Answer:
The correct answer are "smaller; stronger".
Explanation:
In chemistry, hardness is defined as how difficult is to penetrate or scratch a substance. The general rule of thumb for hardness is that the smaller the substance's atoms and the stronger the bonds, the harder the substance will be. For instance, diamonds which are the hardest natural substances in the world, are made of carbon atoms which are strongly attached to each other forming covalent bonds in each atom available (one carbon attached to other four carbons).
