If we divide a drop into smaller and smaller parts, what will be the smallest particle that can be obtained and that will continue to be water?

If it hits a single atom, a "product" can be broken. But only until it is a single molecule will a "molecule" be separated. Of starters, oxygen, H2O, always appears like oxygen in millions. But if you split it into smaller and lesser parts, say half, you can divide it only until you get 2 hydrogen and 1 oxygen. It's no longer water if you divide it further. It's going to be another matter.

Hope this helps!