Janus transition metal dichalcogenide (JTMD) monolayers have broadened the family of two-dimensional (2D) materials. Despite numerous theoretical predictions of JTMDs, the underlying stability mechanisms at the electronic scale remain systematically underexplored. Here, we investigate the group-dependent stability of 1H-phase JTMDs, revealing its origin in the bonding mode competition, coupled with the antibonding orbital splitting at the Fermi level through $d$-orbitals repulsion. Three orbital splitting configurations are identified to explain the group-dependent stability and structural phase preferences. Guided by the electronic origin of stability, we design a family of stable 2D Janus transition metal halides with intrinsic ferrovalley properties. This work bridges the gap between stability predictions and electronic structures, and extends the design guidelines for synthesizing 2D Janus materials.

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• Band gap
• Electronic structure
• Valleytronics
• Janus materials
• Transition metal dichalcogenides
• Density functional theory