How will you measure very small size like size of an atom of a substance

How will you measure surface tension?

What is the size of atom?

How will you measure the height of an accessible object.

How will you measure the distance of a hill by echo method?

How will you prepare atomic hydrogen?

How will you measure the height of an inaccessible object using triangulation method?

How will you express gram atomic mass in terms of moles?

Suppose we have a large number of identical particles, very small in size. Any two of them at 10 cm separation repel with a force of 3 x 10^-10N. Suppose you measure the repulsion and find it 6xx10^-6N . How many particles were there in the group?