If you take chemistry, you will learn about electrons. Electrons are the smallest of the particles that make up an atom, and they carry a negative charge.
The number of protons and electrons is equal in each atom. The hydrogen atom, for example, has just one electron and one proton. The uranium atom, on the other hand, has 92 protons, and therefore, 92 electrons. The atom's atomic number is the number of protons in the atom, which by default is also the number of electrons. An electron is so small that it cannot be seen with the naked eye, but neither can the atom itself.