Question

Write formulas for ionic compounds formed between (a) \(\mathrm{Na}\) and \(\mathrm{Br}\), (b) \(\mathrm{K}\) and \(\mathrm{I},(\mathbf{c}) \mathrm{Ba}\) and \(\mathrm{O},\) (d) \(\mathrm{Mg}\) and \(\mathrm{Br}\), (e) \(\mathrm{Al}\) and \(\mathrm{S},\) and \((\mathbf{f}) \mathrm{Ba}\) and \(\mathrm{F}\)

Short answer

The formulas for the ionic compounds are (a) NaBr, (b) KI, (c) BaO, (d) MgBr2, (e) Al2S3, and (f) BaF2.

Step-by-step solution

Determine the Charges of the Ions

First, determine the charge of each ion. Sodium (Na) forms a +1 ion, bromine (Br) forms a -1 ion. Potassium (K) forms a +1 ion, iodine (I) forms a -1 ion. Barium (Ba) forms a +2 ion, oxygen (O) forms a -2 ion. Magnesium (Mg) forms a +2 ion, aluminum (Al) forms a +3 ion, sulfur (S) forms a -2 ion, and fluorine (F) forms a -1 ion.

Combine Ions to Form Neutral Compounds

Combine the ions in ratios that result in neutral compounds. For Na and Br, a 1:1 ratio is used since they both have charges of +1 and -1, respectively. For K and I, the same 1:1 ratio applies. For Ba and O, a 1:1 ratio is central because their charges of +2 and -2 balance out. For Mg and Br, a 1:2 ratio is needed to balance the +2 charge of Mg with two -1 Br ions. For Al and S, a 2:3 ratio balances the +3 charge of Al with three -2 charges of S. Finally, for Ba and F, a 1:2 ratio is used to balance the +2 charge of Ba with two -1 F ions.

Write the Formulas

Using the ratios, write the formulas for each compound. For Na and Br, the compound is NaBr. For K and I, the compound is KI. For Ba and O, the compound is BaO. For Mg and Br, the compound is MgBr2. For Al and S, the compound is Al2S3. For Ba and F, the compound is BaF2.

Key concepts

Chemical Bonding

Chemical bonding is essential in creating compounds from individual elements. It involves the attraction between atoms allowing the formation of chemical substances that contain two or more atoms. The bond type is determined by the elements involved and their electron configurations.

An ionic bond, for example, is formed when one atom donates an electron to another, creating ions with opposite charges that attract each other. This transfer of electrons typically occurs between metals and nonmetals. Metals, which have few electrons in their outer shell, are likely to lose them and form positively charged ions, or cations. Conversely, nonmetals tend to gain electrons to complete their outer shell, forming negatively charged ions, or anions.

In the given exercise, when sodium (Na), a metal, bonds with bromine (Br), a nonmetal, they form an ionic bond. Sodium donates one electron to bromine, resulting in the positively charged Na+ ion and negatively charged Br- ion that attract and form the ionic compound NaBr. This simple transfer mechanism is the core of ionic bonding, which creates a vast array of compounds.

Chemical Formulas

Chemical formulas represent the elements that compose a compound and the relative number of atoms of each element in the compound. The formula is dictated by the need for electrical neutrality; the total positive charge of the cations must balance the total negative charge of the anions.

For example, in the case of magnesium (Mg) and bromine (Br), Magnesium forms a Mg²⁺ ion, while bromine forms Br^- ions. To achieve neutrality, two Br^- ions are needed to balance the charge of one Mg²⁺ ion, leading to a chemical formula of MgBr₂. This stoichiometry, the relative number of ions needed, is the heart of writing chemical formulas.

Understanding the chemical formula also tells us about the proportions in which the elements combine. For the compound aluminium sulfide, a 2:3 ratio of Al³⁺ and S^2- ions is required to balance the charges, thus the correct chemical formula is Al₂S₃. This notation quickly informs us that each molecule of aluminium sulfide is composed of two aluminium and three sulfur atoms.

Ion Charges

Ion charges are integral to understanding chemical bonding and formula writing for ionic compounds. An ion is an atom or molecule with a net electric charge due to the loss or gain of one or more electrons. Metals typically form cations as they lose electrons while nonmetals usually become anions by gaining electrons.

The charges of common ions should be memorized for efficient formula writing. For example, group 1 elements like sodium (Na) and potassium (K) usually form +1 ions. Group 2 elements like magnesium (Mg) and barium (Ba) typically form +2 ions. Meanwhile, group 16 elements like oxygen (O) form -2 ions, and halogens like fluorine (F) usually form -1 ions.

The charge of the ion dictates how it combines with ions of opposite charge to form a neutral compound. With a foundational understanding of ion charges, predicting and writing the formula of ionic compounds becomes straightforward. For instance, knowing that barium forms Ba²⁺ and oxygen forms O^2-, one can deduce that barium oxide will have the formula BaO, a 1:1 ratio that ensures the charges cancel each other out.