Question

Name the following ionic compounds: (a) CaS, (b) AlBr, (c) \(\mathrm{Na}_{3} \mathrm{P}\) (d) \(\mathrm{Ba}_{3} \mathrm{As}_{2}\) (e) \(\mathrm{Rb}_{2} \mathrm{~S}\).

Short answer

The names of the ionic compounds are: (a) calcium sulfide, (b) presumed aluminum bromide (AlBr3), (c) sodium phosphide, (d) barium arsenide, (e) rubidium sulfide.

Step-by-step solution

- Name CaS

Calcium is a Group 2 element and has a +2 charge as cation. Sulfur is a Group 16 element and forms anion with a -2 charge. The compound is made of one calcium ion and one sulfur ion so it's named calcium sulfide.

- Name AlBr

Here appears to be a typo. For aluminum bromide, it should be AlBr3. The compound made of aluminum cation Al(3+) and bromide anions Br(-). There are three bromide ions for each aluminum ion to balance the charges, resulting in the name aluminum bromide.

- Name \(\mathrm{Na}_{3}\mathrm{P}\)

Sodium is a Group 1 element with a +1 charge and phosphorus forms a 3- anion. The compound is made of three sodium ions and one phosphide ion and is named sodium phosphide.

- Name \(\mathrm{Ba}_{3}\mathrm{As}_{2}\)

Barium is a Group 2 element with a +2 charge and arsenic forms a 3- anion. In order to balance the charges, two arsenic ions pair with three barium ions, so the compound is named barium arsenide.

- Name \(\mathrm{Rb}_{2}\mathrm{~S}\)

Rubidium is a Group 1 element with a +1 charge and sulfur forms a 2- anion. Two rubidium ions are needed to balance one sulfide ion leading to the name rubidium sulfide.

Key concepts

Chemical Nomenclature

Understanding chemical nomenclature is essential for students as it is the system used for naming compounds. It helps to convey the elemental composition of the compound and the proportion of each element involved. In the context of ionic compounds, it involves identifying the cation (positively charged ion) and the anion (negatively charged ion), and then naming each part. The name of the metallic cation comes first, followed by the name of the nonmetallic anion, which typically ends in '-ide'. For example, in the compound CaS, 'calcium' is the name of the cation and 'sulfide' is the derived name of the anion from sulfur.

When dealing with multivalent metals that can form more than one possible charge, the charge is often indicated in Roman numerals in parentheses right after the cation name; however, this is not necessary for elements like calcium which only have one common ionic charge. Remember, precision is crucial in nomenclature, as you can see from our correction of the AlBr compound to AlBr3, to reflect the proper stoichiometry for aluminum bromide.

Ionic Charges

Ionic charges are the electrical charges found in ions, which are atoms that have lost or gained electrons. Metals typically form cations with a positive charge, while nonmetals often form anions with a negative charge. Understanding the charges is crucial when naming ionic compounds because the total charge must balance to zero. This balance is known as charge neutrality.

For example, aluminum has a 3+ charge and bromide has a 1- charge. In AlBr3, there are three bromide ions (each with a -1 charge) to balance the one aluminum cation (+3 charge), leading to a neutral compound. Similarly, in Na3P, three sodium ions (each +1 charge) balance the one phosphide ion (-3 charge) to keep the compound electrically neutral. If the compound has polyatomic ions—ions made up of more than one atom—the same principles apply, but naming includes the polyatomic ion's established name, such as sulfate or nitrate.

Formula Unit Composition

Formula unit composition refers to the lowest whole number ratio of ions within an ionic compound that results in a net charge of zero. To understand this concept, it's essential to recognize how the charges of ions determine the ratio in which they combine.

For instance, in Barium Arsenide (Ba3As2), barium has a charge of +2 and arsenic has a charge of -3. To achieve charge neutrality, we can't simply combine one barium ion with one arsenic ion. Instead, it takes three barium ions (each +2 for a total of +6) to balance two arsenic ions (each -3 for a total of -6), resulting in the formula Ba3As2 with a neutral overall charge. Understanding formula unit composition is critical when predicting the formula of an unknown compound based on the charges of its constituent ions, and it greatly aids in writing chemical formulas accurately.