Sub-Atomic particles
electron: mass of 0.00055 amu and a relative charge of -1
proton: mass of 1.00728 amu and a relative charge of +1
neutron: mass of 1.00866 amu and charge of zero
(Chadwick discovered the neutron in 1932)
Every atom has equal numbers of electrons and protons and this number is unique for each element. It is called the element's atomic number. Today we order the periodic table by atomic number, not atomic weight. The relationship between the charge on the nucleus and the arrangement of the elements was discovered by Henry Moseley in 1914.