In the vapor phase, BeCl2 exists as a discrete molecule. (a) Draw the Lewis structure of this molecule, using only single bonds. Does this Lewis structure satisfy the octet rule? (b) What other resonance structures are possible that satisfy the octet rule? (c) On the basis of the formal charges, which Lewis structure is expected to be dominant for BeCl2?
In the vapor phase, BeCl2 exists as a discrete molecule. (a) Draw the Lewis structure of this molecule, using only single bonds. Does this Lewis structure satisfy the octet rule? (b) What other resonance structures are possible that satisfy the octet rule? (c) On the basis of the formal charges, which Lewis structure is expected to be dominant for BeCl2?
The Correct Answer and Explanation is:
Correct Answers:
(a) The Lewis structure of BeCl₂ with only single bonds is:
nginxCopyEditCl — Be — Cl
Beryllium has only two single bonds, giving it 4 electrons in its valence shell, not satisfying the octet rule.
(b) To satisfy the octet rule, resonance structures with double bonds between Be and Cl are proposed. These include:
- One structure with a double bond to the left Cl and a single bond to the right Cl, with a lone pair on the right Cl.
- A second structure where the double bond is with the right Cl and the left Cl has a single bond.
These two resonance structures look like this:
iniCopyEditCl=Be–Cl ↔ Cl–Be=Cl
Each structure gives Be 6 electrons from bonding and 2 from lone pairs (if we hypothetically consider electron donation from Cl), satisfying the octet.
(c) The dominant Lewis structure is the one with single bonds only:
nginxCopyEditCl — Be — Cl
Because in this structure, all atoms have formal charges of zero, and Be does not exceed its preferred coordination number (Be is small and prefers 4 electrons). The double-bond resonance structures give Cl a formal charge of +1 (due to 6 valence electrons in bonding instead of 7), which is less stable.
Explanation (300 words):
In the vapor phase, beryllium chloride (BeCl₂) exists as a linear, discrete molecule. In part (a), the simplest Lewis structure includes a beryllium atom at the center with two single bonds to chlorine atoms. Beryllium contributes 2 valence electrons, and each chlorine contributes 7, totaling 16 valence electrons. Distributing electrons to fulfill chlorine’s octet first, each Cl atom gets 6 non-bonding electrons, and Be forms two single bonds. However, Be is left with only 4 valence electrons around it—violating the octet rule, which ideally requires 8.
In part (b), alternative Lewis structures aim to satisfy the octet rule for Be by introducing double bonds. In such resonance forms, one of the Cl atoms forms a double bond with Be, allowing Be to reach 8 electrons. The two resonance forms alternate the position of the double bond between the two Cl atoms. These satisfy the octet rule, but they come at a cost.
In part (c), we consider formal charges to determine the most stable (dominant) structure. In the single-bonded structure, all atoms have formal charges of zero: Cl has 7 valence electrons—6 as lone pairs and 1 in the bond, and Be uses its 2 valence electrons in bonding. In contrast, the resonance structures with double bonds impose a +1 formal charge on Cl and a -1 on Be, which is less favorable.
Therefore, even though Be does not achieve an octet, the single-bonded structure is dominant due to its minimized formal charges and consistency with Be’s tendency to be electron-deficient.
