Beryllium chloride exhibits mixed covalent and dative bonding

Steps:

• Identify bonding types in group 2 and 13/14 chlorides: ionic for Mg, covalent for Al/Si/Be.
• Examine structures: BeCl2 forms polymeric chains with tetrahedral Be, involving both regular covalent Be-Cl bonds and dative bonds from Cl to Be.
• Compare with AlCl3 (dimeric with dative bridges), SiCl4 (pure covalent), MgCl2 (ionic lattice).
• Confirm BeCl2 uniquely shows both in its solid-state polymer, per VSEPR and Lewis acid-base theory.

Why D is correct:

• BeCl2 structure (BeCl2)n has terminal covalent Be-Cl bonds and bridging dative Cl→Be bonds, satisfying Be's octet via coordination.

Why the others are wrong:

• A. AlCl3: Primarily covalent with dative bonds only in specific dimers/adducts, not inherently mixed in monomer.
• B. SiCl4: Pure covalent tetrahedral molecule; no dative bonding as Si achieves octet without coordination.
• C. MgCl2: Ionic lattice with electrostatic bonds; no covalent or dative character.

Final answer: D