Question

In the vapor phase, beryllium chloride consists of discrete \(\mathrm{BeCl}_{2}\) molecules. Is the octet rule satisfied for Be in this compound? If not, can you form an octet around Be by drawing another resonance structure? How plausible is this structure?

Short answer

The octet rule is not satisfied for Be in \(\mathrm{BeCl}_2\). A possible resonance structure with double bonds might satisfy the octet, but it's unlikely and implausible for Be.

Step-by-step solution

Review Electronic Configuration of Be

Beryllium (Be) has an atomic number of 4, resulting in an electron configuration of 1s² 2s². This means Be has two valence electrons in the 2s orbital.

Count Electrons Around Be in BeCl₂

In the \(\mathrm{BeCl}_2\) molecule, each chlorine (Cl) atom, with a higher electronegativity, forms a single covalent bond with Be by using one of its unpaired electrons to pair with one of Be's valence electrons. This sharing results in four electrons (two bonds) around Be.

Verify Satisfaction of Octet Rule

The octet rule states that elements tend to achieve eight electrons in their valence shell. Beryllium in \(\mathrm{BeCl}_2\) only achieves four electrons, consisting of two covalent bonds with two chlorine atoms, which violates the octet rule.

Draw Resonance Structure for Octet Completion

To satisfy the octet rule for Be, consider forming double bonds between Be and Cl, thus: Cl=Be=Cl. This implies Be shares two electrons with each Cl, achieving eight electrons around Be.

Assess Plausibility of Resonance Structure

While double bonds in the Cl-Be-Cl resonance structure hypothetically satisfy the octet, it is highly unlikely given Be’s small size and low electronegativity. Additionally, such a structure is rare due to the instability compared with the already stable, conventional geometry of \(\mathrm{BeCl}_2\) through simple single bonds.

Key concepts

Beryllium Chloride

Beryllium chloride (\(\mathrm{BeCl}_2\)) is a fascinating compound that deviates from the classic octet rule when it forms in the vapor phase. In this state, discrete molecules of \(\mathrm{BeCl}_2\) exist. The central beryllium atom bonds with two chlorine atoms, each forming a single covalent bond. This creates a linear molecular structure which is quite different from many other covalent compounds that aim to fulfill the octet rule. Though these bonds satisfy the valence requirements of chlorine, they leave beryllium with only four valence electrons, falling short of the conventional octet.

Electron Configuration

Understanding the electron configuration of beryllium is key to comprehending why it doesn't follow the octet rule in \(\mathrm{BeCl}_2\). Beryllium, with an atomic number of 4, has an electron configuration of \(1s^2 2s^2\). This indicates that beryllium has two electrons in its outermost shell. These are the valence electrons available for bonding. When forming \(\mathrm{BeCl}_2\), beryllium uses these two valence electrons to make single bonds with two chlorine atoms. This means only two electron pairs are shared, leading to a valence shell with merely four electrons for beryllium, hence deviating from the octet rule.

Resonance Structures

In chemistry, resonance structures are used to depict molecules where the electrons can be arranged in more than one valid configuration. For \(\mathrm{BeCl}_2\), the concept of drawing resonance structures to fulfill the octet rule comes into question. If beryllium were to form double bonds with each chlorine, as a resonance structure would suggest (i.e., \(\text{Cl}=\text{Be}=\text{Cl}\)), beryllium could gain eight electrons in its valence shell. However, this is hypothetically unstable due to beryllium's small atomic size and low electronegativity, making it improbable in reality. Thus, while drawing such resonance structures might theoretically satisfy the octet, they do not constitute plausible structures under typical conditions.

Covalent Bonding

Covalent bonding in \(\mathrm{BeCl}_2\) is a classic example of electron sharing between atoms. Each chlorine atom in \(\mathrm{BeCl}_2\) shares one of its unpaired electrons with one of beryllium's valence electrons to form a single covalent bond. To compare:

• Chlorine, having a high electronegativity, readily attracts electrons, achieving an octet with a single bond.
• Beryllium, however, only forms two bonds, resulting in fewer than eight valence electrons.

This creates a conundrum known as the octet rule exception, prevalent in some lighter elements and illustrating that while the octet rule is a useful guideline, it does not universally apply to all elements or compounds.