Why can't non metals be scratched?

Answer 1

It depends on which non metals you mean. The ability to scratch a material depends on its hardness relative to the material you use to scratch it with. With your nails you can scratch materials softer than your nails, like some plastics and minerals. But nothing can scratch in diamond because it's the hardest material there is, but in turn diamond can scratch all other materials, since they're softer that diamond including all metals and non metals.

So although most metals are relatively soft, there are many non metals that are softer that some metals.

Trivia:

the softest metal is Mercury with a hardness of 0 (it's liquid at room temperature)

The hardest metal is carbon steel with a hardness of around 8 to 9

The softest mineral is talc with a hardness of 1

The hardest mineral is diamond with a hardness of 10 (a form of pure carbon)

Answer 2

Non-metals are generally brittle and have weakly bonded atomic structures, which make them resistant to scratching. Unlike metals, non-metals do not have a lattice structure that allows for deformation without breaking, which makes them more prone to breakage rather than scratching when subjected to mechanical stress.