On the basis of the expected charges of the monatomic ions, give the chemical formula of each of the following compoundss (a) iron(II) sulfide; (b) cobalt(III) chloride; (c) lithium phosphide.

Ionic compounds are substances composed of positive and negative ions that are held together by strong electrostatic forces known as ionic bonds. These compounds are typically formed between metals, which tend to lose electrons and become positively charged cations, and nonmetals, which tend to gain electrons and become negatively charged anions. For example, in iron(II) sulfide, iron acts as the cation with a +2 charge, and sulfur as the anion with a -2 charge.

The formation of ionic compounds is driven by the need for atoms to achieve a stable electron configuration, often resembling that of the nearest noble gas. Metals will lose electrons to achieve this stability, whereas nonmetals will gain electrons. The ions produced as a result will generally arrange themselves in a lattice structure, maximising the attractions between opposite charges to create a neutral compound.

The charge balance in compounds is crucial for their stability and formation. It mandates that the total positive charge from the cations must be equal to the total negative charge from the anions. This ensures that the compound is electrically neutral. To achieve this, ions combine in ratios that neutralize their charges. The formula of an ionic compound reflects this balance. For instance, cobalt(III) chloride (CoCl₃) forms when one Co³⁺ cation combines with three Cl⁻ anions, balancing out the +3 charge with three instances of a −1 charge, yielding a neutral compound.

Whenever a new ionic compound is conceived, the chemist must ensure the charges are balanced. Sometimes this may require multiple ions of one type; for cobalt(III) chloride, a single cobalt cation requires three chloride anions, while for lithium phosphide (Li₃P) three lithium cations balance one phosphide anion. This balance is fundamental to the chemical stability of the compound and dictates the ratios of ions in the chemical formula.

The periodic table is designed in such a way that elements within the same group often share similar properties, including the common charges of their ions. For instance, group 1 elements (alkali metals) typically have a +1 charge when they become cations. Lithium (Li), from group 1, will most commonly form a Li⁺ ion in ionic compounds. Conversely, elements in group 17 (halogens) such as chlorine (Cl) commonly form anions with a -1 charge, becoming Cl⁻.

Understanding the typical ionic charges of elements based on their group can be a significant help when writing chemical formulas. Group 2 elements (alkaline earth metals) usually form +2 cations, while group 16 elements (oxygen group) form -2 anions. Transition metals can have multiple possible charges, which is why notation like iron(II) or cobalt(III) is used to indicate their charge directly. Being familiar with these trends simplifies the process of predicting and writing the formulas for new ionic compounds. By knowing the typical charge of an element based on its group, one can quickly determine the correct stoichiometric ratios needed for charge balance in forming a neutral compound.