Question

In the vapor phase, beryllium chloride consists of discrete molecular units \(\mathrm{BeCl}_{2}\). 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

No, the octet rule is not satisfied for Beryllium in \(\mathrm{BeCl}_{2}\). Possible resonance structures that form an octet around Beryllium are not plausible due to its inherent stability with only four electrons, making such structures unlikely.

Step-by-step solution

Understand the Octet Rule

The Octet rule states that atoms prefer to have eight valence electrons in their outermost shell. Beryllium (Be), being a second period element, has two valence electrons. Its electronic configuration is 1s2 2s2. Thus Beryllium 'wishes' to have only four electrons (including valence electrons) to mimic Helium's electronic configuration, not eight as the Octet rule suggests.

Analyze Beryllium Chloride's Structure

Beryllium Chloride (\(\mathrm{BeCl}_{2}\)) in the vapor phase, is a diatomic molecule. Each Chlorine (Cl) atom shares its unpaired electron with Beryllium (Be) to form two covalent bonds, in hopes of completing its own octet, resulting in an electron-deficient Beryllium that does not follow the Octet rule.

Draw a Possible Resonance Structure Attempting to Fulfill the Octet Rule

A plausible resonance structure could involve shifting one of the electron pairs in the lone pairs on a Chlorine (Cl) atom to form a double bond with Beryllium (Be). The new structure, though, will not be very stable because Beryllium is more stable with four electrons in total. Therefore, such a resonance structure is not likely to exist under regular conditions.

Conclusion

While it is technically possible to create an octet around Beryllium in Beryllium Chloride, it is not plausible due to Beryllium's inherent stability with only four electrons. Therefore, the Octet Rule is not satisfied for Beryllium in this compound.

Key concepts

Chemical Bonding

Chemical bonding is the force that holds atoms together in molecules and can come in various forms, including ionic, covalent, and metallic bonds.

In the context of beryllium chloride (\( \text{BeCl}_2 \)), we encounter what is known as a covalent bond. Covalent bonds form when two atoms share one or more pairs of valence electrons. Beryllium has two valence electrons, while each chlorine atom has seven. Beryllium shares one electron with each chlorine atom, leading to the formation of two single covalent bonds.

This fulfillment of sharing with covalent bonds is different from reaching an octet; in beryllium's case, it is more energetically favorable for the atom to share only two electrons rather than striving for an octet, which contradicts the commonly taught octet rule, showing that real-world scenarios can differ from simplistic rules.

Resonance Structures

Resonance structures are a way of describing the delocalization of electrons within molecules where the actual structure is a blend of multiple possible structures. To visualize this, chemists draw different structures or 'resonance forms' that have the same arrangement of atoms but differing arrangements of electrons.

When looking at beryllium chloride, the idea might arise to create a resonance structure where the octet rule is satisfied by shifting an electron pair from a chlorine atom to form a double bond with beryllium. However, such a resonance structure for \( \text{BeCl}_2 \) is not favored since it forces beryllium into an energetically less stable state. Realistically, \( \text{BeCl}_2 \) doesn't adopt resonance structures since the single bonds present are the most stable arrangement for this molecule. This example serves as a prime teaching moment for students to understand that resonance structures are only relevant when they represent a real possibility for the stabilization of the molecule.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a pivotal role in chemical bonding and reactivity because they can be lost, gained, or shared during chemical reactions. The octet rule is based on the stability associated with having eight valence electrons, similar to that of noble gases.

In the case of beryllium chloride, the octet rule is not met for beryllium as it is surrounded by only four valence electrons - two from itself and one from each chlorine atom. The rule doesn't fully apply to beryllium because it is in the second period of the periodic table, where the energy cost to add additional electrons to satisfy the octet exceeds the energy stabilization gained from forming new chemical bonds. It's a clear demonstration that despite the rule's usefulness, there are exceptions that students must be aware of, and recognizing the stability of elements like beryllium with less than eight valence electrons is crucial for a deeper understanding of chemical bonding.