The nucleus of an atom is its central body, containing a net positive charge due to the presence of protons in it. The nucleus also contains neutrons that are electrically neutral and have negligible mass. The number of protons in the nucleus also represent the atomic number of the element. Different neutrons in the different atoms of the same element make isotopes of the element. Protons and neutrons are held together inside a nucleus by strong forces of attraction.
Protons are the only positively charged subatomic element in an atom. Unlike electrons, they do not revolve in atomic orbitals and are around 1,837 times heavier than electrons. Protons are always equal in number to the electrons, which keeps an atom in an electrically neutral state. Protons are made up of two "up" quarks and one "down" quark. Quarks are, so far as we now know, the fundamental particles that make up neutrons and protons.
Electrons, usually represented by the symbol "e," are the only negatively charged elements found in an atom. The negative charge of an electron is of the magnitude 1.6022 x 10-19 coulomb, and the mass of an electron is 1.1 x 10-31 kg. Electrons are placed in atomic orbitals -- s, p,d and f -- that can hold a maximum of two, six, ten and fourteen electrons respectively.
Neutrons reside inside the nucleus, along with protons, and have no electrical charge. The mass of a neutron is 1.6749 x 10-27 kg, making it 1,842 times heavier than an electron. Because of the presence of neutrons and protons, the nucleus is by far the densest part of an atom. Like a proton, a neutron is made up of three quarks. A free neutron not contained inside a nucleus can be radioactive. It undergoes beta decay, giving rise to a proton and an electron.