Question

Use Lewis symbols to determine the formula for the compound that forms between each pair of elements. $$ \text { a. }\mathrm{Ca} \text { and } \mathrm{N} \quad \text { b. } \mathrm{Mg} \text { and } \mathrm{I} \quad \text { c. }\mathrm{Cand} \mathrm{S} \quad \text { d. } \mathrm{Cs} \text { and } \mathrm{F} $$

Short answer

The chemical formulas for the compounds are a. \(\text{Ca}_3\text{N}_2\), b. \(\text{MgI}_2\), c. \(\text{CS}_2\), and d. \(\text{CsF}\).

Step-by-step solution

Identify Valence Electrons for Calcium and Nitrogen

Firstly, identify the number of valence electrons for each element using the periodic table. Calcium (Ca) has 2 valence electrons as it is in group 2. Nitrogen (N) has 5 valence electrons as it is in group 15.

Determine the Lewis Symbols for Calcium and Nitrogen

The Lewis symbol for calcium is a Ca symbol with two dots representing its valence electrons. For nitrogen, the Lewis symbol is an N symbol with five dots around it.

Combine Calcium and Nitrogen to Achieve Noble Gas Configuration

Calcium can donate its two electrons to two different nitrogen atoms. Each nitrogen atom requires three more electrons to achieve a noble gas configuration. This means three calcium atoms will combine with two nitrogen atoms to both achieve stable octets.

Write the Formula for the Compound Formed Between Calcium and Nitrogen

The chemical formula for the compound formed between calcium and nitrogen is \( \text{Ca}_3\text{N}_2 \), indicating that three calcium atoms combine with two nitrogen atoms.

Repeat Steps for Magnesium and Iodine

Magnesium (Mg) has two valence electrons, and iodine (I) has seven valence electrons. Magnesium can donate its two electrons to two separate iodine atoms, forming \( \text{MgI}_2 \), where one magnesium atom combines with two iodine atoms.

Repeat Steps for Carbon and Sulfur

Carbon (C) has four valence electrons, and sulfur (S) has six valence electrons. A carbon atom can share its four electrons with a sulfur atom to form a double bond, resulting in the compound \( \text{CS}_2 \).

Repeat Steps for Cesium and Fluorine

Cesium (Cs) has one valence electron, and fluorine (F) has seven valence electrons. Cesium donates its one electron to a fluorine atom, forming the compound \( \text{CsF} \), where one cesium atom combines with one fluorine atom.

Key concepts

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. They are the key players in chemical reactions because they are involved in the bonds atoms form with each other. For instance, in the case of calcium (Ca), which belongs to group 2 of the periodic table, it has 2 valence electrons. These electrons are the ones calcium can 'donate' to other elements in the process of forming chemical bonds.

In contrast, nitrogen (N) has 5 valence electrons, found by recognizing its position in group 15. Nitrogen is eager to gain more electrons to obtain a stable configuration. When elements like calcium and nitrogen react, calcium gives up its two valence electrons—one to each of two nitrogen atoms—a process beautifully represented by their Lewis symbols. Here, the Lewis symbol is simply the element's symbol surrounded by dots equal to the number of valence electrons. For calcium, it's 'Ca' with two dots, and for nitrogen, it's 'N' with five dots.

Understanding the valence electrons of individual atoms provides a foundation for predicting the behavior of atoms during chemical bonding and is crucial when determining the correct chemical formula for a compound.

Chemical Formula

The chemical formula of a compound tells us the exact number of different atoms of each element it contains. It's a symbolic representation wherein the elemental symbols are followed by numerical subscripts indicating the respective numbers of atoms in the compound. For example, with the compound formed between calcium and nitrogen, the formula is written as \( \text{Ca}_3\text{N}_2 \).

This signifies that three atoms of calcium react with two atoms of nitrogen to form the compound. Similarly, magnesium (Mg) combines with iodine (I) in a ratio of 1:2, resulting in the formula \( \text{MgI}_2 \). These ratios come from the need of each element to achieve the most stable electron configuration possible—usually that of the nearest noble gas. To write the correct chemical formula, one must consider not only the actual charges of the ions involved but also the ratio in which they combine to cancel their charges, leading to a neutral compound.

Noble Gas Configuration

The noble gas configuration refers to an atom's electron configuration that resembles the electron configuration of the noble gases, the most stable elements in the periodic table. This configuration is characterized by a full valence shell, which means that the outermost electron orbitals are fully occupied. Noble gases such as helium, neon, and argon achieve this naturally. Other elements attempt to reach a full valence shell through the loss, gain, or sharing of electrons when they form compounds.

For instance, calcium prefers to lose two valence electrons, transforming into a cation (positively charged ion), reminiscent of the stable electron configuration of argon. On the flip side, nitrogen aims to gain three electrons, to match the configuration of neon. This drive for stability compels elements to form chemical bonds, creating compounds with neutral overall charge but individual atoms that exhibit noble gas-like electron arrangements.

Through chemical bonding, particularly ionic and covalent bonds, elements achieve the coveted noble gas configuration, resulting in the diverse compounds we encounter in chemistry. This underlying principle helps explain the ratios found in chemical formulas, as these ratios ensure that each participating atom achieves a stable, noble gas-like state.