Formulas of Elements

In Section 2.1, atoms and molecules are identified as two of the particles studied in chemistry. They appear again in the introduction to chemical formulas in Section 2.6. 

The formula of most elements is simply the elemental symbol. In a chemical reaction, the macroscopic sample of the element behaves as if it were a collection of individual particles, or atoms. This indicates that the tiniest individual unit in a sample of the element is one atom. We call this individual unit a formula unit. 

Some elements do not behave as if their smallest particle is a single atom. For example, at common temperatures seven elements form diatomic molecules. A molecule is the tiniest independent particle of a pure substance—element or compound— in a sample of that substance. Di- is a prefix that means two. Thus, a diatomic molecule consists of two atoms that are chemically bonded to each other. This molecule is a formula unit of the element. Its chemical formula is the elemental symbol followed by a subscript 2. For example, the chemical formula of hydrogen (symbol H) is H2. 

You must be able to recognize the elements that form diatomic molecules and write their formulas correctly. Listed in a way that will help you remember them, they are 

Fluorine, chlorine, bromine, and iodine, the first fonr elements in Group 7A/17 

---

Note in the previous equations that the formulas of elemental hydrogen and oxygen are written as diatomic molecules H2 and O2. Before equations are balanced, the formulas for all reactants and products must be written according to the rules given in Chapter 5. 

The language we use to describe the forms of matter and the changes in its composition is not limited to use in chemistry courses it appears throughout the scientific world. Chemical symbols, formulas, and equations are used in such diverse areas as agriculture, home economics, engineering, geology, physics, biology, medicine, and dentistry. In this chapter we describe the simplest atomic theory. We shall use it as we represent the chemical formulas of elements and compounds. Later, after additional facts have been introduced, this theory will be expanded. 

Introduction to Nomenclature Formulas of Elements Compounds Made from Two Nonmetals Names and Formulas of Ions Formed by One Element... 

In Chapter 2, we took one mole of an element to be 6.02214 x 10 atoms of the element. This is the only definition possible for such elements as iron, magnesium, sodium, and copper, in which enormous numbers of individual spherical atoms are clustered together, much like marbles in a can. But the atoms of some elements are joined together to form molecules. Bulk samples of these elements are composed of collections of molecules. The molecules of P4 and 83 are represented in Figure 3-5. The molecular formulas of elements that you should become familiar with are... 

In empirical formulas of inorganic compounds, electropositive elements are listed first [3]. The stoichiometry of the element symbols is indicated at the lower right-hand side by index numbers. If necessary, the charges of ions are placed at the top right-hand side next to the element symbol (e.g., S "). In ions of complexes, the central atom is specified before the ligands are listed in alphabetical order, the complex ion is set in square brackets (e.g., Na2[Sn(OH)+]). 

The analyses which follow are arranged in the order in which they would be applied to a newly discovered substance, the estimation of the elements present and molecular weight deter-minations(f.e., determination of empirical and molecular formulae respectively) coming first, then the estimation of particular groups in the molecule, and finally the estimation of special classes of organic compounds. It should be noted, however, that this systematic order differs considerably from the order of experimental difficulty of the individual analyses. Consequently many of the later macro-analyses, such as the estimation of hydroxyl groups, acetyl groups, urea, etc. may well be undertaken by elementary students, while the earlier analyses, such as estimation of elements present in the molecule, should be reserved for more senior students. 

A double-focusing mass spectrometer can mea.sure mass accurately to several decimal places, thus enabling the determination of molecular formulae and elemental compositions of fragment ions. 

An appropriate set of iadependent reference dimensions may be chosen so that the dimensions of each of the variables iavolved ia a physical phenomenon can be expressed ia terms of these reference dimensions. In order to utilize the algebraic approach to dimensional analysis, it is convenient to display the dimensions of the variables by a matrix. The matrix is referred to as the dimensional matrix of the variables and is denoted by the symbol D. Each column of D represents a variable under consideration, and each tow of D represents a reference dimension. The /th tow andyth column element of D denotes the exponent of the reference dimension corresponding to the /th tow of D ia the dimensional formula of the variable corresponding to theyth column. As an iEustration, consider Newton s law of motion, which relates force E, mass Af, and acceleration by (eq. 2) ... 

From the plant Escallonia pulvemlenta (Escalloniaceae), which grows in Chile, an iridoid gluco-side of elemental composition CigZ/j On was isolated. Formula / gives the structure of the iridoid glucoside skeleton... 

A little explanation is in order. A set is a collection of elements with a defining formula for example, the set of positive integers, the set of all integers, the set of all real numbers and so on. Figure 13.1 shows a mapping / between two sets ... 

To this point, our study of chemistry has been largely qualitative, involving very few calculations. However, chemistry is a quantitative science. Atoms of elements differ from one another not only in composition (number of protons, electrons, neutrons), but also in mass. Chemical formulas of compounds tell us not only the atom ratios in which elements are present but also the mass ratios. 

As you will see shortly, the formula of a compound can be used to determine the mass percents of the elements present. Conversely, if the percentages of the elements are known, the simplest formula can be determined. Knowing the molar mass of a molecular compound, it is possible to go one step further and find the molecular formula. In this section we will consider how these three types of calculations are carried out. 

Knowing the formula of a compound, Fe203> you can readily calculate the mass percents of its constituent elements. It is convenient to start with one mole of compound (Example 3.4a). The formula of a compound can also be used in a straightforward way to find the mass of an element in a known mass of the compound (Example 3.4b). 

The most complex problem of this type requires you to determine the simplest formula of a compound given only the raw data obtained from its analysis. Here, an additional step is involved you have to determine the masses of the elements present in a fixed mass of the compound (Example 3.6). 

The compound that gives vinegar its sour taste is acetic acid, which contains the elements carbon, hydrogen, and oxygen. When 5.00 g of acetic acid are burned in air, 7.33 g of C02 and 3.00 g of water are obtained. What is the simplest formula of acetic acid ... 

Chemical analysis always leads to the simplest formula of a compound because it gives only the simplest atom ratio of the elements. As pointed out earlier, the molecular formula is a whole-number multiple of the simplest formula. That multiple may be 1 as in H20, 2 as in H202, 3 as in CjHg, or some other integer. To find the multiple, one more piece of data is... 

Chemical reactions are represented by chemical equations, which identify reactants and products. Formulas of reactants appear on the left side of the equation those of products are written on the right In a balanced chemical equation, there are the same number of atoms of a given element on both sides. The same situation holds for a chemical reaction that you carry out in the laboratory atoms are conserved. For that reason, any calculation involving a reaction must be based on the balanced equation for that reaction. 

Write the formula of a compound of each of the following elements that cannot act as an oxidizing agent. 

Simplest formula A formula of a compound which gives the simplest atom ratio of the elements present The simplest formula of hydrogen peroxide is HO, 57-60... 

Here are the names of some common chemicals and their formulas. What elements does each contain ... 

Table 6-IX. the formulas of some compounds of the third-row elements...

There is little new to be said about the bonding capacity of a lithium atom. With just one valence electron, it should form gaseous molecules LiH and LiF. Because of the vacant valence orbitals, these substances will be expected only at extremely high temperatures. These expectations are in accord with the facts, as shown in Table 16-1, which summarizes the formulas and the melting and boiling points of the stable fluorides of the second-row elements. In each case, the formula given in the table is the actual molecular formula of the species found in the gas phase. 

---