![]() Compare this to Figure 1, which shows the even distribution of electrons in the H 2 nonpolar bond. Note that the shaded area around Cl is much larger than it is around H. Thus, in an HCl molecule, the chlorine atom carries a partial negative charge and the hydrogen atom has a partial positive charge. Figure 2 shows the distribution of electrons in the H–Cl bond. For example, the electrons in the H–Cl bond of a hydrogen chloride molecule spend more time near the chlorine atom than near the hydrogen atom. The atom that attracts the electrons more strongly acquires the partial negative charge and vice versa. This unequal distribution of electrons is known as a polar covalent bond, characterized by a partial positive charge on one atom and a partial negative charge on the other. Instead, the bonding electrons are more attracted to one atom than the other, giving rise to a shift of electron density toward that atom. When the atoms linked by a covalent bond are different, the bonding electrons are shared, but no longer equally. (b) Symbols δ+ and δ– indicate the polarity of the H–Cl bond. The small, black dots indicate the location of the hydrogen and chlorine nuclei in the molecule. The electron density is greater around the chlorine nucleus. (a) The distribution of electron density in the HCl molecule is uneven. Care must be taken when calculating formula masses for formulas containing multiple polyatomic ions because the subscript outside the parentheses refers to all the atoms in the polyatomic ion.Figure 2. The formula mass of an ionic compound is the sum of the masses of each individual atom in the formula. An example is the common name for FeCl 2, which is ferrous chloride. These suffixes are attached to a stem representing the element (which frequently comes from the Latin form of the element name). In the common system, the suffixes - ous and - ic are used to stand for the lower and higher possible charge of the cation, respectively. An example is the name for FeCl 2, which is iron(II) chloride. In the Stock system, a roman numeral in parentheses indicates the charge on the cation. If an element can form cations of different charges, there are two alternate systems for indicating the compound’s name. Names of ionic compounds are derived from the names of the ions, with the name of the cation coming first, followed by the name of the anion. Ionic compounds typically form between metals and nonmetals or between polyatomic ions. Writing formulas with polyatomic ions follows the same rules as with monatomic ions, except that when more than one polyatomic ion is present in a chemical formula, the polyatomic ion is enclosed in parentheses and the subscript is outside the right parenthesis. Some ions are groups of atoms bonded together and having an overall electrical charge. A subscript to the right of the ion indicates that more than one of that ion is present in the chemical formula. Chemical formulas for ionic compounds are determined by balancing the positive charge from the cation(s) with the negative charge from the anion(s). Instead, they exist as crystals, three-dimensional arrays of ions, with cations surrounded by anions and anions surrounded by cations. Formula unit is considered the basic unit of an ionic compound because ionic compounds do not exist as discrete units. ![]() In a formula, the symbol of the cation is written first, followed by the symbol of the anion. Ionic compounds are represented in writing by a chemical formula, which gives the lowest ratio of cations and anions present in the compound. Electron dot diagrams, or Lewis diagrams, can also be used to illustrate the formation of cations and anions. Because elements in a column of the periodic table have the same valence shell electron configuration, atoms in the same column of the periodic table tend to form ions having the same charge. The formation of both cations and anions can be illustrated using electron configurations. Positively charged ions are called cations, while negatively charged ions are called anions. This attraction is called ionic bonding, and the compounds formed are called ionic compounds. Ions having opposite charges attract each other. When this happens, the atoms take on an electrical charge. One way atoms obtain eight electrons in the valence shell is for some atoms to lose electrons while other atoms gain them. A survey of stable atoms and molecules leads to the octet rule, which says that stable atoms tend to have eight electrons in their outermost, or valence, shell. To ensure that you understand the material in this chapter, you should review the meanings of the following bold terms and ask yourself how they relate to the topics in the chapter.Ītoms combine into compounds by forming chemical bonds. ![]()
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