The neutron is the other type of particle found in the nucleus. It carries no electrical charge. Due to the lack of charge it is not repelled when aimed at a nucleus, and therefore readily penetrates it. It has the same mass  as a proton, 1.67 x 10      kg.

In 1897, J.J. Thomson discovered the existence of the electron, marking the beginning of modern atomic physics. The negatively charged electrons follow a random pattern within the defined energy shells around the nucleus. An electron has a mass of 9.1 x 10      kg.