Molecules and Molecular Compounds

Out of all the elements, only the six noble gases in Group 8A of the periodic table (He, Ne, Ar, Kr, Xe, and Rn) exist in the form of isolated atoms under ordinary conditions. For this reason, they are called monatomic (meaning a single atom) gases. Most matter is composed of molecules, which we discuss in this section, or ions, which we will discuss in Section 2.7. 

A molecule is a combination of at least two atoms in a specific arrangement held together by electrostatic forces known as covalent chemical bonds [W Section 8.3]. A molecule may contain atoms of the same element, or it may contain atoms of two or more elements joined in a fixed ratio, in accordance with the law of definite proportions [W Section 2.1]. Thus, a molecule can be an element or it can be a compound, which, by definition, is made np of two or more elements [W Section 1.2], Hydrogen gas, for example, is an element, but it consists of molecules, each of which is made up of two H atoms. Water, on the other hand, is a compound that consists of molecules, each of which contains two H atoms and one O atom. 

The hydrogen molecule, symbolized as H2, is called a diatomic molecule because it contains two atoms. Other elements that normally exist as diatomic molecules are nitrogen (N2), oxygen (O2), and the Group 7A elements— fluorine (F2), chlorine (CI2), bromine (Br2), and iodine (I2). These are known as homonuclear diatomic molecules because both atoms in each molecule ate of the same element A diatomic molecule can also contain atoms of different elements. Examples of these het-eronuclear diatomic molecules include hydrogen chloride (HCl) and carbon monoxide (CO). 

Most molecules contain mote than two atoms. They can all be atoms of the same element, as in ozone (O3) and white phosphoras (P4), or they can be combirrations of two or mote different elements, as in water (H2O) and methane (CH4). Molecttles containing mote than two atoms ate called polyatomic molecules. 

In Chapters 8 and 9, you will learn how to use the molecular formttla to deduce the stmctural formula and the three-dimerrsional arrangement of a molecule. We will use all of these methods for representing molecules throughout the book, so you should be familiar with each method and with the information it provides. 

Even though the atom is the smallest representative sample of an element, only the noble-gas elements are normally found in nature as isolated atoms. Most matter is composed of molecules or ions. We examine molecules here and ions in Section 2.7. 

Several elements are found in nature in molecular form— two or more of the same type of atom bound together. For example, most of the oxygen in air consists of molecules that contain two oxygen atoms. As we saw in Section 1.2, we represent this molecular 

Oxygen also exists in another molecular form known as ozone. Molecules of ozone consist of three oxygen atoms, making the chemical formula O3. Even though normal oxygen (O2) and ozone (O3) are both composed only of oxygen atoms, they exhibit very different chemical and physical properties. For example, O2 is essential for life, but O3 is toxic O2 is odorless, whereas O3 has a sharp, pungent smell. 

The elements that normally occur as diatomic molecules are hydrogen, oxygen, nitrogen, and the halogens (H2, O2, N2, F2, CI2, Br2, and I2). Except for hydrogen, these diatomic elements are clustered on the right side of the periodic table. 

Compounds composed of molecules contain more than one type of atom and are called molecular compounds. A molecule of the compound methane, for example, consists of one carbon atom and four hydrogen atoms and is therefore represented by the chemical formula CH4. Lack of a subscript on the C indicates one atom of C per methane molecule. Several common molecules of both elements and compounds are shown in FIGURE 2.18. Notice how the composition of each substance is given by its chemical formula. Notice also that these substances are composed only of nonmetallic elements. Most molecular substances we will encounter contain only nonmetals. 

---

MOLECULES AND MOLECULAR COMPOUNDS We discuss the assemblies of atoms called molecules and how their compositions are represented by pineal and molecular formulas. 

The Periodic Table, Molecules and Molecular Compounds, and Ions and Ionic Compounds (Sections 2.5 and 2.7)... 

Atomic Number, Mass Numbed and Isotopes The Periodic Table The Atomic Mass Scale and Average Atomic Mass Molecules and Molecular Compounds... 

---