Problem 160 - Chemistry The Science in Context

Question

Antacid Tablets Antacids commonly contain calcium carbonate, magnesium hydroxide, or both. Draw the Lewis structures for calcium carbonate and magnesium hydroxide.

Step-by-Step Solution

Answer

Answer: The Lewis structure for calcium carbonate (CaCO3) is Ca^2+ [O=C=O]^2-, where calcium is the central atom surrounded by the carbonate ion, which consists of carbon double-bonded to three oxygen atoms. The Lewis structure for magnesium hydroxide (Mg(OH)2) is [Mg^2+] [O - H]^- [O - H]^- , where magnesium is the central atom surrounded by two hydroxide ions (OH^-), each having oxygen single-bonded to hydrogen.

Step 1: Identify the elements and valence electrons

Firstly, let's identify the elements present in the calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2) molecules. The elements in CaCO3 are calcium (Ca), carbon (C), and oxygen (O). The elements in Mg(OH)2 are magnesium (Mg), oxygen (O), and hydrogen (H). Now, let's find the number of valence electrons for each element: - Calcium (Ca): 2 valence electrons (from group 2) - Carbon (C): 4 valence electrons (from group 14) - Oxygen (O): 6 valence electrons (from group 16) - Magnesium (Mg): 2 valence electrons (from group 2) - Hydrogen (H): 1 valence electron (from group 1)

Step 2: Draw the skeleton structure of calcium carbonate (CaCO3)

In CaCO3, calcium is the least electronegative and forms the central atom, with the carbonate ion (CO3^2-) surrounding it. In the carbonate ion, carbon is the central atom surrounding by three oxygen atoms. We draw them like this: Ca - O - C - O | O

Step 3: Add valence electrons to the calcium carbonate (CaCO3) skeleton structure

Now, add the valence electrons to the skeleton structure, remembering that calcium loses its two valence electrons to become stable and the carbonate ion gains these two electrons: Ca^2+ [O=C=O]^2- | O In this Lewis structure, each oxygen atom has two lone pairs of electrons and has a double bond with carbon.

Step 4: Draw the skeleton structure of magnesium hydroxide (Mg(OH)2)

In Mg(OH)2, magnesium is the least electronegative and forms the central atom, surrounded by two hydroxide ions (OH^-). In each hydroxide ion, oxygen is the central atom attached to hydrogen. We draw them like this: Mg - O - H | O - H

Step 5: Add valence electrons to the magnesium hydroxide (Mg(OH)2) skeleton structure

Now, add the valence electrons to the skeleton structure, remembering that magnesium loses its two valence electrons to become stable, and each hydroxide ion gains one electron: [Mg^2+] [O - H]^- [O - H]^- In this Lewis structure, each oxygen atom has two lone pairs of electrons and a single bond with hydrogen. These are the Lewis structures for calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2).

Key Concepts

Understanding Valence Electrons

Valence electrons are the outermost electrons of an atom. These are the electrons involved in making chemical bonds. The number of valence electrons can determine an atom's chemical reactivity. For example:

• Calcium (Ca) has 2 valence electrons because it is in group 2 of the periodic table.
• Carbon (C) has 4 valence electrons, as seen in group 14.
• Oxygen (O), seated in group 16, carries 6 valence electrons.
• Magnesium (Mg), like calcium, presents 2 valence electrons since it's also in group 2.
• Hydrogen (H) has a single valence electron belonging to group 1.

Valence electrons are crucial to forming Lewis structures. They help determine how atoms connect to form stable molecules. In the case of calcium carbonate (CaCO extsubscript{3}) and magnesium hydroxide (Mg(OH) extsubscript{2}), the sharing or transfer of valence electrons leads to their particular structures.

Calcium Carbonate: Molecule Analysis

Calcium carbonate, commonly found in rocks and as the active ingredient in antacids, consists of calcium ions and carbonate ions. In its Lewis structure, Ca loses its 2 valence electrons to become the positively charged Ca extsuperscript{2+}. This electron release aids the carbonate ion (CO extsubscript{3} extsuperscript{2-}), which effectively uses electrons to stabilize with carbon central bonding to three oxygens.
One oxygen forms a double bond with carbon, providing stability and spreading out electron pairs efficiently. The remaining electrons localize as lone pairs on the other two oxygen atoms. This electron distribution results in a balanced molecule, making it highly effective as an antacid by neutralizing stomach acidity.

Magnesium Hydroxide: Molecule Analysis

Magnesium hydroxide, another common antacid component, is structured with Mg extsuperscript{2+} as the central figure. Here, magnesium loses its 2 valence electrons, which are then absorbed by each two hydroxide ions (OH extsuperscript{-}). Each "OH" unit displays oxygen as the central atom with a single bond leading to hydrogen.
The extra electrons from magnesium settle on the oxygens as lone pairs, giving each its charge stabilization. This relatively simple structure makes magnesium hydroxide effective in neutralizing excess stomach acids. The ionic form helps it dissolve well in water, further increasing its antacid efficacy. Understanding the role of valence electrons in this compound reveals the reasons behind its beneficial properties.