Naming and Writing Ionic Formulas

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Ionic compounds consist of positive and negative ions. Positively charged ions are called cations while negatively charged ions are called anions. In ionic compounds, ions form when atoms lose or gain electrons to achieve a stable electron configuration. Ions are formed when the valence electrons of atoms of a metal are transferred to atoms of a nonmetal. Ionic bonds result when those ions are then attracted to each other, creating an ionic compound with a neutral charge. Let's consider the ionic compound, sodium chloride. To achieve stable electron configurations, one Na atom, which is a metal, loses its one valence electron to form a sodium cation, Na+, and one chlorine atom, which is a nonmetal, gains one electron to form a chloride anion, Cl-. The formula NaCl indicates that the compound has the positive and negative charges balanced because there is one sodium ion, Na+, for every chloride ion, Cl-. In the formula of an ionic compound, the sum of the ionic charges is always zero. Thus, the sum of positive charge is equal in magnitude but opposite in sign to the sum of negative charge. When we look at the periodic table, we can see that there are patterns for the formation of charge for some ions. For example, all the elements in Group 1A will form 1+ ions because they all have one valence electron to lose. However, when we look at the transition elements, lead, tin, and bismuth, we see that some elements can form more than one type of cation. These metals are ions with variable positive charges. When these metals are part of an ionic compound, they require special naming and may require extra identification of the cation needed to create a neutral ionic compound. Which of the following pairs of elements are likely to form an ionic compound? Is it a, b, c or d? The correct answer is (c). An ionic compound is formed between a metal cation and a nonmetal anion. Barium is a metal and oxygen is a nonmetal. In answers (a) and (b), both elements are metals. Formation of an ionic compound between two metals would mean two cations combining with no negative charge, which would not result in a neutral ionic compound. In answer (d), both elements are nonmetals. Formation of an ionic compound between two nonmetals would mean two anions combining with no positive charge, which would not result in a neutral ionic compound. An ionic bond requires an attraction between a positively charged cation, which results from a metal, and a negatively charged anion, which results from a nonmetal. Now consider an ionic compound composed of sodium and sulfur. To achieve a stable electron configuration, sodium will lose a valence electron to form Na+ ions and sulfur will gain two electrons to form an S2- ion. For these two ions to create a neutral ionic compound, two Na+ ions are needed to balance the negative charge of S2-. This gives the formula Na2S. The subscript 2 shows that two Na+ ions are needed for one S2- ion in order to balance the charge. Select the correct ionic formula for the compound formed between the ions Al3+ and Cl- Is it a, b, c or d? The correct answer is (d). To balance the ionic charge of the Al3+ ion, we need 3 Cl- ions in the formula. This gives the formula AlCl3, which has an overall net charge of 0. The name of an ionic compound made up of two elements consists of the name of the metal cation followed by the name of the nonmetal anion. An example is the ionic compound KI. The name of the metal cation, K+, is the same as its element name, potassium. The name of the nonmetal, I-, is obtained by using the first syllable of its element name, with the suffix -ide added. For example, iodine is the element name and iodide is the name of the anion. The name of the ionic compound, KI, is potassium iodide. In the name of any ionic compound, a space separates the name of the cation from the name of the anion. There is no need to indicate the number of each of the ions in the name of the compound because it is understood that the charge balance of a neutral ionic compound will dictate the number of each ion. Compounds containing an element with a variable charge must use Roman numerals to indicate the charge on that ion. This Roman numeral appears in parentheses after the name of the cation. To determine the charge of the variable charge cation, you can use the charge of the anion and the neutral charge of the ionic compound to calculate the charge of the cation. Let's go through determining the charge of the cation for NiCl2, and then see how to properly name this ionic compound. Step 1 is to determine the charge of the cation, in this case the transition element nickel, from the anion. Nickel is an element with a variable charge, either Ni2+ or Ni3+. There are two chloride ions, each with a charge of 1-, to give a total negative charge of 2-. Since there is only one nickel ion, it must carry all of the positive charge. Thus, the charge on the nickel ion must be 2+. Step 2 is to name the cation by its element name. Since the metal is a transition element with a variable charge, we must use a Roman numeral in parentheses to indicate the charge on the cation. We need the ion with the 2+ charge to balance the 2- charge from the two chloride ions. The cation in this compound is named nickel(II). Step 3 is to name the anion by using the first syllable of its element name followed by the suffix -ide. The element is named chlorine and the anion in this compound is named chloride. Step 4 is to write the name for the cation first with the Roman numeral for the variable charge and the name for the anion second. Therefore, the name of the compound, NiCl2, is nickel(II) chloride. Choose the correct name for the ionic compound, PbO2. Is it a, b, c or d? The correct answer is (d), lead(IV) oxide. Lead is a transition element with a variable charge. For this compound, lead has a 4+ charge. This will balance the overall 4- charge from the two oxide ions, each with a 2- charge.