Problem 17 - Chemistry: The Central Science in SI Units, Global Edition

Question

Predict the chemical formula of the ionic compound formed between the following pairs of elements: (a) Al and Cl, (b) \(\mathrm{Mg}\) and \(\mathrm{O},(\mathbf{c}) \mathrm{Zn}\) and \(\mathrm{Cl}\), (d) \(\mathrm{Li}\) and \(\mathrm{O}\).

Step-by-Step Solution

Answer

The chemical formulas for the ionic compounds between the given pairs of elements are: (a) \(AlCl_3\), (b) \(MgO\), (c) \(ZnCl_2\), and (d) \(Li_2O\).

Step 1: (a) Al and Cl

First, we need to find the valencies of Al and Cl. Aluminium (Al) belongs to group 13 in the periodic table, which means it has 3 valence electrons. It tends to lose these 3 electrons to obtain a stable electron configuration, and forms an Al^3+ ion. Chlorine (Cl) belongs to group 17 in the periodic table, which means it has 7 valence electrons. It tends to gain 1 electron to obtain a stable electron configuration, and forms a Cl^- ion. Now, to write the chemical formula, we need to balance the charges by having 3 Cl^- ions for each Al^3+ ion. The chemical formula for the ionic compound between Al and Cl is: \(AlCl_3\)

Step 2: (b) Mg and O

First, we need to find the valencies of Mg and O. Magnesium (Mg) belongs to group 2 in the periodic table, which means it has 2 valence electrons. It tends to lose these 2 electrons to obtain a stable electron configuration, and forms an Mg^2+ ion. Oxygen (O) belongs to group 16 in the periodic table, which means it has 6 valence electrons. It tends to gain 2 electrons to obtain a stable electron configuration, and forms an O^2- ion. Now, to write the chemical formula, we need to balance the charges by having 1 Mg^2+ ion for each O^2- ion. The chemical formula for the ionic compound between Mg and O is: \(MgO\)

Step 3: (c) Zn and Cl

First, we need to find the valencies of Zn and Cl. Zinc (Zn) has a unique property where it forms a stable Zn^2+ ion. Chlorine (Cl) belongs to group 17 in the periodic table, which means it has 7 valence electrons. It tends to gain 1 electron to obtain a stable electron configuration, and forms a Cl^- ion. Now, to write the chemical formula, we need to balance the charges by having 2 Cl^- ions for each Zn^2+ ion. The chemical formula for the ionic compound between Zn and Cl is: \(ZnCl_2\)

Step 4: (d) Li and O

First, we need to find the valencies of Li and O. Lithium (Li) belongs to group 1 in the periodic table, which means it has 1 valence electron. It tends to lose this electron to obtain a stable electron configuration, and forms an Li^+ ion. Oxygen (O) belongs to group 16 in the periodic table, which means it has 6 valence electrons. It tends to gain 2 electrons to obtain a stable electron configuration, and forms an O^2- ion. Now, to write the chemical formula, we need to balance the charges by having 2 Li^+ ions for each O^2- ion. The chemical formula for the ionic compound between Li and O is: \(Li_2O\)

Key Concepts

Valency

Valency is a key concept in understanding ionic compound formation. It refers to the ability of an atom to gain, lose, or share electrons to achieve a stable electron configuration. This typically means attaining a full outer shell of electrons. Elements in the periodic table display predictable valency patterns based on their group number.

For example:

• Elements in Group 1, like lithium (Li), have one valence electron and a valency of +1 because they tend to lose that electron easily.
• Aluminium (Al), found in Group 13, has three valence electrons and typically forms a +3 valency by losing these electrons.
• In contrast, chlorine (Cl) from Group 17 has a valency of -1 because it tends to gain one electron, completing its valence shell.

Understanding valency helps predict how different elements will interact to form stable ionic compounds. By knowing an element's valency, we can deduce the ratios of ions in an ionic compound to ensure overall neutrality.

Electron Configuration

Electron configuration is the distribution of electrons in an atom's or ion's atomic orbitals. When atoms form ions, they adjust their electron configurations to mimic the nearest noble gas. This gives them a stable electronic structure.

The electron configuration follows the order of filling based on the energy levels: 1s, 2s, 2p, 3s, 3p, and so on.

• For example, magnesium (Mg) begins with the electron configuration of \([Ne] 3s^2\). When it forms an \({Mg}^{2+}\) ion, it loses its 3s electrons, achieving the \([Ne]\) configuration.
• Oxygen (O), which starts with \([He] 2s^2 2p^4\), will gain 2 electrons to form an \({O}^{2-}\) ion with configuration \([Ne]\).

This electron transfer results in ions with positive and negative charges, leading to an attraction that forms ionic bonds. Hence, understanding electron configurations helps explain why certain combinations of elements form stable compounds.

Chemical Formula Determination

Determining the chemical formula of an ionic compound requires balancing the total positive and negative charges of the constituent ions. This ensures that the compound is electrically neutral.

Here's how to determine an ionic compound's formula:

• Identify the valency of each element in the compound. For example, with zinc (Zn) forming \({Zn}^{2+}\) and chlorine (Cl) forming \({Cl}^{-}\), we know their charges.
• Balance the overall charges by adjusting the ratio of ions. For zinc chloride (\({ZnCl}_2\)), two chlorine ions are needed to balance the charge of one zinc ion.
• For magnesium oxide (MgO), where both \({Mg}^{2+}\) and \({O}^{2-}\) ions have charges of \(+2\) and \(-2\), only a 1:1 ratio is needed to balance the charges.

Accurate chemical formula determination relies heavily on understanding both valency and electron configuration. This ensures a stable arrangement of ions in the resulting compound.