Question

The designations IA through 8 A used for certain families of the periodic table are helpful for predicting the charges on ions in binary ionic compounds. In these compounds, the metals generally take on a positive charge equal to the family number, while the nonmetals take on a negative charge equal to the family number minus eight. Thus the compound between sodium and chlorine contains \(\mathrm{Na}^{+}\) ions and \(\mathrm{Cl}^{-}\) ions and has the formula \(\mathrm{NaCl}\). Predict the formula and the name of the binary compound formed from the following pairs of elements. a. \(\mathrm{Ca}\) and \(\mathrm{N}\) e. \(\mathrm{Ba}\) and \(\mathrm{I}\) b. \(\mathrm{K}\) and \(\mathrm{O}\) f. \(\mathrm{Al}\) and Se c. \(\mathrm{Rb}\) and \(\mathrm{F}\) g. Cs and P d. \(\mathrm{Mg}\) and \(\mathrm{S}\) h. In and \(\mathrm{Br}\)

Short answer

The formulas and names of the binary compounds are: a. \(Ca_{3}N_{2}\) - Calcium Nitride b. \(K_{2}O\) - Potassium Oxide c. \(RbF\) - Rubidium Fluoride d. \(MgS\) - Magnesium Sulfide e. \(BaI_{2}\) - Barium Iodide f. \(Al_{2}Se_{3}\) - Aluminum Selenide g. \(Cs_{3}P\) - Cesium Phosphide h. \(InBr_{3}\) - Indium Bromide

Step-by-step solution

a. Ca and N

In this pair, calcium (Ca) belongs to the 2A family, which means it will have a positive charge of 2 (+2), and nitrogen (N) belongs to the 5A family, which means it will have a negative charge of 5 - 8 = -3. Since Ca has a charge of +2 and N has a charge of -3, the formula of this binary compound will be \(Ca_{3}N_{2}\). The name of the compound is Calcium Nitride.

b. K and O

In this pair, potassium (K) belongs to the 1A family, which means it will have a positive charge of 1 (+1), and oxygen (O) belongs to the 6A family, which means it will have a negative charge of 6 - 8 = -2. Since K has a charge of +1 and O has a charge of -2, the formula of this binary compound will be \(K_{2}O\). The name of the compound is Potassium Oxide.

c. Rb and F

In this pair, rubidium (Rb) belongs to the 1A family, which means it will have a positive charge of 1 (+1), and fluorine (F) belongs to the 7A family, which means it will have a negative charge of 7 - 8 = -1. Since Rb has a charge of +1 and F has a charge of -1, the formula of this binary compound will be \(RbF\). The name of the compound is Rubidium Fluoride.

d. Mg and S

In this pair, magnesium (Mg) belongs to the 2A family, which means it will have a positive charge of 2 (+2), and sulfur (S) belongs to the 6A family, which means it will have a negative charge of 6 - 8 = -2. Since Mg has a charge of +2 and S has a charge of -2, the formula of this binary compound will be \(MgS\). The name of the compound is Magnesium Sulfide.

e. Ba and I

In this pair, barium (Ba) belongs to the 2A family, which means it will have a positive charge of 2 (+2), and iodine (I) belongs to the 7A family, which means it again will have a negative charge of 7 - 8 = -1. Since Ba has a charge of +2 and I has a charge of -1, the formula of this binary compound will be \(BaI_{2}\). The name of the compound is Barium Iodide.

f. Al and Se

In this pair, aluminum (Al) belongs to the 3A family, which means it will have a positive charge of 3 (+3), and selenium (Se) belongs to the 6A family, which means it again will have a negative charge of 6 - 8 = -2. Since Al has a charge of +3 and Se has a charge of -2, the formula of this binary compound will be \(Al_{2}Se_{3}\). The name of the compound is Aluminum Selenide.

g. Cs and P

In this pair, cesium (Cs) belongs to the 1A family, which means it will have a positive charge of 1 (+1), and phosphorus (P) belongs to the 5A family, which means it again will have a negative charge of 5 - 8 = -3. Since Cs has a charge of +1 and P has a charge of -3, the formula of this binary compound will be \(Cs_{3}P\). The name of the compound is Cesium Phosphide.

h. In and Br

In this pair, indium (In) belongs to the 3A family, which means it will have a positive charge of 3 (+3), and bromine (Br) belongs to the 7A family, which means it again will have a negative charge of 7 - 8 = -1. Since In has a charge of +3 and Br has a charge of -1, the formula of this binary compound will be \(InBr_{3}\). The name of the compound is Indium Bromide.

Key concepts

Periodic Table

The periodic table is a tabular arrangement of chemical elements, organized based on their atomic number, electron configuration, and recurring chemical properties. It is designed such that elements with similar properties fall into the same columns, known as groups or families. Each group in the periodic table has a number and often a letter designation, like 1A, 2A, and so on. These groups help predict how elements will behave in chemical reactions.

For binary ionic compounds, knowing the family number of an element helps predict the charge it will likely carry when it becomes an ion. This predictable pattern is one of the reasons the periodic table is such a valuable tool for chemists.

Ion Charges

Ion charges play a crucial role in forming binary 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 lose electrons to achieve a full outer electron shell, forming positively charged ions, or cations. On the other hand, nonmetals tend to gain electrons, forming negatively charged ions, or anions.

The group number of a metal often indicates its charge as an ion. For instance, Group 1 metals like sodium (Na) form +1 ions, while Group 2 metals like calcium (Ca) form +2 ions. Nonmetals follow a different rule: their charge can typically be calculated by subtracting eight from their group number; for example, oxygen in Group 6 would have a charge of -2. Understanding these charges is key to correctly predicting the formulas of ionic compounds.

Formula Prediction

Predicting the formula of a binary ionic compound involves balancing the total positive and negative charges. This balance ensures the compound is electrically neutral overall. The crisscross method is a common approach for achieving this:

• Write down the metal and nonmetal with their respective charges.
• Crisscross the numbers (ignoring the signs) to become the subscripts of the other element.
• Simplify the subscripts to the smallest whole numbers that maintain charge balance.

For example, with calcium (Ca) and nitrogen (N), Ca is +2 and N is -3. The charges crisscross to yield the formula \(Ca_3N_2\). This ensures that the total positive charge \((3 \times +2)\) equals the total negative charge \((2 \times -3)\).

Compound Naming

Naming binary ionic compounds is relatively straightforward once their formula is determined. The name generally consists of two parts: the name of the cation (metal) followed by the name of the anion (nonmetal) with an 'ide' ending.

For example, the compound \(Ca_3N_2\) is named *calcium nitride* because it is composed of calcium ions and nitride ions. Similarly, \(K_2O\) is called *potassium oxide*. Despite variations in charges and formulas, this naming convention remains consistent across binary ionic compounds, making it easier to identify and classify them consistently.