Answer:
Make-up of Hydrogen
Hydrogen atoms are extremely simple. They are the first atom on the periodic table, this means that they only have 1 proton. The majority of hydrogen atoms don't even have a neutron. As for electrons, hydrogen atoms with no charge have 1 electron. All of this means that for almost all hydrogen atoms the nucleus is made of one singular proton.
Diagram
The diagram shows a hydrogen atom. It looks like one circle because it is just showing the singular proton found in the nucleus. Electrons are extremely small compared to protons, so the electron is not shown in the picture. It is very hard to accurately depict electrons on diagrams like this because of their size and the fact that we can never know where they are. So, the diagram may look too simple but it just depicts how most hydrogen nuclei look.