An atom is made up from three types of particles, electrons, protons and neutrons. Two types of them, protons and neutrons, are located in nucleus of the atom and these particles are held together within nucleus by strong nuclear forces.
The protons are positively charged, while neutrons are not charged. The size of charge of protons is the same with the size of charge of electrons. Since electrons are negatively charged and based on the phenomenon that the opposite charges attract, electrons and protons attract each other.
Hence, completing the empty space, you need to say that the nucleus of an atom consists of nucleons called protons and neutrons.