Ionic Compounds: Formulas and Nomenclature
An element composed of atoms that readily lose electrons (a metal) can react with an element composed of atoms that readily gain electrons (a nonmetal…
Ionic compounds form ionic bonds between positively charged metal ions, or cations, and negatively charged nonmetal ions, or anions. These compounds are represented by chemical formulas.
Consider calcium fluoride. First, specify the ions of each element and their respective ionic charges. Start with the cations followed by anions.
Ionic charges can be determined from the group number of the element in the periodic table. Calcium from group 2 forms cations with an ionic-charge of 2+. Conversely, fluorine from group 17 forms anions with an ionic-charge of 1−.
Next, the overall charge within the formula is balanced by employing a crisscross method. The cationic subscript is numerically matched to the anionic charge and vice versa, generating a charge-neutral chemical formula.
All ionic compounds are named by following a systematic naming convention called the chemical nomenclature. The chemical formula helps determine the name of the compound.
Binary ionic compounds name the metal cation first, succeeded by a parenthesized Roman-numeral indicating the metal charge if the metal can exist in different cationic states. The anion base-name ending with the suffix ‘–ide’ follows last.
Consider the following three molecules. The cations are sodium or iron. The anions are iodide or bromide. Since sodium forms only a single type of cation, the parenthesized Roman-numeral is excluded. Iron forms two types of cations, which are displayed using parenthesized Roman-numerals. Thus, the names are sodium iodide, iron(II) bromide, and iron(III) bromide.
Polyatomic ionic compounds follow the same nomenclature rules, but apply the polyatomic anion name instead of the suffix ‘-ide’. Polyatomic anions containing oxygen, or oxyanions, are named based on the number of oxygen atoms.
For example, multiple polyatomic ionic compounds are formed between sodium and oxyanions of bromine.
The most common oxyanion of an element is named by combining the element’s base-name with the suffix ‘–ate’. The suffix changes to ‘–ite’ with one less oxygen, or it is given the prefix ‘per–’ with one oxygen atom more.
Lastly, the oxyanion with one oxygen atom fewer than the oxyanion ending with ‘–ite’ obtains the prefix ‘hypo–’. Therefore the ionic compounds are sodium perbromate, sodium bromate, sodium bromite, and sodium hypobromite.
Hydrated ionic compounds or hydrates like CaCl2∙2H2O contain chemically-associated water molecules. The ionic part is named first. The number of water molecules is indicated using Greek-numeral-prefixes followed by the suffix ‘–hydrate.’ The chemical name of CaCl2∙2H2O is calcium chloride dihydrate.
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Q1: How do you determine the ionic charges of elements in an ionic compound?
Ionic charges are determined from the group number of the element in the periodic table. Metals in group 2, like calcium, form cations with a 2+ charge. Nonmetals in group 17, like fluorine, form anions with a 1− charge. Group 16 elements gain two electrons to form anions with a 2− charge. Transition metals often exhibit variable charges that are not predictable by their location in the periodic table.
Q2: What is the crisscross method used for in ionic compound formulas?
The crisscross method balances the overall charge within an ionic compound formula. The cationic subscript is numerically matched to the anionic charge and vice versa, generating a charge-neutral chemical formula. For example, in calcium fluoride, calcium's 2+ charge becomes fluorine's subscript, and fluorine's 1− charge becomes calcium's subscript, producing CaF2.
Q3: Why do ionic compounds require Roman numerals in their names?
Roman numerals indicate the metal ion charge when a metal can form multiple cations with different charges. Transition metals like iron form two or more cations; iron forms both 2+ and 3+ ions. The compounds FeCl2 and FeCl3 are named iron(II) chloride and iron(III) chloride to distinguish them. Metals like sodium, which form only one cation type, do not require Roman numerals.
Q4: How are oxyanions named based on oxygen atom count?
Oxyanions are named using suffixes and prefixes based on oxygen content. The most common oxyanion uses the suffix '–ate'. With one fewer oxygen, the suffix changes to '–ite'. One additional oxygen atom receives the prefix 'per–'. The oxyanion with one oxygen fewer than the '–ite' form gets the prefix 'hypo–'. For example, sodium bromate, bromite, perbromate, and hypobromite represent bromine oxyanions with varying oxygen counts.
Q5: What are polyatomic ions and how do they appear in ionic formulas?
Polyatomic ions are groups of bonded atoms acting as discrete units with an overall charge. In ionic formulas, parentheses indicate polyatomic ions that behave as a unit. For example, calcium phosphate contains the polyatomic ion PO43− and is written as Ca3(PO4)2. This formula shows three Ca2+ ions for every two PO43− groups, maintaining electrical neutrality with equal positive and negative charges.
Q6: How are hydrated ionic compounds named and represented?
Hydrated ionic compounds contain chemically-associated water molecules. The ionic part is named first, followed by a Greek numeral prefix indicating water molecule count and the suffix '–hydrate'. For example, CaCl2∙2H2O is calcium chloride dihydrate. The formula uses a centered dot, a coefficient for water molecules, and H2O to show the water association with the ionic compound.
Q7: How do you write the formula for an ionic compound with polyatomic ions?
Write the formula by treating polyatomic ions as discrete units and balancing charges. Determine the charges of the cation and polyatomic anion, then use subscripts to balance total positive and negative charges. For compounds containing polyatomic ions like CaSO4, the formula represents the empirical ratio. For sodium oxalate with Na+ and C2O42− ions in a 2:1 ratio, the formula is Na2C2O4, showing the relative numbers of constituent ions.