Molecular orbitals have been used to quantitatively describe defects in several specific examples including native defects in silicon, the anion vacancy in binary II-VI compounds, substitutional defects in III-V compounds, and recently in qubit technologies. Yet, the qualitative framework described here can be used more generally to understand the charge states of defects and explain chemical trends. This can be a valuable guide for applications in which doping is essential for tuning the electrical properties of materials, such as thermoelectrics and transparent conducting oxides. In fact, the present molecular orbital analyses for Mg 2 Si can directly be applied to compounds with structures having the same atomic positions, such as half-Heusler TiNiSn and fluorite CeO 2 .