Question

Calcium carbonate \(\left(\mathrm{CaCO}_{3}\right)\) shells are used by mollusks, corals, and snails to form protective coverings. Draw the Lewis structure for \(\mathrm{CaCO}_{3} .\) Be sure to include any resonance structures.

Short answer

The Lewis structure of Calcium carbonate (CaCO₃) consists of a Calcium (Ca²⁺) ion and a carbonate (CO₃²⁻) ion. To draw the Lewis structure, first, arrange Carbon as the central atom with three Oxygen atoms surrounding it. Create single bonds between each Carbon-Oxygen pair and distribute the remaining valence electrons as lone pairs on the Oxygen atoms. Then, draw 3 resonance structures for the CO₃²⁻ ion by promoting one lone pair of electrons from an Oxygen atom into a double bond with Carbon in each possible pair. Combine the Ca²⁺ ion and CO₃²⁻ ion to form the complete CaCO₃ molecule with resonance structures, showing the ionic bond between them.

Step-by-step solution

Count the valence electrons

First, we need to count the valence electrons for each atom in the CaCO3 molecule. Calcium (Ca) has 2 valence electrons, Carbon (C) has 4 valence electrons, and Oxygen (O) has 6 valence electrons. Since there are three Oxygen atoms, the total valence electrons for them will be 18 (6 x 3). So, the total number of valence electrons in the CaCO3 molecule is 2 + 4 + 18 = 24 electrons.

Arrange atoms in the molecule

Next, we arrange the atoms in the molecule considering that the least electronegative atom will be the central atom. In this case, Carbon (C) is the least electronegative atom compared to Calcium and Oxygen, so it will be the central atom. Calcium (Ca) will form an ionic bond with the CO3 2- ion since it's a metal, and its valence electrons will be transferred to the CO3 2- ion. We will have Ca 2+ ion and CO3 2- ion. For the CO3 2- ion, we will arrange the atoms as follows: C is the central atom, and three Oxygen atoms (O) will be surrounding the Carbon atom.

Distribute the electrons

Now, we will distribute the electrons to the atoms in the CO3 2- ion according to the octet rule to achieve a stable configuration for each atom. Starting with the Carbon atom (central atom): Put a single bond (2 shared electrons) between each Carbon-Oxygen pair, which uses 2 x 3 = 6 of the 24 electrons. Your structure should now show 3 single bonds between the Carbon and each of the Oxygen atoms. Next, we distribute the remaining electrons to the Oxygen atoms: Each Oxygen atom currently has 2 electrons from the single bond with Carbon. Since we have used 6 electrons to form the single bonds, we are left with 18 electrons. We will distribute these 18 electrons around the Oxygen atoms to complete their octet. Each Oxygen atom will receive 6 electrons (3 lone pairs). Now, you may notice that the Carbon atom does not yet have an octet (only 6 electrons). We have no more electrons to distribute; this is where resonance structures come into play.

Draw resonance structures

We can solve the incomplete octet issue for Carbon by considering resonance structures in the CO3 2- ion. To do this, we will promote one of the lone pairs from one of the Oxygen atoms to create a double bond between Carbon and that Oxygen atom. This will use two electrons from one of the Oxygen atoms' lone pairs and complete the octet for the Carbon atom. We can do this for each of the Oxygen atoms one at a time, resulting in 3 resonance structures for the CO3 2- ion. Now, we have the 3 resonance structures for the CO3 2- ion and the separate Calcium (Ca2+) ion.

Combine the Ca2+ ion and CO3 2- ion

Combine the Ca 2+ ion and CO3 2- ion to form the complete Calcium carbonate (CaCO3) molecule. Show theirionic bond by placing the Ca 2+ ion near the CO3 2- ion. This completes the Lewis structure of CaCO3 with resonance structures. Remember that the complete representation of the Lewis structure for CaCO3 should include all three resonance structures for the CO3 2- ion, indicating that they contribute equally to the overall molecular structure.

Key concepts

Calcium carbonate

Calcium carbonate, denoted as \( \text{CaCO}_3 \), is a chemical compound that is prevalent in nature. It can be found in rocks as minerals such as calcite and aragonite, and it is the main component of pearls and the shells of marine organisms, snails, and eggs. When drawing the Lewis structure of calcium carbonate, it's important to understand that it consists of a calcium ion (\( \text{Ca}^{2+} \)) and a carbonate ion (\( \text{CO}_3^{2-} \)).

- The calcium ion forms due to calcium's readiness to lose two electrons and achieve a stable electronic configuration similar to noble gases. Calcium, being a metal, tends to form ionic bonds which are indicated in Lewis structures but are not shown with lines like covalent bonds.- The carbonate ion (\( \text{CO}_3^{2-} \)) is the part that undergoes resonance and consists of one carbon atom covalently bonded to three oxygen atoms.

Understanding calcium carbonate is crucial in fields such as geology, biology, chemistry, and even environmental science, due to its ubiquitous presence and role in Earth systems.

Resonance structures

Resonance structures are a critical concept when understanding molecules like the carbonate ion \( \text{CO}_3^{2-} \). They describe a situation where a single Lewis structure does not sufficiently represent a molecule. Instead, multiple structures (resonance structures) are drawn to depict the molecule’s real chemical configuration. These structures are hypothetical but help illustrate where electrons may be found in a molecule.

- In calcium carbonate's carbonate ion, resonance structures are used to properly describe the distribution of electrons among the carbon and three oxygen atoms. Each resonance structure highlights a different oxygen atom forming a double bond with the carbon atom, alternating the position of single and double bonds across the structural depictions.- The reality is that each oxygen shares bonding equally due to this resonance and is represented as partial bonds and charges, which stabilize the ion.

Using resonance structures ensures the depiction of a more accurate picture of the electron distribution and helps in explaining reactivity and molecular stability.

Valence electrons

Valence electrons play a pivotal role in determining how atoms interact and bond within compounds like calcium carbonate. These are the outermost electrons of an atom and are primarily responsible for chemical bonding. Knowing the number of valence electrons can help predict the types of bonds an atom will form.

- For the calcium carbonate molecule, - Calcium (Ca) has 2 valence electrons. - Carbon (C) possesses 4 valence electrons. - Oxygen (O) in the carbonate ion has 6 valence electrons per atom, contributing to a total of 18 valence electrons from the three oxygen atoms.

Understanding how these electrons are distributed in the Lewis structure helps illustrate how atoms achieve full outer shells as per the octet rule and whether they will engage in ionic or covalent bonding. In the case of \( \text{Ca}^{2+} \), it loses its valence electrons to achieve a more stable electron configuration.

Octet rule

The octet rule is a guiding principle that helps in constructing the Lewis structure of molecules like the carbonate ion in calcium carbonate. This rule states that atoms are more stable when they have eight electrons in their valence shell, mirroring the electronic configuration of noble gases.

- Oxygen atoms in \( \text{CO}_3^{2-} \) each achieve the octet configuration by forming single and double bonds with carbon, distributing valence electrons around them as lone pairs.- The carbon atom initially lacks a full octet after forming single bonds with each oxygen. By leveraging resonance, the carbonate ion achieves stability as the electron pairs shift among the oxygen atoms, forming a series of alternating single and double bonds until the octet is satisfied.

The octet rule demonstrates why certain resonance structures are possible and explains the chemical stability of molecules like \( \text{CO}_3^{2-} \). In summary, it helps us understand how the elements in \( \text{CaCO}_3 \) interact during the formation of stable compounds.