Problem 83 - Chemistry An Atoms First Approach

Question

One type of exception to the octet rule are compounds with central atoms having fewer than eight electrons around them. \(\mathrm{BeH}_{2}\) and \(\mathrm{BH}_{3}\) are examples of this type of exception. Draw the Lewis structures for \(\mathrm{BeH}_{2}\) and \(\mathrm{BH}_{3}\)

Step-by-Step Solution

Answer

The Lewis structures for the given compounds are as follows: For BeH2: Be -- H -- H For BH3: B -- H | H | H Both central atoms exhibit fewer than 8 electrons around them, making them exceptions to the octet rule.

Step 1: Count the total number of valence electrons

To determine the valence electrons, we'll count the valence electrons for each atom within the compound. For example, Beryllium (Be) and Boron (B) have 2 and 3 valence electrons respectively, while Hydrogen (H) has 1 valence electron. For BeH2: Be: 2 valence electrons H: 1 valence electron each (2 Hydrogens present) Total valence electrons: \(2 + 2(1) = 4\) For BH3: B: 3 valence electrons H: 1 valence electron each (3 Hydrogens present) Total valence electrons: \(3 + 3(1) = 6\)

Step 2: Determine the central atom and arrange the surrounding atoms

The central atom in a compound is typically the one with the lowest electronegativity, excluding Hydrogen. For BeH2, the central atom is Be, and for BH3, the central atom is B. Arrange Hydrogen atoms around the central atom.

Step 3: Distribute the electrons

Now, we will distribute the valence electrons to form bonds and fill the outer shells of the atoms. Hydrogen requires 2 electrons to be stable, while Be and B can accommodate fewer than 8 electrons. For BeH2, we need 4 electrons. Each electron pair will create a bond between Be and one H atom: Be -- H -- H For BH3, we need 6 electrons. Each electron pair will create a bond between B and one H atom: B -- H | H | H

Step 4: Verify the structure and check for exceptions

We have now drawn the Lewis structures for both compounds. Let's verify whether they are exceptions to the octet rule. The central atom Be has 4 electrons around it, and B has 6 electrons. Both central atoms have fewer than 8 electrons around them, which confirms that these structures are indeed exceptions to the octet rule.

Key Concepts

Lewis Structure

A Lewis structure is a diagrammatic way of representing molecules. It shows how atoms are connected through bonds and how valence electrons are arranged around them. This handy tool helps visualize and understand the configuration of atoms and electrons in a molecule.
To draw a Lewis structure, follow these simple steps:

• Count the total number of valence electrons available in the molecule.
• Determine the central atom, typically the one with lower electronegativity, except for hydrogen which is never a central atom.
• Connect surrounding atoms to the central atom using electron pairs to form bonds.
• Distribute remaining electrons to complete all atom's octet, if possible.

In \(\mathrm{BeH}_2\), for instance, we sketch a structure where each hydrogen atom (with 1 electron each) forms a bond with the central beryllium atom (with 2 electrons). These bonds use all 4 electrons available, filling hydrogen's requirement of 2 electrons for stability but showing beryllium as an exception to the octet rule.

Valence Electrons

Valence electrons are the electrons available for bonding and are found in the outermost shell of an atom. They play a crucial role in Lewis structures as they determine how atoms in a molecule will connect and interact with each other.
To determine the number of valence electrons, consider the group number of the element in the periodic table:

• Beryllium (Be) is in group 2, thus having 2 valence electrons.
• Boron (B) belongs to group 13, contributing 3 valence electrons.
• Hydrogen, in group 1, has 1 valence electron.

When dealing with \(\mathrm{BeH}_2\) and \(\mathrm{BH}_3\), add up the valence electrons for all atoms in the molecule to get the total electrons available for bond formation. For example, \(\mathrm{BeH}_2\) has a total of 4 valence electrons, while \(\mathrm{BH}_3\) has 6 valence electrons. This counting helps in setting up the initial framework of the Lewis structure.

Central Atom

The central atom in a molecule's Lewis structure is often the one with the lowest electronegativity, making it less greedy for electrons. Typically, the central atom is where all surrounding atoms will connect.
Choosing the correct central atom is crucial and follows a basic rule: always exclude hydrogen from being the central atom due to its tendency to only form one bond. Instead, look for atoms with lower electronegativity.
In \(\mathrm{BeH}_2\), the central atom is Beryllium because of its lower electronegativity. For \(\mathrm{BH}_3\), Boron takes the center stage. Once chosen, hydrogen atoms will be placed around the central atom, using available valence electrons to establish connections.

Electronegativity

Electronegativity is a measure of how strongly an atom can attract shared electrons in a chemical bond. It plays a key role in deciding the central atom in Lewis structures, where atoms with lower electronegativity usually serve as the central atom.
The concept of electronegativity helps explain why some atoms form exceptions to the typical octet rule:

• Beryllium and Boron have relatively low electronegativity compared to many other elements, which permits them to function as central atoms even with fewer than 8 electrons.
• Due to their desire to balance electron sharing, Beryllium in \(\mathrm{BeH}_2\) and Boron in \(\mathrm{BH}_3\) do not require a complete octet.

This understanding of electronegativity helps explain why Beryllium and Boron, despite lacking a full set of 8 valence electrons, still form stable compounds under certain conditions.