Question

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

Short answer

The Lewis structures for BeH2 and BH3 are as follows: BeH2: H - Be - H BH3: H | H - B - H In both structures, the central atom (Be in BeH2, and B in BH3) has fewer than 8 electrons around them, thus representing an exception to the octet rule.

Step-by-step solution

Determining Valence Electrons

Determine the valence electrons for the elements involved in the molecules BeH2 and BH3: Be, B, and H. Beryllium (Be): Group 2, 2 valence electrons Boron (B): Group 13, 3 valence electrons Hydrogen (H): Group 1, 1 valence electron

Drawing BeH2 Lewis Structure

To draw the Lewis structure for BeH2, first put Be atom (the central atom) in the center and draw two H atoms on either side of Be. 1. Since Be has 2 valence electrons and each H has 1 valence electron, the total number of valence electrons in BeH2 is 4. 2. Use 2 of these 4 valence electrons to form a single bond with each of the two H atoms. 3. Now, all the valence electrons are used up, and we have the Lewis structure for BeH2: H - Be - H

Drawing BH3 Lewis Structure

To draw the Lewis structure for BH3, first put B (the central atom) in the center and draw three H atoms around it. 1. B has 3 valence electrons, and each H has 1 valence electron, so the total number of valence electrons in BH3 is 6. 2. Use these 6 valence electrons to form single bonds with each of the three H atoms. 3. Now, all the valence electrons are used up, and we have the Lewis structure for BH3: H | H - B - H In both BeH2 and BH3, the central atom does not obey the octet rule, having only 4 and 6 electrons around them, respectively. These are examples of exceptions to the octet rule.

Key concepts

Lewis structures

Lewis structures are visual representations that show how atoms in a molecule share electrons. They help illustrate the arrangement of valence electrons around atoms in a molecule. This is pivotal in understanding molecular geometry and bonding. The basic idea is to place the valence electrons around the element symbols as dots and to represent shared pairs of electrons or bonds between atoms withlines.
For example, in the case of the molecule BeH₂, the central beryllium atom is connected to hydrogen atoms by single bonds. Each bond consists of one pair of shared electrons. Similarly, in BH₃, the central boron atom forms three single bonds with hydrogen atoms.

• It helps visualize which atoms are bonded together
• Depicts the distribution of valence electrons in a molecule
• Provides insights into the bonding properties of the molecule

Previously, you might have learned that atoms tend to form bonds to achieve a stable electronic configuration similar to noble gases. The Lewis structure helps in identifying how this configuration can be achieved, even in molecules that are exceptions to the octet rule.

Valence electrons

Valence electrons are the outermost electrons of an atom that are involved in forming chemical bonds. They play a crucial role in determining the chemical properties and reactivity of the atom. For most elements, these electrons are found in the outermost shell of an atom's electron cloud.
To figure out the number of valence electrons available for bonding, check the group number of the element in the periodic table. For example, Beryllium (Be), which is in group 2, has two valence electrons. Boron (B), from group 13, has three valence electrons. And Hydrogen (H), being a part of group 1, has one valence electron.

• Valence electrons determine how atoms will bond with each other
• They are indicated in Lewis structures, showing how they share or transfer between atoms
• Understanding valence electrons helps explain exceptions to the octet rule, like in BeH₂ and BH₃

In compounds like BeH₂ and BH₃, the central atoms don't achieve a complete set of eight valence electrons. Instead, they form stable compounds with fewer electrons, showcasing that while the octet rule is a useful guideline, it doesn't cover all cases.

Chemical bonding

Chemical bonding refers to the attractive force that holds atoms together in a compound. This force arises due to the sharing or transfer of valence electrons between atoms, which leads to a more stable electron configuration. There are several types of bonds, but in BeH₂ and BH₃, we are primarily dealing with covalent bonds, where electrons are shared between atoms.
In the molecule BeH₂, each hydrogen atom forms a single bond with the central beryllium atom by sharing a pair of valence electrons. Similarly, in BH₃, three hydrogen atoms form single bonds with boron. Despite beryllium and boron not completing an octet, their bonds are stable and form a complete shape for the molecules.

• Chemical bonds allow atoms to achieve stable electron configurations
• Covalent bonds involve the sharing of electrons between atoms
• Even when the octet rule is not fulfilled, atoms can still form stable compounds through bonding

Understanding chemical bonding, especially in terms of exceptions to the octet rule, is key to predicting molecule behavior and properties. It showcases the diversity and adaptability of chemical interactions beyond conventional rules.