Problem 17 - Chemistry The Central Science In Si Units

Question

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

Step-by-Step Solution

Answer

(a) AlCl₃, (b) MgO, (c) ZnCl₂, (d) Li₂O.

Step 1: Understand Ionic Compounds

Ionic compounds are formed between metals and nonmetals. The metal donates electrons to become a positively charged cation, while the non-metal accepts electrons to become a negatively charged anion.

Step 2: Determine Charges

Identify the valency of the elements: - Al forms a 3+ cation, Cl forms a 1- anion. - Mg forms a 2+ cation, O forms a 2- anion. - Zn forms a 2+ cation, Cl forms a 1- anion. - Li forms a 1+ cation, O forms a 2- anion.

Step 3: Write Formulas Using Charge Balance

Create a neutral compound by balancing the total positive and negative charges: - For Al and Cl: Al forms AlCl₃ (3 chloride ions are needed to balance 1 aluminum ion). - For Mg and O: Mg forms MgO (one magnesium ion balances with one oxide ion). - For Zn and Cl: Zn forms ZnCl₂ (two chloride ions are needed to balance one zinc ion). - For Li and O: Li forms Li₂O (two lithium ions are needed to balance one oxide ion).

Key Concepts

Predicting Chemical Formulas

Predicting the chemical formula of an ionic compound is an essential skill in chemistry. It involves understanding the interaction between metal and nonmetal elements, where the metal loses electrons, forming a cation, while the nonmetal gains electrons, forming an anion.
To predict a chemical formula, it's crucial to know the charges of these ions, which allows us to balance them in a compound. For example, aluminum (Al), a metal, forms a cation with a charge of +3, and chlorine (Cl), a nonmetal, forms an anion with a charge of -1. To balance these, the formula becomes AlCl₃, meaning one aluminum ion combines with three chloride ions.

• Always identify the charge of each ion first.
• Use the charges to determine the ratio needed to achieve charge neutrality.

Hence, predicting chemical formulas includes examining these charges carefully.

Cation and Anion Charges

In ionic compounds, understanding the charges of cations and anions is fundamental. A cation is a positively charged ion, and an anion is a negatively charged ion. Metals typically form cations, and nonmetals form anions.
For example:

• Aluminum (Al) forms a cation with a charge of +3.
• Chlorine (Cl), a nonmetal, forms an anion with a charge of -1.
• Magnesium (Mg) has a valency of +2, while oxygen (O) forms a -2 anion.

Recognizing these charges helps in forming correct ionic compounds, like balancing Al with Cl to form AlCl₃. The charges dictate how many of each ion are needed to achieve a neutral compound. Remember, the total charge from cations must equal the total charge from anions for the compound to be stable.

Metal and Nonmetal Reactions

The reaction between metals and nonmetals is the basis for forming ionic compounds. Metals, which are on the left side of the periodic table, tend to lose electrons and form cations.
Nonmetals, found on the right side of the table, gain these electrons to become anions. For instance, lithium (Li) loses an electron to form Li⁺, while oxygen (O) gains two electrons to form O²⁻. This electron exchange is what creates the ionic bond, resulting in compounds like Li₂O.
Some key points include:

• Metals are electron donors (e.g., Li = Li⁺).
• Nonmetals are electron acceptors (e.g., O = O²⁻).
• The exchange of electrons forms a stable ionic bonding.

This predictable exchange pattern makes it easier to determine the products of metal and nonmetal reactions.

Charge Balancing in Compounds

In ionic compounds, charge balancing is vital to ensure neutrality. The positive and negative charges must cancel each other out.
For example, in aluminum chloride (AlCl₃), one Al³⁺ cation balances with three Cl⁻ anions. The overall charge of the compound must equal zero.
Consider these steps when balancing charges:

• Calculate total positive charge (e.g., from metal cations).
• Calculate total negative charge (e.g., from nonmetal anions).
• Adjust the ratio of ions to equalize positive and negative charges.

In zinc chloride (ZnCl₂), Zn²⁺ pairs with two Cl⁻ ions to form a neutral compound. This precise balancing ensures electrical neutrality, which is crucial for the formation of stable compounds.