Protons, neutrons and isotopes

Every nucleus is built from two kinds of nucleon:

• the proton: charge +e, mass about 1.0073 u
• the neutron: no charge, mass about 1.0087 u. It was identified by James Chadwick in 1932.

Three numbers describe a nucleus:

• Z, the atomic number: how many protons. This decides the element. Z = 6 is always carbon; Z = 92 is always uranium.
• N, the neutron number.
• A, the mass number: the total number of nucleons.

A = Z + N.

The standard way to write a nuclide is with A at the top left and Z at the bottom left of the symbol: ¹⁴₆C is carbon with A = 14 and Z = 6, so N = 14 − 6 = 8. You will also see it written as carbon-14 or C-14, which leaves Z to be looked up because the symbol already tells you the element.

Isotopes are nuclei with the same Z but a different N. They are the same element with different masses.

• Carbon-12 (6 protons, 6 neutrons) is by far the most common, about 99 percent of natural carbon.
• Carbon-13 (6 protons, 7 neutrons) makes up about 1 percent.
• Carbon-14 (6 protons, 8 neutrons) is radioactive and extremely rare, about one carbon atom in a trillion.

Hydrogen has three well-known isotopes: ordinary hydrogen-1 (one proton), deuterium (hydrogen-2, one proton and one neutron) and tritium (hydrogen-3, one proton and two neutrons). Water made with deuterium is called heavy water. You will meet both D and T again in the fusion world.

Chemistry depends on how an atom's electrons are arranged, and a neutral atom has as many electrons as protons. Isotopes of one element have the same number of electrons, so they behave almost identically in chemical reactions. What differs is the mass and, often, the stability of the nucleus.

Counting mass. Nuclear masses are written in atomic mass units: 1 u is exactly one twelfth of the mass of a carbon-12 atom, which is about 1.6605 × 10⁻²⁷ kg. That is why the mass number A is close to the mass in u. Carbon-12 is exactly 12 u by definition.

The atomic mass printed on a periodic table is an average over the natural mixture of isotopes. Chlorine is listed as 35.45 because natural chlorine is roughly three parts chlorine-35 to one part chlorine-37. No single chlorine atom actually has that mass.