Ionic compounds smallest unit

Ionic compounds result from the combination of a positive ion known as a cation and a negative ion called an anion. Salt is an ionic compound in which sodium cations and chloride anions chemically combined. Molecular compounds contain discrete molecular units. Molecular units or molecules are the smallest unit of a molecular compound. Atoms in a compound are held together by covalent bonds. Bonds dictate how atoms are held together in a compound or molecule, but for now, just think of ionic compounds as compounds composed of ions, and molecular compounds as compounds composed of molecules. Sugar, water, and carbon dioxide are examples of molecular compounds. 

Not all ionic compounds crystallize in the same way as rock salt. Figure 10 shows the lattice of CsGl in which each positive ion is not surrounded by six, but by eight Cl- ions. The smallest unit, or unit cell of the crystal, is a cube in which the Cs+ ions occupy the corners,... 

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

Substances can be molecular (linked together by covalent bonds) or ionic (associated by ionic electrical attraction). Molecular substances are described by their molecular formula (e.g., H2O or CH4. Ionic substances are described by the formula unit (e.g., NaCl or MgF2). When dealing with ionic compounds, the smallest whole-number subscripts are always used. 

Because there are no molecules of ionic substances, we should not refer to a molecule of sodium chloride, NaCl, for example. Instead, we refer to a formula unit of NaCl, which consists of one Na ion and one CC ion. Likewise, one formula unit of CaCl2 consists of one Ca ion and two CC ions. As you will see in the next section, we speak of the formula unit of all ionic compounds as the smallest, whole-number ratios of ions that yield neutral representations. It is also acceptable to refer to a formula unit of a molecular compound. One formula unit of propane, C3Hg, is the same as one molecule of CjHg it contains three C atoms and eight H atoms bonded together into a group. 

As stated earlier, ionic compounds are not made up of molecules rather, the smallest unit of an ionic compound is the formula unit that is given by the formula of the compound. Barium fluoride, BaF2, is an ionic compound and its formula unit, BaF2, is composed of one barium ion, Ba2+, and two fluoride ions, F . The molar mass of BaF2 is 175.3 g. 

In order for ionic compounds to be electrically neutral, the sum of the charges on the cation and anion in each formula unit must be zero. If the charges on the cation and anion are numerically different, we apply the following rule to make the formula electrically neutral The subscript of the cation is numerically equal to the charge on the anion, and the subscript of the anion is numerically equal to the charge on the cation. If the charges are numerically equal, then no subscripts are necessary. This rule follows from the fact that because the formulas of ionic compounds are empirical formulas, the subscripts must always be reduced to the smallest ratios. Let us consider some examples. 

Chemical compounds are either molecular compounds (in which the smallest units are discrete, individual molecules) or ionic compounds (in which positive and negative ions are held together by mutual attraction). Ionic compounds are made up of cations and anions, formed when atoms lose and gain electrons, respectively. 

Ionic compounds form extended systems or lattices of alternating positive and negative charges, such as that shown in Figure 2.9. Although the formula NaCl correctly indicates that sodium and chlorine are present in a 1 1 ratio, we cannot really identify an individual molecule of NaCl. To emphasize this distinction, we sometimes refer to a formula unit, rather than a molecule, when talking about ionic compounds. The formula unit is the smallest whole number ratio of atoms in an ionic compound. 

Formula unit (2.4) The smallest whole number ratio of anions and cations in an ionic compound. 

In Section 1.8, Avogadro s number was mentioned as an example of a huge number, 6.02 X 10 . Avr adro s number is the number of specified entities in a mole of substance. The specified entities may consist of atoms or molecules or they may be groups of ions making up the smallest possible unit of an ionic compound, such as 2 Na+ ions and one S in Na2S. (It is not really correct to refer to NaaS as a molecule, because the compound consists of ions arranged in a crystalline structure such that there are 2 Na+ ions for each ion.) A general term that covers all these possibilities... 

In Section 1.3, a molecule was defined as the smallest unit of a pure substance capable of a stable, independent existence. As you will see later, some compound formulas are used to represent single molecules. True molecular formulas represent the precise numbers of atoms of each element that are found in a molecule. However, as we have seen, formulas for ionic compounds represent only the simplest combining ratio of the ions in the compounds. 

Ionic compounds contain one or more cations paired with one or more anions. In most cases, the cations are metals and the anions are nonmetals. When a metal, which has a tendency to lose electrons (see Section 4.6), combines with a non-metal, which has a tendency to gain electrons, one or more electrons transfer from the metal to die nonmetal, creating positive and negative ions that are then attracted to each otiter. You can assume that a compoimd composed of a metal and a nonmetal is ionic. The basic xmit of ionic compoimds is die formula unit, the smallest electrically neutral collection of ions. Formula units are different from molecules in that diey do not exist as discrete entities, but ratiier as part of a larger lattice. For example, salt (NaCl) is composed of Na and d ions in a 1 1 ratio. 

Remember, ionic compounds do not contain individual molecules. In loose language, the smallest electrically neutral collection of Ions Is sometimes called a molecule but Is more correctly called a formula unit. 

In a chemical formula, element symbols and, often, numerical subscripts show the type and number of each atom in the smallest unit of the substance. In this section, you ll learn how to write the names and formulas of ionic and simple covalent compounds, how to calculate the mass of a compound from its formula, and how to visuahze molecules with three-dimensional models. 

In Chapter 6, we discuss solid state structures of metals and ionic compounds, and detail the unit cells of a number of prototype stuctures. The unit cell is the smallest repeating unit in a crystal lattice, and its dimensions are characteristic of a particular polymorph of a compound. A unit cell is characterized by three cell edge lengths (a, b and c) and three angles (a, (3 and 7). Distances are often given in the non-SI unit of the angstrom (A), because lA=10 °m and... 

Some covalent compounds do not consist of individual molecules. Instead, each atom is joined to aU its neighbors in a covalently bonded, three-dimensional network. There are no distinct units in these compounds, just as there are no such units in ionic compounds. The subscripts in a formula for a covalent-network compound indicate the smallest whole-number ratio of the atoms in the compound. Naming such compounds is similar to naming molecular compounds. Some common examples are given below. 

When a new substance is synthesized or is discovered, it is analyzed quantitatively to reveal its percentage composition. From these data, the empirical formula is then determined. An empirical formula consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole-number mole ratio of the different atoms in the compound. For an ionic compound, the formula unit is usually the compound s empirical formula. For a molecular compound, however, the empirical formula does not necessarily indicate the actual numbers of atoms present in each molecule. For example, the empirical formula of the gas diborane is BHg, but the molecular formula is BgHg. In this case, the number of atoms given by the molecular formula corresponds to the empirical ratio multiplied by two. 

Ionic compounds are composed of cations (usually a metal) and anions (usually one or more nonmetals) bound together by ionic bonds. The basic unit of an ionic compound is the formula unit, the smallest, electrically neutral collection of ions. A formula unit is not a molecule—it does not usually exist as a discrete entity but rather as part of a larger lattice. For example, the ionic compound table salt, with the formula unit NaCl, is composed of Na and CF ions in a one-to-one ratio. In table salt, Na and CF ions exist in a three-dimensional alternating array. Because ionic bonds are not directional, no one Na ion pairs with a specific CF ion. Rather, as you can see in Figure 3.6(b) a, any one Na cation is surrounded by CF anions and vice versa. 

Compounds can be divided into two types molecular compounds, formed between two or more covalently bonded nonmetals, and ionic compounds, usually formed between a metal ionically bonded to one or more nonmetals. The smallest identifiable unit of a molecular compound is a molecule, and the smallest identifiable unit of an ionic compound is a formula unit the smallest electrically neutral collection of ions. 

We observe that each Na ion in sodium chloride is surrounded by six CP ions, and vice versa, and we cannot say that any one of these six CP ions belongs exclusively to a given Na ion. Yet, the ratio of CP to Na ions in sodium chloride is 1 1, and so we arbitrarily select a combination of one Na ion and one CP ion as a formula unit. The formula unit of an ionic compound is the smallest electrically neutral collection of ions. The ratio of atoms (ions) in the formula unit is the same as in the chemical formula. Because it is buried in a vast network of ions, called a crysfal, a formula unit of an ionic compound does not exist as a distinct entity. Thus, it is inappropriate to call a formula imit of solid sodium chloride a molecule. 

Sigma (a) bonds Sigma bonds have the orbital overlap on a line drawn between the two nuclei, simple cubic unit cell The simple cubic unit cell has particles located at the corners of a simple cube, single displacement (replacement) reactions Single displacement reactions are reactions in which atoms of an element replace the atoms of another element in a compound, solid A solid is a state of matter that has both a definite shape and a definite volume, solubility product constant (/ p) The solubility product constant is the equilibrium constant associated with sparingly soluble salts and is the product of the ionic concentrations, each one raised to the power of the coefficient in the balanced chemical equation, solute The solute is the component of the solution that is there in smallest amount, solution A solution is defined as a homogeneous mixture composed of solvent and one or more solutes. 

---