Atoms are made up-essentially, of three fundamental particles, which differ in mass and electric charge as follows:

 

 

The atomic mass unit (amu) is 1/12th of the mass of an individual atom of ₆C¹², i.e. 1.660565 ´ 10^-27 kg. The neutron and proton have approximately equal masses of 1 amu and the electron is about 1836 times lighter; its mass can sometimes be neglected as an approximation. The electron and proton have equal, but opposite, electric charges; the neutron is not charged.

 

The existence of electrons in atoms was first suggested, by J.J. Thomson, as a result of experimental work on the conduction of electricity through gases at low pressures, which produces cathode rays and x-rays, and a study of radioactivity by Becquerel, the Curies and Rutherford.

 

An atom is electrically neutral, and if it contains negatively charged electrons it must also contain some positively charged particles, and the supposition that they existed within atoms came about as a result of Rutherford's experiments in which he bombarded elements with the α - rays and α-rays were given off by radioactive elements. The neutron was discovered in 1932 by James Chadwick by bombarding beryllium with α rays.