The nucleus is very tiny compared to the cloud of electrons. Each chemical element has a different atomic number, which is the number of its protons. Atoms with different atomic weights but the same atomic number are called isotopes of one another. An electrically neutral atom must have a number of electrons that is equal to its atomic number, since protons are positively charged and electrons are negatively charged. Matter can exist in the different phases.
Most matter is solid at very low temperatures.
At higher temperatures it may become a liquid or a gas. If the matter consists of molecules made from two or more atoms, at relatively high temperatures the molecules will dissociate into individual atoms, a process called molecular dissociation. At very high temperatures, electrons may be stripped from atoms in what we call ionization. Hydrogen has just one electron, and so when it is ionized, only a proton is left. Iron has 26 electrons, and in extremely hot plasma, can have numerous electrons missing.