Is a atom small and hard?

Answer 1

The original Greeks thought of an atom was that it was a small hard chunk of the matter that it represented, not necessarily ball shaped.

As atomic theory developed the models built to represent compounds often used sticks (or springs) and balls to represent the atoms and their bonds.

These days an atom is thought to be mostly not there at all. A very small nucleus occupies the center made up of quarks which are hard to describe in physical terms, with an electron density cloud around it showing where the electron might be at any given time.

So yes, the atoms are small. And no, they are not hard, at least not hard like a marble.