(a) All the protons and neutrons are located in the nucleus. Proton and neutron each have a mass equal to 1 atomic mass unit. Electrons revolving outside the nucleus have negligible mass. Thus, the whole mass of the atom is concentrated in the nucleus. So the nucleus is heavy.
(b) All the positively charged particles, i.e. protons, are located in the nucleus. Negatively charged electrons are present outside the nucleus. Thus, the whole positive charge of the atom is concentrated in the nucleus. So the nucleus is positively charged.
(c) The number of protons and number of electrons in an atom is equal. So the two charges neutralize each other and the atom, on the whole, is electrically neutral.