can represent an atom, a molecule, or the simplest unit of an ionic compound (Figure 5.8). For example. He represents an uncombined atom F2 represents a molecule of an element CO2 represents a molecule of a compound ... 

The formula mass of a compound is the sum of the atomic masses (Chap. 3) of all the atoms (not merely each kind of atom) in the formula. Thus, in the same way that a symbol is used to represent an element, a formula is used to represent a compound or a molecule of an element, such as H2, and also one unit of either. The formula mass of the substance or the mass of 1 mol of the substance is easily determined on the basis of the formula (Sec. 7.4). Note that just as formula unit may refer to uncombined atoms, molecules, or atoms combined in an ionic compound, the term formula mass may refer to the atomic mass of an atom, the molecular mass of a molecule, or the formula mass of a formula unit of an ionic compound. 

The same procedure can be used for molecules and compounds. By adding together the relative atomic masses of the individual elements in a particular compound in their correct proportions, we get the relative molecular mass (RMM) of the compound this is equivalent to one mole of the compound and contains 6 X 10 formula units. 

You have learned that covalent compounds, such as water and hexa-chloroethane, consist of molecules as units. Formulas for covalent compounds show both the elements and the number of atoms of each element in a molecule. Hexachloroethane has the formula QCls. Each molecule has 8 atoms covalently bonded to each other. Ionic compounds aren t found as molecules, so their formulas do not show numbers of atoms. Instead, the formula shows the simplest ratio of cations and anions. 

Chemical formulas reveal composition. The subscripts in the formula give the number of atoms of a given element in a molecule or formula unit of a compound or diatomic element. 

Percentage composition gives the relative contribution of each element to the total mass of one molecule or formula unit. 

Would it surprise you to learn that two or more substances with distinctly different properties can have the same percent composition and the same empirical formula How is this possible Remember that the subscripts in an empirical formula indicate the simplest whole-number ratio of moles of the elements in the compound. But the simplest ratio does not always indicate the actual number of moles in the compound. To identify a new compound, a chemist must go one step further and determine the molecular formula, which specifies the actual number of atoms of each element in one molecule or formula unit of the substance. Figure 11-11 shows an important use of the gas, acetylene. It has the same percent composition and empirical formula, CH, as benzene which is a liquid. Yet chemically and structurally acetylene and benzene are very different. 

The empirical formula for a substance is the simplest ratio of the number of atoms or moles of each element. The molecular formula gives the exact number of each atom or moles of atoms in a molecule, whereas the formula unit is the empirical formula for a solid for which no discrete molecules exist. Chemical equations are balanced by inspection, ensuring that the same number of atoms of each element appears on both sides of the equation. 

We have already discussed that fact that you can find the atomic masses for each of the elements on the Periodic Table of Elements. In this lesson, we will examine the process of taking these individual elemental masses and determining the mass of a compound. In reality, you have already needed to do some of these calculations, but now we will cover them formally. When dealing with molecular compounds we talk about molecular mass. The molecular mass is the mass of one molecule of a molecular compound. When we are dealing with an ionic compound, we can t use the term molecular mass, because ionic compounds are not made up of individual molecules. For ionic compounds, we use the term formula mass, which is the mass of one formula unit. The term formula unit refers to the simplest ratio of the cations to anions that are found in the compound. NaCl, the formula unit of table salt, for example, indicates that there is one Na+ ion for every Cl ion. 

When you want to determine the number of atoms of a particular element found in a certain number of molecules, it is often easiest to determine the number of atoms of that element in one molecule, and then multiply by the number of molecules. The same is true when you are working with formula units and, as was the case in our previous lesson, working with formula units or molecules doesn t change the method for solving these particular problems. 

In Figure 3.2 an arbitrary object, here the set of discrete atoms belonging to the backbone structure of a small protein, is chosen as the fundamental unit of construction. This object is termed the asymmetric unit because, in the completed crystal, no part of this object will be systematically related to any other of its parts by crystallographic properties. That is, it has no inherent symmetry or symmetry elements or, if present, they do not coincide with any symmetry operators of the crystal (i.e., the elements generate only internal or local symmetry). In general, the asymmetric unit is one formula unit of a compound, a molecule, or a protein subunit. It can be a small integral number of these, or it may be a fraction such as j or i if the molecule does posses self-symmetry. The essential property of the asymmetric unit for our purposes is the set of relative x, y, z coordinates of the atoms, which comprise its structure. These are, of course, what we are ultimately interested in. 

The general term formula unit applies to molecular or ionic compounds, whereas the more specific term molecule appUes only to elements and compounds that exist as discrete molecules. 

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