Write a correct chemical formula for each of the following ionic compounds: a. Sodium phosphide b. Iron (II) nitrite c. Calcium hydrogen phosphate d. Chromium (III) oxide

Ionic compounds are formed when atoms of different elements exchange electrons to achieve a full outer electron shell, resulting in ions that are held together by electrostatic forces. These compounds are typically comprised of metals, which lose electrons and form positively charged cations, and non-metals, which gain electrons and form negatively charged anions. In a stable ionic compound, the total positive charge of the cations equals the total negative charge of the anions, ensuring overall charge neutrality.

For instance, sodium (Na), a metal, loses one electron to form a Na⁺ ion, while phosphorus (P), a non-metal, gains three electrons to form a P^3- ion. When combining these ions to form sodium phosphide, three Na⁺ ions are needed to balance the charge of one P^3- ion, resulting in the compound Na₃P.

Polyatomic ions are ions that consist of more than one atom covalently bonded together, behaving as a single charged unit. Many of these ions are essential components of various ionic compounds. Understanding how they bind with other ions to form compounds is crucial for writing correct chemical formulas.

One such example is the nitrite ion, NO₂⁻, a polyatomic ion with a single negative charge. When combined with the Iron(II) cation, Fe²⁺, which has a double positive charge, two nitrite ions are required to balance one Iron(II) ion, leading to the compound's formula Fe(NO₂)₂.

The principle of charge balance in compounds is essential in determining the correct stoichiometry of an ionic compound. The total positive charge from the metal cations must be equal to the total negative charge from the non-metal anions or polyatomic ions, to ensure that the compound is electrically neutral. In the formation of compounds like calcium hydrogen phosphate, the charge balance is indeed simple: one Ca²⁺ ion pairs with one HPO₄²⁻ ion to give a neutral compound, hence the formula CaHPO₄.

Consider chromium (III) oxide, where the chromium (III) ion has a +3 charge and the oxide ion has a -2 charge. A 2:3 ratio of chromium to oxide ions results in the neutral compound Cr₂O₃.

Determining the chemical formula of an ionic compound involves identifying the charges of the respective ions and then combining them in a ratio that ensures neutrality of the overall compound. This often requires using subscripts in the chemical formula to denote the number of each ion present.

When determining the formula for compounds such as Chromium (III) oxide, where a single cation does not neatly balance a single anion, you'll need to find the lowest whole number ratio that will balance the charges. For Cr₂O₃, this means needing two Cr³⁺ ions to balance with three O²⁻ ions. The use of parentheses becomes important when dealing with polyatomic ions; for example, in Iron (II) nitrite, the nitrite ion is enclosed in parentheses with a subscript outside to indicate two nitrite ions per iron ion, resulting in Fe(NO₂)₂.