Problem 37 - Chemistry Matter and Change

Question

Draw the Lewis structure for BH \(_{3}\)

Step-by-Step Solution

Answer

BH\(_3\) has 6 valence electrons forming three B-H bonds, with boron as the central atom.

Step 1: Count Total Valence Electrons

To draw the Lewis structure of a molecule, start by determining the total number of valence electrons. Boron (B) is in group 13 and has 3 valence electrons. Each hydrogen (H) atom has 1 valence electron. Therefore, for BH\(_3\), we have 1 boron atom and 3 hydrogen atoms: \(3 + 3 = 6\) valence electrons in total.

Step 2: Choose the Central Atom

In drawing Lewis structures, the least electronegative atom is usually the central atom. Here, boron is less electronegative than hydrogen, so boron will be the central atom with the three hydrogen atoms surrounding it.

Step 3: Arrange Electrons to Form Bonds

Use the valence electrons to form bonds. Each hydrogen will form a single bond with boron. Use a pair of electrons (2) for each B-H bond. With three bonds formed, all 6 valence electrons are used up: \(3 \times 2 = 6\) electrons.

Step 4: Check for Full Octets and Stability

Hydrogen is stable with 2 electrons (duet rule), which it achieves by forming single bonds. Boron, in BH\(_3\), lacks a full octet (it has only 6 electrons around itself), but it is stable with only 6 electrons in some compounds, such as BH\(_3\).

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom, which play a critical role in forming chemical bonds. In a Lewis structure, it's essential to know the number of valence electrons to predict how atoms will bond. For example, boron (B) is in Group 13 of the periodic table, meaning it has three valence electrons.
Each hydrogen (H) atom has one valence electron.
In the molecule BH\(_3\), you sum them up: three electrons from boron and one from each of the three hydrogens, giving a total of six valence electrons.

• Boron: 3 valence electrons
• Hydrogen (x3): 3 valence electrons
• Total: 6 valence electrons for BH\(_3\)

Understanding the distribution of these electrons helps us draw Lewis structures and determine how atoms share electrons in a molecule.

Central Atom

When drawing a Lewis structure, identifying the central atom is a key step. The central atom is typically the least electronegative element, as it is more willing to share electrons.
In the compound BH\(_3\), boron is less electronegative than hydrogen, making it the central atom around which hydrogen atoms are arranged.
Picking the correct central atom is crucial for properly visualizing molecule geometry and predicting bonding.

• Less electronegative elements often serve as central atoms.
• For BH\(_3\), boron takes the central position.

Making the right choice ensures accurate representation of the molecular structure.

Electronegativity

Electronegativity is the measure of an atom's ability to attract and hold onto electrons. It's an essential concept when understanding chemical bonds and molecule behavior. In any given bond, the more electronegative atom tends to pull shared electrons closer, affecting molecule polarity and interactions.
In the case of BH\(_3\), boron is less electronegative compared to hydrogen, making it suitable to serve as the central atom.

• More electronegative atoms attract electrons more strongly.
• Electronegativity differences influence bond polarity.
• Boron's lower electronegativity leads it to be the central atom in BH\(_3\).

Understanding electronegativity helps predict how atoms will interact in a molecule.

Octet Rule

The octet rule is a guideline used in chemistry to predict atom behavior in bonds, stating that atoms tend to bond in a way that gives them eight electrons in their outer shell, similar to noble gases. However, there are important exceptions to this rule. Boron, for instance, is known for being electron-deficient and does not always follow the octet rule.
In BH\(_3\), boron is stable with just six valence electrons, which highlights one such exception.
Although each hydrogen atom achieves stability with a duet of two electrons (the duet rule), boron accommodates fewer than eight electrons while still forming stable compounds.

• Most atoms aim for an octet for stability.
• Boron often forms stable structures with fewer than eight electrons.
• Hydrogen follows the duet rule as a stable exception.

Recognizing these rules and exceptions is key to correctly predicting molecule stability and reactivity.