A hydrogen nucleus is just a single proton. All the other nuclei contain protons and neutrons. The nucleus gets bigger and heavier as we move down the list of elements, until we come to element 92 - uranium. This has 92 protons and 146 neutrons. Surrounding the nucleus there is a cloud of electrons .
Electrons have a negative charge.
This charge will just cancel out the positive charge on a proton. The number of electrons in an atom is equal to the number of protons. This atomic number decides which element the atom belongs to, and how it will behave chemically. It would take about 2,000 electrons to weigh as much as a proton, so nearly all the mass of an atom is in the nucleus. Look at 1 millimetre on a ruler.