Problem 148 - Introductory Chemistry Atoms First

Question

Draw dot diagrams for \(\mathrm{Cl}_{2}\) and for \(\mathrm{MgCl}_{2}\) (put \(\mathrm{Mg}\) in the middle) that agree with the type of bonding occurring in each. (That is, show electrons as being shared or transferred, and if they are transferred, show the charges of the resulting ions.)

Step-by-Step Solution

Answer

In the dot diagram of covalent-bonded Cl₂, both chlorine atoms share one valence electron, resulting in the structure: :Cl - Cl: For ionic-bonded MgCl₂, magnesium transfers two valence electrons, one to each chlorine atom, and forms Mg²⁺ and two Cl⁻ ions, shown as: [Mg]²⁺ / \ [ :Cl⁻ ] [ :Cl⁻ ]

Step 1: Recall the basics of dot diagrams

In dot diagrams, also known as Lewis structures, the chemical symbols represent the atomic nuclei and inner shell electrons, whereas the valence electrons are shown as dots or x's surrounding the symbol. For covalent bonding, valence electrons are shared between atoms, while in ionic bonding, electrons are transferred from one atom to another.

Step 2: Drawing the dot diagram for Cl₂

For Cl₂, the type of bonding occurring is covalent bonding, where both chlorine atoms share a pair of valence electrons. To draw the dot diagram for Cl₂, we will first note that each chlorine atom has 7 valence electrons. To satisfy the octet rule, each chlorine atom will share one electron, creating a single bond: Cl : Cl The remaining 6 valence electrons for each chlorine atom will be placed around the symbol, such that each atom has a total of 8 electrons (including the shared pair). The final dot diagram for Cl₂ is: :Cl - Cl:

Step 3: Drawing the dot diagram for MgCl₂

For MgCl₂, the type of bonding occurring is ionic bonding. Magnesium (Mg) atom will transfer two electrons to two different chlorine (Cl) atoms, which will result in the formation of Mg²⁺ and two Cl⁻ ions. First, let's recall that Mg has 2 valence electrons, and Cl has 7 valence electrons. When Mg transfers its 2 valence electrons to two Cl atoms, they will all achieve stable electron configurations: Mg → Mg²⁺ + 2e⁻ Cl + e⁻ → Cl⁻ (this process occurs twice) Now we can draw the dot diagram for MgCl₂. We will represent the magnesium ion (Mg²⁺) in the middle, surrounded by the two chloride ions (Cl⁻). The transferred electrons will be shown as part of the chloride ions' electron configurations: [Mg]²⁺ / \ [ :Cl⁻ ] [ :Cl⁻ ] This dot diagram represents the ionic bond in MgCl₂, showing the transferred electrons and the charges of the resulting ions. Keep in mind that the square brackets indicate the complete valence shell of the ions.

Key Concepts

Dot Diagrams

In chemistry, dot diagrams are visual tools used to represent the valence electrons of atoms. These diagrams, also known as Lewis structures, help us understand how atoms bond together. The chemical symbol of an element stands for the nucleus and core electrons, while dots or X's around the symbol show the valence electrons. Valence electrons are crucial because they participate in bonding.

Dot diagrams are helpful in predicting the type of bond:

• Covalent bonds occur when atoms share electrons.
• Ionic bonds involve the transfer of electrons between atoms.

Creating dot diagrams involves understanding how many valence electrons each atom has and how they are used in bonding. This powerful tool makes complex molecular structures easier to visualize.

Covalent Bonding

Covalent bonding happens when two atoms share a pair of valence electrons. It's the type of bond that forms in \( \mathrm{Cl}_2 \). Chlorine atoms need one additional electron to fulfill the octet rule, which means having eight electrons around them for stability.

By sharing one electron each, two chlorine atoms form a covalent bond: \[ : \text{Cl} - \text{Cl} : \] Each chlorine atom ends up with a complete octet.

This sharing creates a stable arrangement, ensuring both atoms reach a more stable energy state. Covalent bonds are typically found in molecules where two or more nonmetals bond together. They can vary from single, double, to triple bonds, depending on the number of shared pairs of electrons.

Ionic Bonding

Ionic bonding occurs through the complete transfer of valence electrons from one atom to another, resulting in positive and negative ions. In \( \mathrm{MgCl}_2 \), magnesium (Mg) transfers two electrons to chlorine atoms, forming an ionic compound.

Here's how it works:

• Magnesium loses two electrons to form \( \text{Mg}^{2+} \).
• Each chlorine atom gains one electron to form \( \text{Cl}^- \).

Creating the ionic compound \( \mathrm{MgCl}_2 \) involves drawing square brackets around each ion: \[[\text{Mg}]^{2+} \hspace{2mm} / \hspace{2mm} \{\text{:Cl}^-\} \hspace{2mm} \{\text{:Cl}^-\}\]
The ionic bond forms due to the attraction between positively charged \( \text{Mg}^{2+} \) and negatively charged \( \text{Cl}^- \) ions. This type of bonding is typical in metal and nonmetal combinations.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a key role in chemical bonding. These electrons determine how atoms interact and form bonds with one another. The number of valence electrons can be identified from an element's group number in the periodic table.

Why are valence electrons important?

• They are involved in forming bonds, either by being shared as in covalent bonds or transferred as in ionic bonds.
• They determine the reactivity of an element.
• Understanding the number of valence electrons helps predict the type of bonds and the molecule's shape.

For example:
- Chlorine has seven valence electrons and forms one covalent bond to satisfy the octet rule.
- Magnesium has two valence electrons, which it can lose to form strong, stable ionic bonds. Recognizing the role of valence electrons helps us understand an atom's bonding behavior and predict the properties of the compounds they form.