The binary heteroelemental monolayersHeteroelemental monolayers of group-IVA elements have many similarities with the corresponding elemental monolayers. Nevertheless, the monolayers of several combinations of group-IVA elements have novel interesting properties, such as the planar honeycomb carbides SiC, GeC, and SnC, which complement those observed for the elemental compounds and therefore are attracting considerable attention. Of particular interest are the heteroatomic monolayers containing carbon of the second period of the periodic table, due to the small atom size. A well-studied example is graphene-like SiC, which has a series of outstanding properties, such as a direct and wide band gap, high thermal and mechanical stability, suitability for high-power electronics, and hostile-environment electronics. The optical absorption spectra and visible light emission data qualify SiC monolayers for applications in optoelectronicsOptoelectronics, e.g., for blue light-emitting diodesLight-Emitting Diode (LED). The recent success of bottom-up syntheses of 2D SiC is a major step forward to various technical applications. Unfortunately, synthesisSynthesis of planar GeC and SnC monolayers and low-buckled SiGe, SnSi, and SnGe compounds, which are also promising candidates for various applications in electronics, optoelectronicsOptoelectronics, spintronicsSpintronics, sensoricsSensorics, and catalysiscatalysis, is still lacking. The three planar carbides and the three low-buckled compounds exhibit an inverse relationship between cohesive energyCohesive energy and bond length, as predicted by tight-binding theory. The mechanical properties, which depend strongly on the cohesive energy, exhibit a similar inverse behavior, elucidated for Young’s moduli and ultimate strengthsUltimate strength with respect to bond lengths. The calculated fractureFracture strengths are only partly in good agreement with Griffith’s ruleGriffith´s rule. The functionalization of these monolayers, e.g., by H, F or Cl atoms on one or both sides, transforms the planar monolayers into stable buckled structures. The functionalization introduces topological features of potential interest for applications in spintronicsSpintronics.