Ionic compounds formula unit

Ionic Compounds, Formula Units, and Formula Mass... 

Many sohd ionic compounds have a specific number of water molecules incorporated into their crystal structure. Such compounds are called hydrates. For example, in its normal state, each unit of copper(ll) sulfate has five water molecules associated with it. In the naming of hydrates, we use Greek prefixes (see Table 0.4) to denote the number of water molecules associated with each ionic compound formula unit. 

Ionic compounds, however, are composed of ions, not covalently bonded atoms. For ionic compounds formula mass should be used instead of molecular mass and empirical formula, simplest formula or formula unit should be used instead molecular formula. 

As we discussed in Chapter 1 (Section 1.4.2), all of the more than 13 million chemical compounds that exist consist of elements in chemical combination. Compounds that are covalent or molecular have molecules as their fundamental particles. Confounds that are ionic have formula units as then-fundamental particles. Molecules are composed of various numbers of different atoms in covalent chemical combination formula units are composed of various numbers of different ions. 

The chemical formula of an ionic compound shows the ratio of the numbers of atoms of each element present in one formula unit. A formula unit of an ionic compound is a group of ions with the same number of atoms of each element as appears in its formula. 

Ionic compounds are named by starting with the name of the cation (with its oxidation number if more than one charge is possible), followed by the name of the anion hydrates are named by adding the word hydrate, preceded by a Greek prefix indicating the number of water molecules in the formula unit. 

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. 

FIGURE E.5 Each sample contains 1 mol of formula units of an ionic compound. From left to right are 58 g of sodium chloride (NaCl), 100 g of calcium carbonate (CaCO,), 278 g of iron(ll) sulfate heptahydrate (FeS04-7H.0), and 78 g of sodium peroxide (Na. O,). 

Every ionic compound contains discrete ionic units with specific charges. In addition, ionic compounds must always contain equal amounts of positive and negative charge. These requirements dictate the ratio of cations to anions in an ionic substance. The following guidelines ensure uniformity in writing ionic formulas ... 

Many ionic compounds can have water molecules incorporated into their solid structures. Such compounds are called hydrates. To emphasize the presence of discrete water molecules in the chemical structure, the formula of any hydrate shows the waters of hydration separated from the rest of the chemical formula by a dot. A coefficient before H2 O indicates the number of water molecules in the formula. Copper(II) sulfate pentahydrate is a good example. The formula of this beautiful deep blue solid is C11SO4 5 H2 O, indicating that five water molecules are associated with each CuSOq unit. Upon prolonged heating, CuSOq 5 H2 O loses its waters of hydration along with its color. Other examples of hydrates include aluminum nitrate nonahydrate, A1 (N03)3 9 H2 O,... 

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... 

Now that we know how to name the cations and anions, we merely have to put the two names together to get the names of ionic compounds. The cation is named first and the anion is named next. The number of cations and anions per formula unit need not be included in the name of the compound because anions have characteristic charges, and the charge on the cation has already been established by its name. There are as many cations and anions as needed to get a neutral compound with the lowest possible subscripts. 

Many compounds in nature, particularly compounds made of Ccirbon, hydrogen, and oxygen, are composed of atoms that occur in numbers that are multiples of their empirical formula. In other words, their empirical formulas don t reflect the actual numbers of atoms within them instead, they reflect only the ratios of those atoms. What a nuisance Fortunately, this is an old nuisance, so chemists have devised a means to deal with it. To account for these annoying types of compounds, chemists are Ccireful to differentiate between an empirical formula and a moleculcir formula. A molecular formula uses subscripts that report the actual number of each type of atom in a molecule of the compound (a formula unit accomplishes the same thing for ionic compounds). 

Metal cations team up with nonmetal anions to form ionic compounds. What s more, the ratio of cations to anions within each formula unit depends on the charge assumed by the fickle transition metal. The formula unit as a whole must be electrically neutral. The rules you follow to name an ionic compound must accommodate the whims of transition metals. The system of Roman numerals or suffixes applies in such situations ... 

We do not speak of a molecule of an ionic compound. However, it is useful to be able to refer to a representative group of ions with the number of atoms given by the formula. This group of ions- is called a formula unit. For example, the formula unit of sodium chloride, NaCl, consists of one Na+ ion and one Cl ion and the formula unit of ammonium sulfate, (NH4)2S04, consists of two NH4+ ions and one S042- ion. 

EXAMPLE C.2 Predicting the formula unit of a binary ionic compound... 

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... 

Lewis formula (for an ionic compound) A representation of the structure of an ionic compound showing the formula unit of ions in terms of their Lewis symbols. Lewis structure A diagram showing how electron pairs are shared between atoms in a molecule. Examples H-C1 0=C=0. 

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