Nonmetals, monatomic cations

To predict the electron configuration of a monatomic cation, remove outermost electrons in the order np, ns, and (n — l)d fora monatomic anion, add electrons until the next noble-gas configuration has been reached. The transfer of electrons results in the formation of an octet (or duplet) of electrons in the valence shell on each of the atoms metals achieve an octet (or duplet) by electron loss and nonmetals achieve it by electron gain. 

Because nonmetals do not form monatomic cations, the nature of bonds between atoms of nonmetals puzzled scientists until 1916, when Lewis published his explanation. With brilliant insight, and before anyone knew about quantum mechanics or orbitals, Lewis proposed that a covalent bond is a pair of electrons shared between two atoms (3). The rest of this chapter and the next develop Lewis s vision of the covalent bond. In this chapter, we consider the types, numbers, and properties of bonds that can be formed by sharing pairs of electrons. In Chapter 3, we revisit Lewis s concept and see how to understand it in terms of orbitals. 

Nonmetal atoms accept electrons from metal atoms if the metal atoms are available, or else they share electrons they never donate electrons to form monatomic cations. The largest charge on any monatomic cation is 4 -I-, and on any monatomic anion, it is 3 —. 

Metals with high oxidation numbers tend to act somewhat like nonmetals. For example, many transition metals form oxoanions, such as permanganate ion, chromate ion, and dichromate ion, in which the metal is covalently bonded to oxygen. The ability to form covalent bonds to oxygen is evidence of these metals more covalent nature. (In their low oxidation states, most metals typically exist in ionic compounds as monatomic cations.) Titanium(lV) chloride is an example of a compound in which the... 

In Chap. 6 we placed Roman numerals at the ends of names of metals to distinguish the charges on monatomic cations. It is really the oxidation number that is in parentheses. This nomenclature system is called the Stock system. For monatomic ions, the oxidation number is equal to the charge. For other cations, again the oxidation number is used in the name. For example, Hg2 + is named mercury(I) ion. Its charge is 24- the oxidation number of each atom is 4-1. Oxidation numbers are also used for other cations, such as dioxovanadium(V) ion, V02". The prefix 0x0- stands for oxygen. Oxidation numbers can be used with nonmetal-nonmetal compounds, as in sulfur(VI) oxide for SO3, but the older system using prefixes (Table 6-2) is still used more often. 

Metals form cations nonmetals form anions C, P, and the metalloids do not form monatomic ions. 

Now we can work out the formula of an ionic compound formed between the monatomic ions of two main-group elements, one a metal and the other a nonmetal. Unless a lower oxidation number is specified (as for the p-block metals), the metal atom loses all its valence electrons, and the nonmetal atom gains enough electrons to complete its valence shell. Then we adjust the numbers of cations and anions so that the resulting compound is electrically neutral. A simple example is calcium chloride. The calcium atoms ([Ar]4s2) each lose two electrons, to form... 

Each manganese cation must therefore be +2 to balance the -2 of the oxide to yield an uncharged ionic formula. The systematic name for Mn + is manganese(II). Monatomic anions are named with the root of the nonmetal followed by -ide, so is oxide. MnO is named manganese(II) oxide. Example 3.8 provides other examples. 

The iron ions must be +3 in order for them to balance the -6 from three -2 oxide ions, so the cation name is iron(III). Because is a monatomic anion, its name includes the root of the name of the nonmetal and -ide. Therefore, Fe203 is iron(III) oxide. 

An atom can lose or gain more than one electron. Examples of ions formed by the loss or gain of more than one electron are Mg, Fe, S , and. These ions, as well as Na" and Cl, are called monatomic ions because they contain only one atom. Figure 2.11 shows the charges of a number of monatomic ions. With very few exceptions, metals tend to form cations and nonmetals form anions. 

In effect, the metal atoms transfer electrons to the nonmetal atoms. The resulting large numbers of cations and anions attract each other and form the ionic compound. A cation or anion derived from a single atom is called a monatomic ion we ll discuss polyatomic ions, those derived from a small group of atoms, later. 

Across a period. The decrease in metallic behavior across a period is consistent with a decrease in size, an increase in IE, and a more favorable (more negative) EA. Consider Period 3 elements at the left tend to form cations, and those at the right tend to form anions. Sodium and magnesium are metals that occur as Na and Mg - in seawater, minerals, and organisms. Aluminum is a metallic element and occurs as Al + in some compounds, but it bonds covalently in most others. Silicon (Si) is a shiny metalloid that does not occur as a monatomic ion. Phosphorus is a white, waxy nonmetal that occurs rarely as P, whereas crumbly, yellow sulfur forms S in many compounds, and gaseous, yeUow-green chlorine occurs in nature almost always as Cl . 

The metallic elements generally have low ionization energies and low electronegativities compared with the nonmetalUc elements. As a result, the metals tend to lose their valence electrons to form cations (Na, Ca, Al " ) in compounds or in aqueous solution. Nonmetals, on the other hand, form monatomic anions (O, Cl ) and oxoanions (N03, S04 ). 

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