Naming Inorganic Compounds Ionic compounds Chemical nomenclature The names and chemical formulas of compounds are essential in chemistry. The system used in naming substances is called chemical nomenclature, from the Latin words “nomen" (name) and “calare" (to call). Many important substances that have been known for a long time, such as water (H2O) and ammonia (NH3), do have traditional names (called common names). For most substances, however, there are a set of rules that leads to a unique name for each substance. The rules for chemical nomenclature are based on the division of substances into categories. The major division is between organic and inorganic compounds. Organic compounds contain carbon-carbon bonds and/or carbon-hydrogen bonds, often in combination with oxygen, nitrogen, or other elements. All others are inorganic compounds. Early chemists associated organic compounds with plants and animals and inorganic compounds with the nonliving portion of our world. In this section we will see the basic rules for naming three categories of inorganic compounds: • ionic compounds • molecular compounds • acids. 54 CHAPTER 2 Atoms, Molecules, and Ions GO FIGURE Picturing Molecules What advantage does a ball-and- The molecular formula of a substance summarizes the composition of the substance but stick model have over a space- does not show how the atoms are joined together in the molecule. A structural formula filling model? shows which atoms are attached to which, as in the following examples: CH 4 H H Molecular formula O O O H C H HHH H H WaterHydrogen peroxide Methane H C H The atoms are represented by their chemical symbols, and lines are used to represent the H bonds that hold the atoms together. Structural formula A structural formula usually does not depict the actual geometry of the molecule, that is, the actual angles at which atoms are joined together. A structural formula can be Dashed Solid line is written as a perspective drawing (ǡ FIGURE 2.19), however, to give some sense of three- wedge is bond H bond in plane behind page of page dimensional shape. C Scientists also rely on various models to help visualize molecules. Ball-and-stick H H Wedge is models show atoms as spheres and bonds as sticks. This type of model has the advantage H bond out of accurately representing the angles at which the atoms are attached to one another in of page the molecule (Figure 2.19). Sometimes the chemical symbols of the elements are super- Perspective drawing imposed on the balls, but often the atoms are identified simply by color. A space-filling model depicts what the molecule would look like if the atoms were scaled up in size (Figure 2.19). These models show the relative sizes of the atoms, but the angles between atoms, which help define their molecular geometry, are often more diffi- cult to see than in ball-and-stick models. As in ball-and-stick models, the identities of the atoms are indicated by color, but they may also be labeled with the element’s symbol. GIVE IT SOME THOUGHT The structural formula for ethane is H H Ball-and-stick model H C C H H H a. What is the molecular formula for ethane? b. What is its empirical formula? c. Which kind of molecular model would most clearly show the angles between atoms? | Space-filling model 2.7 IonicIONS ANDCompounds IONIC COMPOUNDS SECTION 2.7 Ions and Ionic Compounds 55 į FIGURE 2.19 Different The nucleusMany of an atoms atom is unchangedgain or loseby chemical electrons processes, to but end some up atoms with can read-the same number of electrons as representations of the methane (CH ) ily gain or lose electrons. If electrons are removed from or added to an atom, a charged 4 The net charge on an ion is represented by a superscript. The superscripts +, 2+, molecule. Structural formulas, perspective particle calledthe closestan ion is formed. noble An gasion with in thea positive periodic charge is table. a cation We(pronounced might deduce that this is because their and 3+, for instance, mean a net charge resulting from the loss of one, two, and three drawings, ball-and-stick models, and space- CAT-ion); a negatively charged ion is an anion (AN-ion). filling models correspond to the molecular electronic configuration is very stable.electrons, Nearby respectively. elements The superscripts can obtain , 2 , these and 3 samerepresent stable net charges resulting To see how ions form, consider the sodium atom, which has 11 protons and 11 elec- - - - formula, and each helps us visualize the from the gain of one, two, and three electrons, respectively. Chlorine, with 17 protons trons. Thisarrangements atom easily loses byone losingelectron. or The gainingresulting cationelectrons. has 11 protons and 10 ways atoms are attached to each other. and 17 electrons, for example, can gain an electron in chemical reactions, producing the electrons, which means it has a net charge of1+ . Cl- ion: ϩ ᎐ 11p 11e ϩ ᎐ ϩ ᎐ ϩ ᎐ 17p 17e 17p 18e 11p 10e Loses an Gains an electron electron ϩ Na atom Na ion Cl atom Cl᎐ ion In general, metal atoms tend to lose electrons to form cations and nonmetal atoms tend to For example, the loss of one electrongain electrons from to forman atom anions. of Thus, Na ionic leaves compounds it with tend theto be composedsame of metals bonded with nonmetals, as in NaCl. number of electrons as in a Ne atom. SAMPLE EXERCISE 2.7 Writing Chemical Symbols for Ions Similarly, when Cl gains an electron,Give itthe ends chemical up symbol, with including 18, the superscript same numberindicating mass of number, for (a) the ion with 22 protons, 26 neutrons, and 19 electrons; (b) the ion of sulfur that has 16 neutrons and electrons as in Ar. 18 electrons. SOLUTION (a) The number of protons is the atomic number of the element. A periodic table or list of elements tells us that the element with atomic number 22 is titanium (Ti). The mass number (protons plus neutrons) of this isotope of titanium is 22 + 26 = 48. Because the ion has three more protons than electrons, it has a net charge of3+ :48Ti3+ . (b) The periodic table tells us that sulfur (S) has an atomic number of 16. Thus, each atom or ion of sulfur contains 16 protons. We are told that the ion also has 16 neutrons, meaning the mass number is 16 + 16 = 32. Because the ion has 16 protons and 18 electrons, its net charge is 2- and the ion symbol is 32S2-. In general, we will focus on the net charges of ions and ignore their mass numbers unless the circumstances dictate that we specify a certain isotope. PRACTICE EXERCISE How many protons, neutrons, and electrons does the 79Se2- ion possess? Answer: 34 protons, 45 neutrons, and 36 electrons In addition to simple ions such as Na+ and Cl-,there are polyatomic ions,such as + 2- NH4 (ammonium ion) and SO4 (sulfate ion). These latter ions consist of atoms joined as in a molecule, but they have a net positive or negative charge. We consider polyatomic ions in Section 2.8. It is important to realize that the chemical properties of ions are very different from the chemical properties of the atoms from which the ions are derived. Although a given atom and its ion may be essentially the same (plus or minus a few electrons), the behav- ior of the ion is very different from that of its associated atom. Predicting Ionic Charges Many atoms gain or lose electrons to end up with the same number of electrons as the noble gas closest to them in the periodic table. Noble-gas elements are chemically non- reactive and form very few compounds. We might deduce that this is because their elec- tron arrangements are very stable. Nearby elements can obtain these same stable arrangements by losing or gaining electrons. For example, the loss of one electron from an atom of sodium leaves it with the same number of electrons as in a neon atom (10). Similarly, when chlorine gains an electron, it ends up with 18, the same number of elec- trons as in argon. We will use this simple observation to explain the formation of ions until Chapter 8, where we discuss chemical bonding. Ionic Compounds When elemental Na is allowed to react with elemental Cl, an electron transfers from Na to Cl, forming a Na+ ion and a Cl- ion. Because objects of opposite charge attract, the Na+ and the Cl- ions bind together to form the compound sodium chloride (NaCl). Sodium chlorideSECTION (table 2.7salt),Ions is and an Ionicexample Compounds 57 of an ionic compound, a compound made up of cations and anions. 11pϩ 11e᎐ 11pϩ Loses an 10e᎐ electron Naϩ ion Na atom e؊ 17pϩ 17e᎐ 17pϩ 18e᎐ Gains an electron Cl atom Cl᎐ ion (a) (b) (c) In generalį FIGURE ionic 2.21 compoundsFormation of an ionic are compound. combinations(a) The transfer of of anmetals electron fromand a Nanonmetals . atom to a Cl atom leads to the formation of a Na+ ion and a Cl- ion. (b) Arrangement of these In contrast,ions in solid molecularsodium chloride, compounds NaCl. (c) A sample areof sodium generally chloride crystals. composed of nonmetals only, as in H2O. SAMPLE EXERCISE 2.9 Identifying Ionic and Molecular Compounds Which of these compounds would you expect to be ionic: N2O, Na2O, CaCl2,SF4? SOLUTION We predict that Na2O and CaCl2 are ionic compounds because they are composed of a metal combined with a nonmetal. We predict (correctly) that N2O and SF4 are molecular com- pounds because they are composed entirely of nonmetals.