The three particles that atoms are composed of are protons, neutrons, and electrons.
Protons carry positive electric charge and make up a nucleus of an atom. Each atom has to have at least one proton. The mass of a proton is 1.7*10^(-27) kg and its charge is +1.6*10^(-19) C.
The nucleus of an atom might also contain neutrons. Neutrons are electrically neutral (do not carry electric charge) and are similar in mass and size to protons.
Electrons are much (2,000 times) lighter than either protons or neutrons, and carry negative electric charge. The mass of an electron is 9*10^(-31) kg and its charge is -1.6*10^(-19) C. The distance between the electrons and the nucleus in an atom is approximately 10,000 greater than the size of the nucleus. The charge of an electron is equal in magnitude to the charge of proton, so in any atom the number of protons equals the number of electrons, so that the whole atom is electrically neutral.