1. Analyze Octet Fulfillment:

• (CH₃)₂ (Ethane): Carbon (Group 14) has 4 valence electrons. It forms 4 sigma bonds (3 with H, 1 with C), resulting in a complete octet (8 electrons).
• (SiH₃)₂ (Disilane): Silicon (Group 14) acts similarly to Carbon, forming 4 bonds and completing its octet.
• PH₃ (Phosphine): Phosphorus (Group 15) has 5 valence electrons. It forms 3 bonds with H and possesses 1 lone pair. Total valence electrons = $2(lone pair) + 6(bonding) = 8$. Octet is complete.
• (BH₃)₂ (Diborane): Boron (Group 13) has 3 valence electrons. In the monomer $BH_3$, Boron forms 3 bonds, sharing 6 electrons. This is an incomplete octet (6 < 8), making it electron-deficient. It behaves as a Lewis acid (electron acceptor) [Concept of Lewis acids defined in Class 12 Chemistry, Unit 5, Sec 5.2]. To stabilize, it dimerizes to $(BH_3)_2$ using 3-center-2-electron bonds.

(Note: Specific details of diborane structure are from standard chemistry curriculum outside the provided text fragments.)