At the center of every atom is a nucleus, comprised of a number of protons and (with the exception of hydrogen) neutrons. The simplest and lightest chemical element is hydrogen, which has a nucleus containing just one proton and no neutrons. Each distinct chemical element has a unique number of protons, coupled with a number of neutrons, which can vary.
A proton is a subatomic particle with a positive charge equivalent to the negative electrical charge of an electron. Protons and neutrons together form the nucleus of an atom. The number of protons in a nucleus determine the 'atomic number' of the element – the atomic number of hydrogen is 1; carbon has 6 protons so its atomic number is 6; iron has 26 protons, so its atomic number is 26, and so on.
A neutron is subatomic particle similar in mass to a proton, with no electrical charge. The number of neutrons in a nucleus determines the isotope, or particular form, of the element. For instance, iodine 127 and iodine 131 are both forms of iodine, with the same atomic number and the same number of protons, but with different numbers of neutrons and therefore different atomic weights.
An electron is a subatomic particle with a negative electrical charge. Electrons orbit around and exist outside of the nucleus of an atom. They are bound to the atom by electromagnetic forces, but they are very light and can move between atoms creating an electrical current. All electrical and electronic activity is created by moving electrons. As they form the outer shell or shells of an atom, electrons are largely responsible for how chemicals interact with each other.