All modern research and models shows this not to be the case. Most of the community of chemistry educators teaches that formal charge reduction creates hypervalency. Therefore, the true base formula for this ionic compound is Be 1 ( CN ) 2.Ultimately those structures are wrong. Additionally, a subscript that is written outside of a parenthetical unit does not impact the identity of the polyatomic ion, which cannot be modified, by definition. A subscript in this position refers to the entire polyatomic ion, not simply the last element that it contains, which accurately reflects its intended meaning. The subscript that specifies how many of that ion are present within a compound must be written after the closing parenthesis. Because a polyatomic ion is an indivisible unit, its chemical formula must be enclosed inside of two parentheses when incorporated into an ionic chemical formula. As written, the " 2" indicates that 2 nitrogens are present, but the base formula should actually contain 2 cyanide ions. However, this formula does not accurately reflect the intended meaning of the second subscript. Completion of this step results in a base formula of Be 1 CN 2. To apply this information, remove the numerical superscripts from each symbol and instead utilize subscripts to indicate the ratio established above. This result indicates that for every 1 of the ions with the larger superscript value, in this case, Be 2, 2 of the other ions, CN 1, are required to achieve charge-balance between them. In the current example, the larger superscript value is " 2," and the smaller superscript value is " 1." Therefore, To determine this ratio, a mini-equation, in which the larger superscript value is multiplied by 1, and the smaller superscript value is multiplied by a variable, such as x, is solved. The first process, which is considered the more "scientifically-correct" method, is known as the "Ratio Method," as it establishes the correct cation-to-anion ratio by equating the total charges of the cations to the sum of the charges of the anions, in order to ensure that the final compound will be a net-neutral species.Two different processes, both of which are presented below, can be used to determine this information. Use subscripts to indicate how many of each of the given ions are present in the base chemical formula.The revised symbols are written as Be 2 and CN 1, respectively. In this example, the " +" sign is removed from Be +2 and the " –" sign is removed from CN –1. As this information was established in the previous step, the " +" and " –" signs can be removed, so that only the base elemental symbols and numerical superscripts remain. The signs of the ions are only used to determine the relative order in which the ion symbols are written.In this example, Be +2 will be written before CN – 1 in the final chemical formula. In order to ensure consistent formatting in all ionic chemical formulas, the symbol for the cation is written first.Based on the information presented in the previous sections of this chapter, the cyanide ion is symbolized as CN – 1, and beryllium ionizes to form Be +2. Write the ion symbols for any indicated polyatomic ions and for any monatomic ion that results upon the ionization of a main group or transition metal element.After establishing that a pair of chemicals will form an ionic bond, a five-step process can be employed to determine the chemical formula of the resultant ionic compound. In the given combination, the cyanide ion is classified as a polyatomic anion, and beryllium, a metal, ionizes to form a cation. Recall that an ionic bond is produced when a cation exists in close physical proximity to an anion, creating an electrostatic attractive force. The procedure for determining the chemical formula of an ionic compound containing exclusively main group elements or a combination of main group and transition metal elements can also be utilized to establish the chemical formula of an ionic compound that contains a polyatomic ion.įor example, consider the cyanide ion and beryllium.īased on the combinations listed in Section 3.2, the cyanide ion and beryllium will combine to form an ionic compound. Writing Chemical Formulas of Ionic Compounds Containing Polyatomic Ions
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