Problem 41 - Introductory Chemistry Atoms First

Question

Predict the formulas of the ionic compounds that result from combining the following: (a) \(\mathrm{Mg}\) (solid metal) with \(\mathrm{Br}_{2}\) (liquid) (b) Be (solid metal) with \(\mathrm{O}_{2}\) (gas) (c) Na (solid metal) with \(\mathrm{I}_{2}\) (solid)

Step-by-Step Solution

Answer

The resulting ionic compounds are: (a) \(\mathrm{MgBr}_2\), (b) \(\mathrm{BeO}\), and (c) \(\mathrm{NaI}\).

Step 1: (a) Identify the ions and their charges

For \(\mathrm{Mg}\), it is in group 2 of the periodic table, so it will form a \(\mathrm{Mg}^{2+}\) cation. Bromine (Br) is in group 17, so it will form a \(\mathrm{Br}^{-}\) anion.

Step 2: (a) Combine ions to form a neutral compound

To balance the charges, we need 1 \(\mathrm{Mg}^{2+}\) cation and 2 \(\mathrm{Br}^{-}\) anions. Thus, the resulting ionic compound is \(\mathrm{MgBr}_2\).

Step 3: (b) Identify the ions and their charges

For Be, it is in group 2, so it will form a \(\mathrm{Be}^{2+}\) cation. Oxygen (O) is in group 16, so it will form an \(\mathrm{O}^{2-}\) anion.

Step 4: (b) Combine ions to form a neutral compound

To balance the charges, we need 1 \(\mathrm{Be}^{2+}\) cation and 1 \(\mathrm{O}^{2-}\) anion. Thus, the resulting ionic compound is \(\mathrm{BeO}\).

Step 5: (c) Identify the ions and their charges

For Na, it is in group 1, so it will form a \(\mathrm{Na}^{+}\) cation. Iodine (I) is in group 17, so it will form an \(\mathrm{I}^{-}\) anion.

Step 6: (c) Combine ions to form a neutral compound

To balance the charges, we need 1 \(\mathrm{Na}^{+}\) cation and 1 \(\mathrm{I}^{-}\) anion. Thus, the resulting ionic compound is \(\mathrm{NaI}\).

Key Concepts

Forming Ionic Bonds

The process of forming ionic bonds is central to the creation of ionic compounds and involves the transfer of electrons from one atom to another. This typically occurs between a metal and a non-metal element. Metals, such as magnesium (Mg), beryllium (Be), and sodium (Na), tend to lose electrons and become positively charged ions, or cations. In contrast, non-metals like bromine (Br), oxygen (O), and iodine (I) tend to gain electrons and become negatively charged ions, or anions.

When a cation and an anion come into contact, the opposite charges attract, and an ionic bond is formed, creating an ionic compound with a neutral overall charge. The stability of these ionic compounds is derived from the strong electrostatic forces that bind the ions together. In summary, ionic bonds form when electrons are transferred from a metal to a non-metal, resulting in a stable arrangement of ions that are held together by their opposite charges.

Predicting Chemical Formulas

The skill of predicting chemical formulas is crucial for understanding how atoms combine to form compounds. For ionic compounds, the goal is to balance the total positive and negative charges to achieve an electrically neutral entity. Here's a simplified two-step approach to predict the formula of an ionic compound:

• Identify the ions: Determine the charge of the cation (positive ion) and anion (negative ion) formed by the elements in question. The charge often corresponds to the group number in the periodic table for metals and non-metals.
• Combine the ions: Pair cations and anions in ratios that balance their charges, ensuring that the overall electrical charge of the compound is zero. The final formula of the ionic compound represents this neutral combination.

The prediction of the chemical formula is based on the principle of charge neutrality and the commonly known charges of ions derived from their positions in the periodic table.

Cation-Anion Combination

Understanding the cation-anion combination is instrumental when forming the chemical formula of an ionic compound. A cation, usually a metal, has a positive charge and an anion, typically a non-metal, has a negative charge. The cation and the anion combine in a way that their total charges equal zero, leading to a stable compound.

For example, combining Mg (which forms a \(\mathrm{Mg}^{2+}\) ion) with Br (forming two \(\mathrm{Br}^{-}\) ions) results in \(\mathrm{MgBr}_2\), since two bromide ions are needed to balance the double positive charge of one magnesium ion. Similarly, the combination of Be (forming a \(\mathrm{Be}^{2+}\) ion) with O (forming a \(\mathrm{O}^{2-}\) ion) gives us \(\mathrm{BeO}\). Na (forming a \(\mathrm{Na}^{+}\) ion) combines with I (forming a \(\mathrm{I}^{-}\) ion) to produce \(\mathrm{NaI}\).

The ratios in which cations and anions combine depend on their respective charges and can often be determined using a simple cross-multiplication method. This practice allows the prediction of accurate and stable formulas for the compounds created through ionic bonding.