Answer:
3. A dense nucleus of protons and neutrons surrounded by electron clouds of different shapes and sizes.
Step-by-step explanation:
In the nucleus of an atom there are protons an neutrons densely packed together, while the electrons move around this nucleus.
The difference between answer 3. and 4. is that the electrons do not follow an established orbit, like planets do. They are too small and move too quickly for us to precisely determine both their position and their speed, at any given time. This is why they are thought to be electron clouds.