![]() Nonmetals tend to form anions with charges equal to their group number (IUPAC convention) minus 18.Examples: Na + (Group 1), Mg 2+ (Group 2), Al 3+ (Group 13) Main-group metals tend to form cations with charges equal to the last digit of their group number (IUPAC convention)).Metals form cations nonmetals form anions.The following generalizations help determine the charges on the ions in ionic compounds. Binary (two-element) compounds formed between metals and nonmetals are usually ionic e.g., NaCl, CaF 2, Al 2O 3, Na 3N. However, the formula unit has no chemical existence in the way that a molecule does.Ĭompounds consisting of nonmetal elements are usually molecular e.g., H 2O, C 2H 6, N 2O 4, NH 3. It is sometimes convenient to refer to the formula unit of an ionic compound, a minimum collection of ions that corresponds to the empirical formula. The composition of an ionic compound can be represented by an empirical formula, but there is no corresponding molecular formula because there are no molecules of the compound. Ionic compounds are typically crystalline solids with a structure consisting of an orderly three-dimensional array of ions called a crystal lattice. Ionic compounds are made up of electrically equivalent numbers of cations and anions. Note, too, that different compounds, composed of different kinds of molecules, may have the same empirical formula. ![]() But in every case, the molecular formula is a whole number multiple of the empirical formula. Note that sometimes the molecular and empirical formulas are the same, and sometimes they are different. The following examples show the relationship between the molecular formula and empirical formula for some compounds. When formulas of compounds are determined by chemical analysis, the information obtained usually gives only the simplest whole number ratios among the elements, which is expressed as an empirical formula. Not all compounds contain discrete, identifiable molecules, but for those that do a molecular formula indicates the actual numbers and kinds of atoms comprising the molecular unit. Know the molecular formulas and room-temperature states of these common elements. The notations g, l, s stand for gas, liquid, and solid, respectively, and represent the states of these elements at room temperature. The following common elements are composed of molecules with the compositions indicated by their molecular formulas: H 2( g), F 2( g), Cl 2( g), Br 2( l), I 2( s), O 2( g), S 8( s), N 2( g), P 4( s). For a molecular substance (element or compound), the composition of the molecules is indicated by a molecular formula, which shows the kinds and numbers of each atom in the molecule. Molecules of compounds are heteronuclear, because they are composed of two or more different kinds of atoms. Molecules of elements are homonuclear, because they are composed of only one kind of atom. Many elements and compounds - but not all - are composed of molecules. Molecules are combinations of atoms tightly bound together to form a chemically identifiable unit.
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