The general rule of thumb is that the (smaller or larger) a substance's atoms and the (stronger or weaker) the bonds, the harder the substance will be. fill in the parenthesis

Answer is: The general rule of thumb is that the smaller a substance's atoms and the stronger the bonds, the harder the substance will be. 
If the distance between atoms is higher, lesser will be attraction between electrons and protons of atoms, smaller distance means stronger atoms attraction.